I’m trying to design a circuit to test a certain component, so I want to have different pairs of red/green LEDs for indicating whether various parts of the component are working. Indication as to whether or not each component is working will be determined by whether or not a voltage is above/below a certain threshold.
I realize that I can do this with an op-amp/comparator or maybe a relay. However, I’m trying to avoid mechanical or bulky parts (no relay) and I’m not assured that I will have a voltage rail above/below the input voltages (so no op-amp?). Furthermore, since I know exactly what I need from the circuit (just lighting LEDs), I don’t need the nice impedances afforded by an op-amp.
I would like to do this all with some common transistors, diodes, or zeners, and I was able to get something kinda working, but I couldn’t avoid cases where either both LEDs were on or off unless I made the circuit very dependent upon the betas of the transistors.
How simple of a circuit can be made which, for example, given a 12V rail, will toggle between two LEDs when an input voltage crosses, say, 11V? Can it also be done with only a 5V rail?
Best Answer
Here is an arrangement that was on all my industrial power supply modules, and boat switchboards, that let you know if you have No-power, blown-fuse or OK, with a single led.
simulate this circuit – Schematic created using CircuitLab
Over the years I have made quite a few arrangements with bi-colour that transition from green through orange to red giving you an excellent view of what is happening in a single led.