[sdiy] In search of a reasonably precise comparator output

John Speth john.speth at andrews-cooper.com
Wed Jun 20 19:59:24 CEST 2018

Here's an overkill solution but it gives you much flexibility: Get an MCU with an ADC and DAC and easily implement your rule in software.


From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Lloyd Abbott <lloydabbott94 at gmail.com>
Sent: Wednesday, June 20, 2018 8:30:30 AM
To: Guy McCusker
Cc: synth-diy at synth-diy org
Subject: Re: [sdiy] In search of a reasonably precise comparator output

What rails do you have available ? Have you considered a comparator IC (e.g. LM339) rather than using general purpose op-amps ?

If you have 0V, +5V and +15V rails, with an LM339 you can reference the comparator input side to the 0V and +15V rails and have the output side referenced to 0V and +5V. The datasheet has an example circuit using one pullup resistor and one cap per comparator.

On Wed, Jun 20, 2018 at 4:05 PM, Guy McCusker <guy.mccusker at gmail.com<mailto:guy.mccusker at gmail.com>> wrote:
I am "designing" (i.e. cobbling together) a circuit in which I need a
comparator whose output is fairly accurately 0V or 5V.

I can think of many ways to do this. Some are very stable (at least on
paper) but appallingly complicated, which is bad because I need at
least 7 of these;  some are very simple but rely on careful trimming
of the saturation voltage of an op amp, which I would rather not do
for reasons of reproducibility.

It is 100% likely that someone on this list has a very simple and
solid way to do this, so rather than reinvent the wheel I thought I'd
ask... how would you do it?

(What I really want, in fact, is to take an input voltage X and output
X if X<=5, or X-5, if X>5.)

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