Alt-BEAM Archive

Message #00041



To: beam@corp.sgi.com
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Sat, 30 Jan 1999 15:47:18 -0800
Subject: [alt-beam] Re: a question and a comment


This is an example of how easy it is to set up a high pass/variable
threshold detector using a spare HC04 (or HC240) gate:


_______________
| |
[10M] ___ |
C | ___|04 \__|_____ output signal
input signal ----||--+---' |___ /
|
[1M]
|
V
0V----[10K]-----Vcc

A HC04 gives the equivalent of an inverting AC coupled comparator/amplifier
with the 10K pot adjusting the threshold. The output can be biased normally
high or low and if adjusted near the center will turn the HC04 into a linear
amplifier. The capacitor C determines the cutoff frequency, with practical
values of 100pf to 0.1uf. When used as a comparator, the threshold is
sensitive to both frequency and amplitude. When used as a linear amp it can
provide one or more stages of gain for a microphone or other low output
transducer.

If you use a HC245 or other noninverting gate (or two HC04 inverters in
series) you get hysteresis and/or memory of the direction of the last fast
transition depending on the setting of the pot.

An HC14 probably won't work very well unless you remove the 10M resistor
(oscillates!) and then provides typically about 30% hysteresis.

If you want to stretch the output signal (pulses) from the above circuit
just use a series diode and capacitor with another gate (ie HC14) to drive a
LED or other load like so:

Vcc
|
_V_ LED
|
__ [1K]
diode ____|14 \_____|_____ output pulse
--->|--+-[10K]----+ |__ / >1sec
| |
[10M] === 0.1uf
| |
GND GND

The 10K resistor can be used (and changed) to delay the output so that only
a burst of pulses will trigger the output and the 10M resistor sets the
output pulse width.

These circuits tie in nicely with the other BEAM components and the
discussion on signal conditioning so I thought you might be interested.



|
Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
tel: (604)590-7493
fax: (604)590-3411

> -----Original Message-----
> From: RoboDR@aol.com [SMTP:RoboDR@aol.com]
> Sent: Saturday, January 30, 1999 1:34 PM
> To: beam@corp.sgi.com
> Subject: Re: a question and a comment
>
> << Yep! Remember, though, that this circuit will only work if the signal
> swing
> is larger than half of the supply voltage for a chip. ie for a 5 volt
> chip,
> a swing of 2 volts won't trigger it, but a swing of 4 volts will. >>
>
> So if you used a 5V chip, it would ignor a quick change from 4V to 6V, but
> it
> would triger at a cuick change from 2V to 5V? And it would ignor all
> changes
> that take several seconds?

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