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I am looking to use a sensor, such as photoresistor, to activate a circuit only when there is a significant change in the input. Such as when the light incident on the photoresistor changes significantly (e.g. something suddenly blocks or unblocks the light source).

What would I need to make this circuit? Ideally when the resistance changes by a certain amount quickly it would activate a transistor or switch.

I'm not sure where to start with this one without using a computer to process the time taken and calculate a derivative.

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It must be a microcontroller free solution. I'm not fussy about the tolerance or what-ifs from environmental variables, so long as the circuit can detect large resistance changes.

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    \$\begingroup\$ A differentiator can be made with an op-amp, a resistor and a capacitor. There's a lot more to it than that. Look at PIR motion detector circuits. \$\endgroup\$ Sep 9, 2018 at 1:37
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    \$\begingroup\$ motion sensors work best with a pulsed burst signal echo detect. Then integrated with threshold detection. With IR you can easily detect motion in a 60 degree span to 10m or more \$\endgroup\$ Sep 9, 2018 at 1:50
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    \$\begingroup\$ You need to specify your operational requirements. Do you want the detection only when the LDR goes from light to dark ...or do you want to include whenever there is a major change in illumination (either brighter or dimmer). Does a slow change need to be ignored? For example if the LDR was outdoors would you want it to be triggered as you pass through a day to night transition? Without specs it's hard to help. \$\endgroup\$ Sep 9, 2018 at 2:37
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    \$\begingroup\$ @JackCreasey I want it to activate when it goes from light to dark, or dark to light. Bi-directional. And only for significant changes. So yes, slow changes should be ignored. \$\endgroup\$
    – Goulash
    Sep 9, 2018 at 2:44
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    \$\begingroup\$ @NickAlexeev It must be a microcontroller free solution. I'm not fussy about the tolerance or what-ifs from environmental variables, so long as the circuit can detect large resistance changes. \$\endgroup\$
    – Goulash
    Sep 9, 2018 at 16:42

2 Answers 2

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You don't need a computer to calculate a derivative, a small micro-controller will do that just fine. In the simplest case, taking the difference between a value now and a value X seconds ago is enough.

If you expect that any kind of tuning will be required (adjusting the threshold, filtering noise etc.), changing the MCU firmware will be easier than re-soldering RC filters.

Plus, if you use the MCU you get any reasonable processing logic for free. Activate when A and B are true? Keep activated for XX seconds? No problem.

If you insist on using analog components, you should take a look at a diff amplifier:

enter image description here

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Usually comparator circuits are used with sensors like photoresistors, the photoresistor is put in series with another resistor to form a resistor divider. The comparator detects whether the voltage reaches a given level set by the potentiometer.

This allows for a circuit that can detect light levels based on a threshold.

enter image description here Source: http://www.learningaboutelectronics.com/Articles/LM741-night-light-circuit.php

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