Alt-BEAM Archive
Message #07429
To: alt-beam@egroups.com
From: michael.hirtle@ns.sympatico.ca (Michael Hirtle)
Date: Sun, 07 Nov 1999 18:09:37 -0400
Subject: [alt-beam] Re: Virtual BiCORE redeux...
Ummmmmmmm..... wilf my computer is not reading any of the attached
pictures in the file u sent with the message.
BEAM HEADS 101 - a beam circuit tutorial
Rev 1.0 11/99 - wilf rigter
WHAT'S A BEAM HEAD
A BEAM HEAD is a mechanical and electronic assembly (Fig 1) that has the
simple function of detecting and tracking a light source. It does this
by rotating a set of photo sensors so that both receive an equal amount
of light. The head is usually a circuit board containing the electronics
mounted on the body of a small gear motor with the shaft of the motor,
as the neck, attached to fixed support or mobile robot body. While
seeking or tracking a light, a head continuously rotates around its
neck. Since an umbilical power cord could lead to strangulation, the
power source (battery or solar) is usually carried along with the
electronics on the head.
[Image]
By adding a second motor and a duplicate set of electronics, the head
may also look up or down. A head rotating in a 2D plane is referred to
as a 2 Degrees of Freedom (2DOF) head and a head with the additional up
down motion scanning a 3D volume is called a 3DOF head.
ELEMENTARY DESIGN DEAR WATSON
In any design project, we chose and combine elements from various older
designs blended with new ideas to meet specific requirements. The choice
of one element may influence the choice of the other elements. For
example, a large motor may require the addition of a separate motor
driver. The PowerSmart head circuit is a simple solution to meet the
design requirements for a small efficient 2DOF head controller. It has
some unique design ingredients but it owes its concept and general form
to Mark Tilden and others and ultimately to nature itself.
THE BASE
Of all the head elements, the base or support structure is the least
sensitive to influencing the other head elements. It can be almost any
shape that does not obscure "vision" and can made of any material
sufficiently strong to hold up the head. Needless to say, for humans as
well as robots, the overall aesthetics of the head are an important part
of the design and while the simple tripod in this example will suffice,
don't limit your imagination in your own design choices. It is also
possible to wall or ceiling mount a head with the appropriate support.
The details of a mobile base are beyond this introductory tutorial, but
I hope to explore the design of a head mounted on a robot in a future
article.
THE MOTOR
In the PowerSmart head design, an efficient 30 rpm gear motor was chosen
but unmodified and modified hobby servos have also been used albeit with
a different electronics design. Your choice of motors will have an
influence on the design of the electronics and for now we will
concentrate on small gear motors, typically found in video camera lens
assemblies (ie Nihon - BGMicro), which can be driven directly from
74HC/AC logic. The motor I used was mounted with the shaft pointed
vertically down. A small brass collar with a setscrew was attached to
the end of the shaft and soldered to the copper wire tripod base.
THE BRAIN
The head has a brain made with various electronic components including
resistors, capacitors, diodes, LEDs, solar cells, photo sensors, a
voltage trigger and a single IC chip. The brain controls the motion of
the head, in response to the location of a light spot within the field
of vision. There are many ways to construct a head circuit: I used a
Radio Shack 2"x2" square perfboard mounted with two stiff 1/4" wires
connected to the solder terminals on the top of the motor body. The two
photo diodes (PD) or light dependent resistors (LDR) inside small black
tubes are mounted on one side of the board facing out, like a pair of
eyes, in the same direction. To help you understand the basics before
tackling what goes on in the brain of a HEAD, I have also included a
short appendix with a simple description of electronic principles and
useful mechanical models of electronic components used in this tutorial.
WHAT IS A POWER SMART HEAD
The POWERSMART (PS) head circuit was designed as an improved version of
the bicore head. The operation of the PS head circuit will be described
in more detail as an example of BEAM heads in general. The basic PS head
schematic in Fig 2 contains everything except for the SE part. The PS
head uses a 74HC240 Octal inverting buffer organized as 4 inverters in 2
groups each with a separate group output enable. If the second group of
inverters is not yet used, they should have pins 1, 2, 4, 6 and 8
temporarily connected to 0V. Because of its power saving features the PS
head is easily solarized but for some applications, the basic circuit
plus batteries is all you need and of course there is another set of
inverters in case you want to try building a 3DOF head. If want to
solarize the head, one of the two the solar engines can be assembled and
tested. The PS head is controlled with pin 19 connected to the SE enable
output instead of 0V. The operation of the circuits is described in the
next few sections following the order in which the PS head can be build
and tested.
[Image]
Fig 2
THE SOLAR ENGINE
The power supply for the SE head can be 3-6V Alkaline batteries but for
long term autonomous operation power can be generated by a solar cell
and stored in a large capacitor or nicad batteries as shown in Fig 3.
[Image]
Fig 3
Both the bicore heads and PowerSmart heads can be solarized this way.
The solar cell should be chosen to be compatible with the capacitor or
rechargeable battery voltage rating. With sufficient light the solar
cell will slowly charge the nicad battery or capacitor. A Ge diode is
recommended placed in series with the solar cell to prevent the
discharge of stored energy when the light level fluctuates. When the
voltage of the stored charge is sufficient to operate the circuit the SE
triggers. The output of the SE enables the head circuit and head will
attempt to find and lock on to a bright light. For the PS design we look
at two SE options: one using a MC34164 voltage supervisor in a
simplified Chloroplast SE circuit and the other is a 1381 SE adaptation
by Darrell Johnson shown also in Fig 10. Of course only one of the two
SE circuits should be used.
1381 SE
The 1381L voltage supervisor in Fig 4 is used to sense if the voltage on
the power supply capacitor has sufficiently charged up (about 3V) and
its output goes active high when the 1381 triggers. The 1381 has very
small hysteresis and will quickly reset so its output is used to SET a
latch made with two inverters of the second group. The inverted output
of the latch provides and an ENABLE signal to hold the 74HC240 tri-state
control pin19 active low. The latch is held SET by 2 diode in a positive
feedback loop. When the supply voltage drops below the forward voltage
of the 2 feedback diodes, the latch resets disabling pin 19. Because the
PS head uses very little power when locked on, the charge on the solar
cap can last a long time. When the motor runs, the voltage on the
storage capacitor drops until it is less than the forward voltage of the
2 SE latch diodes at which point the latch resets. The voltage on Pin 19
will rise to V+, disabling the PS motion generator circuit and the power
supply capacitor starts to charge up again from the solar cells.
[Image]
Fig 4
THE CHLOROPLAST SE
The Chloroplast SE (CSE) design in Fig 5 is much simpler and uses a
MC34164 voltage trigger instead of a1381. No latch is required for the
CSE since it cleverly uses a series input resistor to increase
hysteresis between the set and reset voltages of the trigger. The 1M
resistor is adjusted to set the trigger point and the reset occurs near
5V. Instead of the usual MPS12A darlington motor driver transistor in
the original design by Craig Maniard, we use a 2N3904 for logic
inversion to drive the 240 tri-state enable pin 19 active low. If you
intend to build a 3DOF PS head, then both pin 1 and pin 19 would be
enabled.
[Image]
Fig 5
PS CIRCUIT DETAILS
In the next paragraphs, we describe each section of the PS head design
as you build it. I recommend that you initially use a 3V to 6V battery
or bench power supply to test and debug each step before proceeding to
the next step. Even if you intend to build the solar version, you should
build and debug the PS head circuit separately. Once the PS head circuit
is set up, add the SE to solarize the circuit.
PS SENSORS
In Fig 6, we see a "photo bridge" with two light dependent resistors
(LDR) or Photo Diodes (PD) used as the PS head "eyes". Either type of
sensor is ok. Even green LEDs can be used instead of photo diodes but
are inferior in terms of sensitivity and can only be used in bright
conditions.
[Image]
Fig 6
The photo bridge serves the same function as photo diodes used in bicore
head circuits but unlike the bicore circuit, the photo bridge acts as a
voltage divider with a midpoint "ratio-metric" output voltage which is
largely independent of the absolute light level. When the top (turn
right) LDR/PD is exposed to more light the midpoint voltage rises and
when the bottom (turn left) LDR/PD is brighter then the voltage at the
midpoint drops. So if both are exposed to the same light level the
midpoint voltage is about 1/2V+. The LDR or PD pair should be carefully
matched so that their resistance is approximately the same value when
each is exposed to the same light level. Radio Shack currently sells a
package of 5 LDRs for a few dollars but you may need to buy two packs to
get good matched pairs. You can use a digital voltmeter connected
between the midpoint of the photo-bridge and 0V to measure sensitivity.
With both sensors pointing at a diffuse light source, the midpoint
should at about 1/2V+. Don't worry if they are not perfectly matched,
since this will only cause a error in the angle of the head when locked
on to a light. One other supplier of LDRs and PDs is Solarbotics who
sell premium matched pairs ready to use. Optional 1K resistors can be
added to each leg of the photo bridge to provide a minimum resistance
between +V and 0V to avoid excessive current flow if the LDR resistances
decrease to very low values from exposure to very bright light. The
important point to remember is that this part of the photo bridge
produces an output voltage at the midpoint, which is proportional to the
difference in light level on each sensor. That makes the circuit work
over a large range of light levels with little change in sensitivity.
The output voltage is used to control the duty cycle (on/off ratio) of
the following oscillator stage in a very interesting and useful way.
PS HEAD HI/LO OSCILLATOR
The brain of the PS head is a classic CMOS astable oscillator with a
difference: it has an additional input resistor, which provides control
of the symmetry of the oscillator waveform proportional to the input
voltage. This is a form of pulse width modulation over a range, which
includes 0-100% on/off ratios. This is sometimes called a saturating
oscillator since the output will be continuously high or low when the
control voltage is higher or lower than the linear control voltage input
range. The resulting circuit in Fig 7 is therefore called a High/Low
Oscillator (HLO).
[Image]
Fig 7
First let's examine how the HLO without using the IN signal, behaves
like a classic CMOS astable oscillator. The oscillator consists of 2
inverters with the input of the first inverter serving as a summing node
for several signals. As can be seen, a 5M potentiometer (R1) is
connected from the output of inverter A back to it's own input. Because
of the signal inversion, this is DC negative feedback, which tends to
drive the input voltage of inverter A towards its 1/2V+ threshold. The
output of inverter A is also directly connected to the input of inverter
B. The output of inverter B is connected with a capacitor back to the
input of inverter A. Because of the even (2) number of inverter in that
feedback path, this is positive AC feedback, which drives the input to
0V or +V. This feedback capacitor (C1) has a similar function to the two
capacitors in the bicore circuit described later. The inverter A
initially has a voltage equal to V+ across the feedback resistor which
discharges the voltage on the capacitor and the input of inverter A and
eventually causes it's own output voltage to change from high or low
towards the center linear region. When the output of inverter A enters
the linear region of the inverter B input the signal passes inverted
through inverter B and its output starts to change. The output signal of
inverter B, capacitively coupled back to the input of inverter A, will
rapidly drive the changing input and output voltages to their saturated,
supply voltage limited, high or low states. Once stable, these levels
are again subject to dc negative feedback and the process repeats but in
the reverse direction.
To recap, the voltage at inverter A output connected to the feedback
resistor is opposite to the voltage on the capacitor at the summing
input of inverter A, so the capacitor discharges through the resistor
with the input voltage of A eventually reaching the threshold. This
causes the output of inverters A and B to change and the capacitor sends
the change back in phase to the summing input of A thereby rapidly
accelerating the change. When the outputs have changed state, the whole
process repeats. The result is two square wave signals at the output
pins of the inverters with one signal inverted with respect to the
other. The frequency of the squarewave signal is inversely proportional
to the product of the capacitor and resistor values. If you adjust the
pot value to a smaller value the frequency increases. Before the final
adjustments leave the pot in the center position
PULSE WIDTH MODULATION
As described in the photo bridge section, that circuit generates a
voltage proportional to the difference in the left and right LDR light
levels. Now we convert this voltage to a useful waveform to control the
motor by connecting the photo bridge output to the IN terminal of the
HLO as shown in Fig 8. While separate fixed resistors can be used, the
adjustable potentiometers R1 and R2 are recommended for reduced parts
count, to explore various head behaviors and to compensate for
variations in voltage/logic thresholds. The 0.022 cap shown in the
schematic can be two 0.01 caps connected in parallel.
[Image]
Fig 8
The potentiometer R1 is connected to pins 15 and 17 and the IN terminal
to the midpoint of the photo-bridge to adjust the sensitivity,
oscillator frequency and degree of pulse width modulation. Since the
adjustment inherently affects all three functions, it requires tuning to
give the right response. The output voltage of the photo-bridge is
proportional to the difference of light on the photocells. That voltage
is connected via part of R1 to the input of inverter where it is
"summed" with the positive and negative feedback of the capacitor and
the feedback resistor, which is the other part of the potentiometer. The
photo-bridge difference voltage (Vp) results in an offset current which
generates equal left and right pulse widths at the oscillator outputs
when the Vp is balanced at 1/2V+ (or near the switching threshold) but
changes the left right pulse width if Vp is unbalanced. It is useful at
this point to also connect R2, C2, inverters C and D and the motor as
shown in Fig 2 to observe the effects of the photo bridge and adjustment
of R1 on the motor. For this part of the circuit testing, potentiometer
R2 must be adjusted to zero ohms so that inverters C and D simply act as
motor drivers. With R1 adjusted to the maximum value, and exactly equal
light falling on the photo sensors, the motor receives equal duration
left/right pulses, it vibrates rapidly but does not move. But when the
photo-bridge is even slightly unbalanced, the effect of the offset
current causes the pulse width to change asymmetrically producing a net
rotation in the direction of the longer motor on time. By adjusting R1,
the photo bridge sensitivity is changed and the motor will respond more
slowly to light imbalance. As a rule, the value of R1 between inverter A
input and output is adjusted between 90 - 50% of its full value before
the sensitivity of the photo bridge becomes zero. Adjusting R1 to a
smaller value yet is not recommended as it will only increase the
frequency of the square wave, which will become audible but motion will
drop to zero and the supply current will increase. As a small
improvement, a 5M resistor can be added between the output of inverter A
and R1. With R1 adjusted between 90 and 80%, the optimum (medium)
sensitivity is achieved. At that point, the motor is stopped when the
light is nearly balanced and the motor moves slowly left or right when
the light is slightly unbalanced. When the photo-bridge is more
unbalanced, the oscillator outputs are forced to a continuous high and
low state and the motor rotates at full speed in the desired direction
to balance the photo-bridge.
THE Nv/Nu OUTPUT STAGE
Let's have a look now at Fig 9, the Nv/Nu output stage. With the value
of R2 reduced to 0 ohms during the adjustment procedure of R1, the two
output inverters C and D have no effect on the complementary square
waves at inputs IN and /IN other than inverting and isolating them from
the motor load. If potentiometer R2 is 0 ohms and the photo-bridge
output is balanced, the left right pulse width is symmetrical, and the
head stops rotating but the power continues to flow through the motor
while stopped and this is wasted energy. A solution to this problem is a
circuit, which detects that condition and shuts off the power to the
motor. . Increasing the R2 pot resistance value to mid point, it forms,
in combination with C2 and inverter C, a "Nv/Nu" circuit. When R2 is
precisely adjusted, the Nv/Nu time constant will match the frequency of
the HLO and when symmetrical or balanced as would be the case when the
head is "locked on", the motor driver output voltages will be in phase,
completely turning off the motor current and reducing supply current to
about 2 ma.
When the photo bridge is slightly unbalanced, the HLO output waveforms
are asymmetrical and the duration of either the left or right pulse
widths will be longer than the Nv/Nu time constant and the motor will
turn at a speed proportional to the difference. When the HLO output is
high or low, those signals will of course quickly pass through the Nv/Nu
circuit to drive the motor left or right at full speed.
[Image]
Fig 9
To recap: when the photo-bridge is greatly unbalanced, the HLO outputs
are forced high or low. This results in a steady state condition in
which the dc voltage from the two HLO outputs are can pass through the
resistor but not the capacitor (in the fashion of a Nu). This results in
the Nv/Nu stage having one driver on and one off and the motor
continuously rotating towards the light source. Now assume that the
light source is near the center of the field of vision and that the
astable is oscillating but with a left pulse on time which is longer
than the right pulse. Now the Nv/Nu stage acts like a Nv monostable but
again with a small twist: unlike its cousin, the classic Nv, this one
can produce positive and negative output pulses. The result is that for
the "right" direction pulses, the two outputs of the Nv/Nu stage are the
same polarity and NO current flows through the motor. But for the "left"
direction pulses, the Nv times out and for a short duration the two
motor drivers have different output polarity. This provides a
proportional output, which turns the head SLOWLY to the left. So when
approaching the balanced condition the PS head slows down accordingly
and the current also starts to drop off saving power. When the light on
both photocells is fully balanced, the lowest power condition exists
where the left and right pulses are the same duration and the Nv
monostable never times out. Therefore the motor driver outputs always
have the same polarity, no motor current flows and the motor is stopped.
This is the low power (2ma) standby mode that gives the PS head its
name. The circuit is now ready to hook up to the SE stage.
A SOLAR POWERSMART HEAD EXAMPLE
Fig 9 shows the solarized PS head layout drawing by Darrell Johnson
using a 1381L solar engine.
[Image]
Fig 10
Note the unused input pins 6 and 8, which have been terminated to reduce
power.
WHAT'S A BICORE HEAD
Most BEAM heads use the ubiquitous bicore circuit with photodiodes (Fig
10), designed by Mark Tilden, also used in walkers and photovores. The
design is elegantly simple but has some shortcomings. One problem is the
effect of the photo diodes on changing the bicore frequency as well as
on/off time. Additional resistors can limit the influence of the PD's
but this effectively limits operation to a narrow range of lighting
conditions. The circuit also consumes a significant amount of power in
proportion to the work of rotating the head and that's an important
factor for battery or solar operation. There are reports of an updated
bicore head design but at this time it has not been published.
[Image]
Fig 11
The bicore operation is as follows:
Assume that the voltage on bicore 240 inverter A input pin is V+ and
that inverter B input pin is 0V. Therefore the difference voltage (V+)
appears across the resistor and the reverse biased PD2. The output pin
of inverter A will be opposite to its input voltage ie 0V and inverter B
output pin will be V+. The current through the resistor and PDs starts
to rapidly discharge the two capacitors with the voltage on both ends of
the resistor changing towards 1/2V+. The rate of discharge is influenced
by increased light level on PD2, which will effectively lower the total
resistance. As the voltage across the resistor and PD2 drops the current
decreases and the rate of discharge slows down. This is the
characteristic exponential discharge curve for a RC network.
Theoretically, if the switching threshold of the inverters (where the
output start to change) was precisely 1/2V+ then it would take a long
time for the caps to completely discharge. But unequal capacitor values,
small threshold offsets, superimposed voltage ripple, thermal noise etc
all contribute to cause the threshold to be crossed before the
theoretical "end" of the discharge curve. When either inverter (ie
inverter A) output voltage starts to change, that change is coupled
through the capacitor to the input of inverter B. This voltage is added
to the input voltage of inverter B pushing it across the threshold which
in turn switches inverter B output voltage. Inverter B changing output
voltage is coupled back through the capacitor to the input of inverter A
and that pushes both inverter A and B input voltages rapidly to their
respective 0V(-0.6V) and V+(+0.6V). The additional 0.6V is caused by the
clamping action of the inverter input protection diodes. The inverter A
and B output voltages will then be V+ and 0V respectively. Now the
process repeats but with the capacitors discharging in the opposite
direction and PD1 influencing the discharge rate in proportion to the
light level. Photodiodes PD1 and PD2 therefore influence the on/off
times of the bicore outputs causing it to rotate the head for a longer
time in one direction compared to the other direction. When the PD light
levels are balanced the head rapidly shakes back and forth equal
distance effectively keeping it pointed in the same direction. For some
sample bicore waveforms check the section on analogical waves in the
appendix.
APPENDIX A
SOME SIMPLE ELECTRONIC CIRCUIT MODELS
I find that I can imagine the signals of a circuit in my mind as I run a
simulation on my mental circuit model. A good model of a circuit and of
its components are the best way to understand the behaviour of a BEAM
design. Since BEAM circuits are so simple you can follow the action
right down to the electron level where physics rules. Imaging you could
do something like that with a PIC? Forget it! The mechanical models I
have chosen are not perfect but they do shed light on what to expect
from electronics components if you consider how the equivalent
mechanical model would behave. Understand the similarity between the
model and reality and pretty soon you will have a good 'feel" for the
behaviour of the real thing.
SOME SIMPLE ELECTRONIC COMPONENT MODELS
I find that I can imagine the signals of a circuit in my mind as I run a
simulation on my mental circuit model. A good model of a circuit and of
it's components are the best way to understand the behaviour of a BEAM
design. Since BEAM circuits are so simple you can follow the action
right down to the electron level where physics rules. Imaging you could
do something like that with a PIC? Forget it! The mechanical models I
have chosen are not perfect but they do shed light on what to expect
from electronics components if you consider how the equivalent
mechanical model would behave. Understand the similarity between the
model and reality and pretty soon you will have a good 'feel" for the
behaviour of the real thing.
RESISTANCE
The most elementary electronic component is the resistor, a device
usually consisting of small tubular body with 2 leads. The body contains
a "resistive" conducting material like Carbon which slows the rate at
which electrons flow (amps) in direct proportion to the electron force
(volts) and inversely proportional to the resistance (ohms). A useful
model is that of current as the flow of water and resistance as a
constriction opposing the flow. eg a faucet would be equivalent to a
variable resistor.
[Image]
There are a number of "active" resistors used for sensors such as light
dependent resistors (LDRs) which change value when exposed to light or
thermistors, which change value when exposed to varying temperatures.
There are also humidity sensors and pressure sensors which change their
resistance value according to changing conditions. Even two electrodes
inserted into plant soil can be used as a resistive moisture sensor.
CAPACITANCE
Capacitors are also simple two leaded devices but they are quite
different in the effect they have on electrons. A capacitor consists of
two thin conducting layers separated by an insulating layer. A typical
capacitor is made with two strips of plastic separating two strips of
metal foil rolled into a multi-layer tube with a wire terminal connected
to each metal foil. The total surface area of the metal foil, the
thickness of the insulation and the quality of the insulation determine
the capacitance (in farads) of a capacitor.
[Image]
A useful model of a capacitor is a water filled cylinder with a flexible
membrane separating the cylinder into two sides and two pipes at the
ends of the cylinder (like the wire terminals). These pipes connect each
side of the cylinder to other parts of the "circuit" allowing the water
to flow back and forth into each side. The equivalent capacitance then
is proportional to the membrane area, thickness and elasticity. When the
pressure on one side of the membrane in increased, the pressure on the
second side increases an equal amount. If water is permitted to flow
from the second side, the pressure of the second side decreases as the
membrane is deflected displacing a volume of water. A voltage applied to
one terminal of a capacitor is transferred to and remains on the other
terminal if that terminal is not connected to the rest of the circuit.
With the other side terminal connected through a resistance to the rest
of the circuit, a voltage (pressure) applied rapidly to one side will
appear on the other side. But the voltage (pressure) will start to drop
as the charge (volume) bleeds off through the resistance back to the
rest of the circuit. You can try this experiment yourself by measuring
the voltage on one terminal of a capacitor with a digital voltmeter
while applying a small voltage on the other side. Keep in mind that the
voltmeter is itself a 10M resistor so it will bleed off the charge of a
capacitor in the same way. There are some sensors that use capacitance
usually in the form of a metal plate ie the capaciflector or E-field
sensor.
DIODES - OF ALL KINDS
The diode is the simplest "active" semiconductor component. It is a
non-linear device, which behaves quite differently depending on the
polarity of the applied voltage.
[Image]
When the cathode (stripe) is negative and the anode is positive, most
common Silicon diodes (1N4148 or 1N4003 etc) have a forward voltage (Vf)
drop of 0.5-0.6V with the current limited at 10-100ma. At higher
currents the voltage can rise to 1V and at much lower currents the
voltage can be as low as 200mV. Germanium diodes are still used for low
frequency signal detector applications since they offer a much lower
forward voltage of about 100mV at 10-100ma and correspondingly even
lower Vf at lower currents. The Schottky diode has similarly low forward
voltage drop but can be designed for much higher currents. These are
frequently used in high frequency switching power supplies because of
the low losses when switching. Although not often used for
"rectification" or signal diode applications, the light emitting diode
(LED) behave quite similar to ordinary diode but with higher forward
voltages of 1.6V-2.0V depending on the color and semiconductor material
and current. LEDs of course also emit light when passing forwards
current. Photo Diodes are designed to generate a voltage or current
proportional to the light falling on the large geometry junction of a Si
diode chip encased in transparent (but sometimes filtered) lens. When
the voltage is applied in the reverse direction, in theory, no current
will flow but, in practice, there is "leakage" current which varies from
0.1ua or less for a Si diode to 100ua for a Ge or Schottky diode
depending on the applied reverse voltage. Green LEDs, like photodiodes,
have a reverse leakage current proportional to the external light level
and be used for some applications instead of Photo diodes. The simple
mechanical model is a pressure operated one way valve that permits the
flow of water in one direction but not in the other. When applying lots
of reverse pressure on the closed valve, it may leak a little.
TRANSISTORS - BASES AND GATES
The transistor is a semiconductor device that amplifies. It has a
control terminal used to control the flow of electrons from input and
output terminals.
There are two types and two polarities of transistors. The two types are
the bipolar transistors like the 2N3904/2N3906 and the other type are
the field effect transistors like the 2N7000. The two polarities are the
bipolar NPN (2N3904) and PNP (2N3906) and the N channel (2N7000) and P
channel mosfets.
[Image]
Bipolar transistors use a current controlled BASE terminal to control
electron flow from the EMITTER input terminal to the COLLECTOR output
terminal. Mosfets use a voltage controlled GATE terminal to control the
electron flow from the SOURCE input terminal to the DRAIN output
terminal. Because transistors are made with diode-like internal
connections they can control the direction of electron flow only one
way. Hence the need for polarities: NPN or N-channel devices to control
the current flow from negative to positive and PNP or P-channel devices
for current flow in the other direction.
OTHER SENSORS
The passive infrared (PIR) motion detector uses a body heat sensitive
device that effectively behaves as a changing capacitance. To be sure,
it actually a semiconductor device with piezo electric behaviour from
differential thermal stress and which results in a momentary charge
transfer at the output. The PIR module used inside motion detectors has
the body heat focuses on the sensor with multiple "freznel" lenses (like
bug eyes), which increase the effect and sensitivity of the sensor.
There are many other sensors that have potential BEAM applications
ranging from ALPHA brain wave sensors to microphones, all subject for
future beam designs and discussion.
MAGNETICS
There are other passive components especially electromagnetic devices
like inductors and motors, which are used in BEAM tech. However they are
more complex in behaviour and will be "modeled" in a future tutorial.
SOME ELECTRONIC TERMS AND PRINCIPLES
AC/DC
The terms "AC" and "DC" will be used in the description. While these
terms stand for "alternating current" and "direct current", in this
discussion, we will say that AC stands for voltage signals connected via
capacitors and DC stands for voltage signals connected directly or
through a resistor to a voltage source. Capacitively coupled AC signals
(like the connections between Nv stages) have a characteristic that fast
changing waveforms can easily pass through capacitors but slow changing
or static voltages cannot. Resistors are used for connecting dc signals
(in combination with capacitors to ground (ie Nu neurons) to delay and
resist rapid change and instead permit slow or unchanging voltage
signals to pass through.
NO GAIN - NO BRAIN
The bicore and PS circuits use a HC240 octal inverting buffer, which as
you will recall has 8 inverters, organized in two separately enabled
groups of 4 inverters. Each inverter is an amplifier with one input pin
and one output pin. The logic function of the inverter is to make the
output pin +V when the input pin is 0V and make the output 0V when the
input is +V. In other words the state of the output pin is opposite to
the state of the input pin. Each group of 4 inverters is controlled by a
tri-state enable pin which when held high (+V) will disable (float) the
corresponding group of 4 inverter outputs by isolating the output pins.
The output pin of each HC240 is the midpoint of a pair of series
connected N channel and P channel mosfets. When the n channel device is
on the output is low and with the P channel device on the output is
high. When both devices are turned off the output floats. Aside from the
inverting logic function, the inverting amplifiers also have "gain"
which causes a small voltage change around the switching threshold of
the input pin to result in a large voltage change on the output pin.
This is very important since many of the "passive" components reduce,
attenuate or slow down voltage waveforms and it is the job of the
amplifiers to compensate for the losses and speed up the waveforms.
TRUE OR FALSE
We mentioned earlier that the voltage on input and output pins can be +V
and 0V logic levels. These levels are often referred to by other names
such as "high and low" or "1 and 0" or "on and off" or "true and false".
These terms are used interchangeably and have the same meaning of
identifying one of two binary states.
MAKING LOGICAL WAVES
The term square wave, rectangular wave or pulse will be frequently used
in the description of these circuits. They graphically describe the two
logic levels with horizontal and vertical lines. A squarewave uses a
lower horizontal line representing 0V which rapidly changes to and
continues along an upper horizontal line representing +5V then drops
down again to continue the lower horizontal line etc. The vertical
position of these lines corresponds to the high or low logic levels and
the length of the horizontal lines represent the duration of each logic
level.
Square waves have upper and lower line segments which are the same
length, meaning the on and off times are the same duration. Rectangular
and pulse waveforms have unequal on/off times.
MAKING ANALOGICAL WAVES
In contrast to binary waveforms, which change rapidly from high to low
etc, other waveforms change relatively slowly. Such waveforms are
described by graphical terms like triangular or exponential waves. One
feature of analog waveforms is that they are records of analog voltage
levels, which can be any voltage between 0V and +V.
[Image]
The figure shows several cases of bicore circuits and the waveforms at
the inverter inputs. Analog waveforms generally only occur at the input
pins of this class of astable oscillators. With voltage gain or
amplification through the inverter gain stages, the input voltages are
amplified and the output voltage waveforms switch rail to rail (0V or
+V) depending on whether the input voltage is above or below the input
trigger or threshold voltage. When a slowly changing input waveform
voltage crosses the trigger it causes the output to change much more
rapidly. This effect of voltage gain is called clipping or limiting
amplification: changing analog waveforms back to digital waveforms.
HASTE MAKES NO WASTE
Moderately slow waveforms are restored by the gain of the amplifiers
back to binary waveforms and for digital circuits, such as the 74HC240,
these speedy binary waveforms waste little or no power quite unlike
their slower moving cousins, the "linear" waveforms. Extremely slow
waveforms, ie the rising voltage on a capacitor being charged by a solar
cell, cross the trigger level so slowly that the output voltage, while
changing more rapidly than the input, will still spend a significant
time in a state between the +V and 0V levels. With the input voltage in
a narrow range around the threshold voltage (about 1/2V+ for a (HC240),
the resulting "in between" output state is said to be in the "linear
region". An output voltage in the linear region is an undesirable state
of affair, which causes high power loss and electrical noise upsetting
other waveforms in the circuit. There are several solutions to
eliminating those linear region output states and they will be discussed
next.
FEED ME!
Now that we have the inverter basics covered let's look at a more
advanced concept: FEEDBACK.
Feedback is a simple idea with a surprisingly complex range of
behaviour. The general concept is that if a fraction of the voltage of
an output pin of an amplifier is connected back to it's own input pin
then, since the input pin determines the state of the output, the output
pin will literally influence itself! Feedback is possibly the most
important concept in electronics, neural networks and general control
theory. There are two kinds of feedback: negative and positive. Negative
feedback is an (inverted) output signal of an amplifier which, when fed
back to it's input, opposes the present state of the output. An
amplifier with negative feedback will be stable only when the output is
equal to the input, which occurs at the switching threshold (1/2V+)
Positive feedback is an (non inverted) output signal which, when fed
back to it's input, promotes or reinforces the present state of the
output. Since the output and input levels are the same (not inverted),
an amplifier with positive feedback is stable only when the output and
input levels are at 0V or +V (high or low).
There are numerous examples in nature of positive and negative feedback
but for simplicity I will use the lowly balance to illustrate DC
feedback. The figures below graphically shows the three different cases
of applying a force (a steel ball) on a balance beam which is suspended
(at its fulcrum of rotation) at the center of gravity (no feedback) or
above the center of gravity (negative feedback) and below the center of
gravity (positive feedback). The arrows show the direction in which the
ball is pushed and the effect of gravity when it crosses the
"threshold". Below that, the equivalent circuits are shown with sample
input and output voltages showing the effect of the input voltage on the
output state.
[Image]
It is quite likely that a circuit will contain several stages of
inverters or amplifiers connected in series, with the output of one
connected to the input of the next stage. Feedback occurs when a sample
of the output of the last stage is fed back to the input of the first or
intermediate stages. In that case, an even number of inversions in the
feedback path results in non-inverting positive feedback while an odd
number of inversions causes overall inverting or negative feedback.
While we have just scratched the surface layer of a very deep subject,
the circuit descriptions should make a little more sense when referring
to the various technical terms in the tutorial.
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