[tog] Help with a circuit

Krzysztof Foltman wdev at foltman.com
Mon Apr 29 09:56:30 CEST 2019


On 26/04/2019 11:43, Stephen Church via tog wrote:
> Ok so heres a puzzle.
> I have two pressure sensors , they have a limear output that increases voltage from 0-4.5v depending on the pressure of the liquid theyre monitoring. I want to (in as analog a capacity as possible, no arduino etc... as simple a circuit as I can , have it that if the pressure difference between them is say greater than 10% that they trigger an output to an LED , im alright with that difference being variable , the pressure of the liquid may change at any point and thats fine as long as theyre roughly the same between sensors but it means I cant use a circuit that has an absolute threshold for either sensor.

I guess you could use something like this:

https://en.wikipedia.org/wiki/Instrumentation_amplifier

and feed the output to a comparator (maybe even a single op-amp) with 
some sort of adjustable voltage reference as the other input.

You may be able to build a simple prototype with just a single LM324 (3 
units for the amplifier, 1 for the comparator), but there are dedicated 
instrumentation amplifier chips and comparators of course, and precision 
voltage references as well. Maybe a transistor to drive the LED, but you 
may get away without it, especially if you limit the drive current for 
the LED to less than 10 mA.

Noise-proofing the thing might be a challenge, depending on 
circumstances. Not sure if I can recommend anything specific there, 
other than trying the standard stuff (using shielded cables for inputs, 
enclosing the sensitive parts in a grounded Faraday cage, adding 
capacitors and/or inductors and/or ferrites where they're needed etc.).

Krzysztof




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