Subject:
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Re: digital ins
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Tue, 26 Aug 1997 21:10:25 GMT
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Original-From:
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Randy Sargent <rsargent@ANTISPAMnewtonlabs.com>
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Viewed:
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1275 times
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> > I want to feed an amplified signal to the digital ins on my HB to
> > exaggerate
> > the response of a photosensor in the 2-3V range of its operation.
> > Unfortunately, this means that at the 5V end of its range, the input
> > signal
> > will be near 10V. Will I fry my HB doing this?
>
> Yes.
...
> The correct solution to your problem is to use an op-amp signal
> conditioner,
> possibly feeding into a schmidt trigger if you need some hysteresis.
> (sp?)
>
> You can also rig a zener diode across the input to clip the input to
> 5volts, be
> sure the diode can handle the current from your amplifier stage.
Chuck is right about the correct solutions.
For a less correct solution, you can take advantage of the fact that HC
parts (including the 6811) have built in voltage limiters that are only
good for fractional milliamps (in some cases multiple milliamps).
Simply hook a large resistor (like around 100K) in series with your signal.
Then, if your signal goes to 10v, the HC input truncates it to 5v, and
around (10v-5v)/100K = .05ma flows through the HC input protection
circuitry. The main problem with this approach is that you must cut the
trace to the 47k pullup resistor on the handyboard, since the 100K in
series will make the "digital" signal too wimpy to overpower the 47k. If
you read the specs of the 6811 closely and find out it can sink more
current, perhaps you could reduce the resistor to 10k or so (the protection
circuitry sinks .5ma now) and mostly forget about the 47k influencing
things (will just change the transition voltage a bit).
Does someone out there remember the current specs for the input protection
circuitry on the 6811?
-- Randy
----------------------------------------------------------------
Randy Sargent Newton Research Labs
Senior Design Engineer Robotic Systems and Software
rsargent@newtonlabs.com http://www.newtonlabs.com/
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