Activate an led based on a 4 volt drop in voltage

Thread Starter


Joined Nov 4, 2011
I have a Magic DaVE 3D Digital Effects Switcher. The tally output has a 5 volt signal on all the pins that have an UN-selected input. The Selected input has a 1 volt signal. What I need to do is activate an led for the corresponding input when it is selected, and deactivate the rest.

The output is a 9 pin D-Sub.
Pin 1 - Pin 4 are used corresponding to each selected/unselected input
Pin 6 is the ground.

If more information is needed, I will get it ASAP.


Joined Jul 17, 2007
Here, see the attached schematic and simulation.

LM324's are very common and cheap quad opamps. You could use most any common voltage feedback opamp EXCEPT the TL0xx or LF3xx series.

Ch1 and Ch2 represent a couple of your inputs, varying from 5v to 1v. When the input goes below the reference voltage, the corresponding LED turns on.

I noticed that I was a bit careless entering the values for R1/R2.
Change R1 to 2K, R2 to 3k. That way, Vref will be 3v instead of 2.5v.

This schematic was drawn assuming that the channel outputs (1v or 5v) won't support much of a load at all. If they WILL support a load of ~10mA, then you could simply connect the cathodes of LEDs (shorter leads) to the outputs, and connect the anodes to a +5v source using a resistor; say 100 Ohms for a green one, 200 for a red one.