I have a sensor that I measure a resistance change from open circuit(off) to about 50Kohms (on) or 0mV (off) to 20mV(on). So either change can be used for an input signal. I would like to have the on-off change trigger my relay. I have spent months building op-amp circuits and voltage dividers to get this to work. I just got a voltage divider by itself to work, then when I hooked it up to the relay, it was problematic being now part of the voltage divider circuit. My thought now is to try a microcontroller but I have never done any of this type of programming. I have an Arduino Uno.
This relay module is working great. It takes an input voltage anywhere from 0.7 to 12V and activates the relay.
20mV tends to be tricky because a small amount of electrical noise will trigger it. And high impedance/resistance (50K) is more prone to noise pickup than lower impedance/resistance.
Did you build a comparator?
You'll need a voltage divider with a comparator to get the reference voltage but you wouldn't want to divide-down that tiny 20mV signal.
Of course the Arduino doesn't work without programming.
But you could program it to use the optional 1.1V analog reference and then 20mV will give you an ADC reading of around 20 and you can use the result of that reading to trigger the relay.
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You are talking about a passive (resistive) element and an active (voltage source) element. How should these descriptions come together?
Please describe that black box element more clearly. What's its nature, what use in your project?
Show the schematics of what you tried, and describe the results.
What are the specifications of your relay module - please give a link.
Also a link to the mystery sensor would help.
Based on the comments, I feel the way to go would be to harness the resistance change rather than the voltage change? I would keep the voltage divider I am using, which is two resistors in series. One is the sensor, and the other is a 1 Mohm resistor. This is the link to the op-amp I have tried that has more background on this sensor:
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