Need help adding status LED

Thread Starter


Joined May 20, 2014
Hi all,

I have a power supply powering a load. I built a detachable connector to connect/disconnect the power supply from the load. When the load is powered, it doesn't make a sound or give visual indication that it is on. I would like to add an indicator LED that lights up to show that the load is receiving power, when the power supply is connected. But I don't know how to do this exactly. I would like to do this without changing the voltage/current to the load.
Power supply is 12 volts
Load draws 0.37 Amps
LED has forward voltage of 2volts, draws 20mA

Simplified diagram:

+ -----LED-----[connector]---|The |
gnd -----------[connector]---|Load|


Joined Feb 19, 2010
You need led and resistor connected in series, call it led circuit. Connect this led circuit in parallel with the load, do it on the load side of the connector.

20 mA is probably led max current, you don't want to use it. Use something like 15-16 mA.
So the resistor for the led circuit is: (12-2)/0.016=625 Ohm
In this setup the resistor will dissipate 0.16 Watts, so you need 625 Ohm 1/4 Watt resistor.


Joined Sep 9, 2010
A more complicated solution for the power supply side - if you don't want to alter the load - is to use a low ohms (eg. 0.1Ω) shunt resistor in series with the load, on the ground return line. Set up a comparator (I use the LM339 quad comparator for such things) to watch the voltage on the load side of the shunt resistor. A voltage above ground indicates the presence of a load.

The LM339 can sink about 5mA of current, and that's plenty for an indicator LED, so you don't need an external transistor. You can power the comparator with your +12V supply.