Alt-BEAM Archive
Message #01839
To: beam beam@corp.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Mon, 29 Mar 1999 07:17:12 -0800
Subject: [alt-beam] uPOPSE
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Hello beamers,
Here is the promised low voltage version of the opamp solar engine with
a nice little write up describing the details of operation. The GIF as
usual is tiny (4K) so I hope you'll forgive the extra bandwidth.
enjoy
wilf
The uPOPSEv2
wilf rigter 1999
Here is a 2.5V version of the micro power operational amplifier solar
engine (uPOPSEv2) This one uses commonly available 2N2222A output
transistors which happily saturate a 100-150 mA motor load. With higher
gain NPN transistors it will perform even better. The uPOPSE switches
on at 2.7V and off at 1.4V.
I tested the circuit with some 15 ohm Namiki motors using a simulated
solar cell from a 5V supply with a 100K series resistor. I also tested
this design with a couple of 45 ohm Sony lens motors. The circuit uses
a minimal number of parts but perhaps it can be further simplified. The
core element is the dual cmos opamp (TLC27L2) made by TI (Cdn $1.49).
Similar CMOS opamps should also work. The photodiodes are generic (5
for $1.99) with build-in IR lens. The absolute value of light current is
not that critical and a 100K pot is used to balance sensitivity. With
components shown, the circuit draws a constant current of about 5uA but
requires 50uA for base current at the switching threshold. A power
mosfet eliminates this threshold current for critical low light
applications. Total cost of all circuit components, not including the
solar cell or motors, is less than Cdn $4.00
The micro Power OPamp Solar Engine (uPOPSEv2) works as follows:
Summary
The basic circuit a threshold detector with hysteresis: the opamp output
switches positive and turns on the output transistor when the (+) input
is more positive than the (-) input (threshold) . When the transistor
turns on the (-) input is pulled to the 0V line reinforcing the
difference between the (+) and (-) inputs (hysteresis)
The transistor remains on until the voltage drops below about 1.4V when
the output turns off driving the (-) input more positive than the (+)
input and the cycle start over again.
Detail
When the main cap starts to charge, the voltage (Vcap) rises ,
rapidly at first and then slowing down as the charge approaches the
maximum available source voltage. This voltage appears at the transistor
collector connected to a 5M resistor with the other end connected to the
inverting input (-) of the opamp. The double inversion provides positive
feedback for hysteresis. In addition a 5M resistor is connected from
the (-) input to ground. This means the main capacitor voltage is
divided by 2 at the (-) input of the opamp which sets the trigger point
at Vcap /2. The capacitor voltage is also connected to the green LED
which acts as a "soft" zener with a 1V to 1.5V voltage drop at less than
1 uA . This means that the voltage at the (+) input is equal Vcap - 1.5V
near the upper trigger threshold and is equal to Vcap -1V near the lower
cut-off voltage. To be sure, the 100K pot and PDs modify this because of
the additional voltage drop across these components. At medium light
levels this additional voltage drop is small and is only used to
differentiate which SE should fire. The PDs have a effective impedance
of 10K to 1M and are connected to the cathode of the green LED through
a 100K resistor which is used to balance the sensitivity of the PDs.
Which SE fires first is determined how the current from the LED is
split in proportion to the light level on each PD. The PD current is
developed into a voltage across the 5.1M resistor to ground at the (+)
input of the opamp. When this voltage is slight lightly higher than
the(-) input the opamp switches the output positive and turns on the
transistor . Since the voltage at the collector drops to 0V, the 5M
resistor pulls the (-) input of the opamp also to 0V increasing the
difference between the (-) and (+) inputs and provides hysteresis
(lowering the switching threshold) The opamp output remains positive,
supplying base current to the output transistor, until the V+ of the
opamp falls below it minimum operating voltage (~1.4V) and the opamp
output impedance starts to rise cutting off the base current. When the
output transistor comes out of saturation for lack of base current the
(-) input rapidly rises to 0.7V (1.4V/2) and since (+) input is below
0.4V(1.4V-1V) , the opamp output switches to 0V holding off the output
transistor and the cycle starts again..
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1840 Mon, 29 Mar 1999 10:52:23 -0500 [alt-beam] OTU!!! beam@corp.sgi.com Jonathan D Rogers Yep, while sifting through my parts collection, I discovered an old GE
oven timer! It still works!
Does anyone want to buy it????
Jonathan
Ps. If no one wants it, then will someone tell me what the heck I'm
supposed to do with it??
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