change detector

[throbscottle]

Can anyone suggest a quick and dirty circuit to detect if a voltage is rising or falling? (not fast changes) With 2 outputs for example so that 10=rising, 01=falling, 00=no change.

I'm thinking the input could charge a capacitor with a resistor connected across it, feeds an op-amp where it's in reference to a mid-point voltage. After that it seems to get complicated, maybe there's a better way?

 

[crutschow]

Can't think of a simper way. So you could connect the voltage to an RC differentiator (capacitor in series with resistor to ground). The output will be a plus voltage for a rising signal, a negative voltage for a falling signal, and zero volts for no change. You could then use a couple of comparators from that to generate the digital signals you want. The amount that the voltage is above or below zero depends upon the rate-of-change of the signal and the RC time-constant.

If you want to use a single supply voltage, you can connect the capacitor to a voltage divider of equal value resistors to the supply voltage. Then the output will be above 1/2 the supply for a positive going signal, below 1/2 the supply for a negative going signal, and equal to 1/2 the supply for no signal change.

 

[jpanhalt]

If you change the criteria from "rising" to above a value; "falling" to below a value, and "no change" if between the two values, then two-comparators and a voltage divider will work (see: window comparator). Interstil makes something that will do the same in a single package: https://www.intersil.com/content/dam/Intersil/documents/isl5/isl55141-42-43.pdf

If you truly need rising or falling detection, then maybe this will work for you: **broken link removed**

John

 

[throbscottle]

Synth module does just what I want, though I'd want to alter it a bit - probably put an LM393 in there and wouldn't need the transistors. Thanks guys