How to detect a change in resistance


I would like to have a circuit that can be controlled by two variable resistors, and it should use the value of the "most-recently-changed" resistor. How can I find out when one of the variable resistors is changing in value? I would like to figure that out and then flip a latching relay such that it uses that resistor for the input. Is this feasible using a circuit without a micro-controller monitoring the two resistors? what would such a circuit look like?

scenario: imagine a lightbulb that is controlled via a three-way switch. Now imagine the switches replaced with dimmers – the most recently adjusted dimmer should be the one used to control the lightbulb, the other one exists simply to allow adjustment from a different physical location.

Best Answer

OK, so you have two variable resistors and you want to use the signal from the variable resistor that last moved? OK so far? You don't want to use a micro so the only real option is to have a circuit that detects rate of change of each resistor value and the one that last moved i.e. created a \$ABS(\frac{dR}{dt}\$) signal, would "trip" a SR flip-flop. The flip flop output drives DPDT analogue switch thus selecting either resistor A or resistor B.

OK so far?

You'll need a fairly sensitive differentiator circuit (probably formed around an op-amp) for each resistor and, each resistor would be biased with a current so that it produces a voltage signal to feed its differentiator. That gives you the ability to generate positive and negative pulses when the resistor value changes - the size of the pulses are dictated by the speed that the resistor changes value. So then, and accepting that small slow changes will not be detected, you can use a window comparator that looks for "going above" and "going below" thresholds - either one will set a SR flip-flop. The other resistor drives a similar circuit that resets the flip-flop.

That should work I think.