# Drive LEDs from 4-20mA source

I have a task to drive four LEDs from a 4-20mA loop. I have 24V for the LED side so I need to convert the current into something that can drive the LEDs.

I know I can use a 250$\Omega$ resistor as the converter in the loop which will give me 1-5V from 4-20mA respectively, now I need to get this voltage to drive the LEDs.

Do I simply connect a transistor base to the 1-5V point, ground the emitter and drive the LEDs from the 24V via dropping resistors via the collector??

It needs to be a variable drive as the brightness of the LEDs must vary with the 4-20mA signal.

• Thanks all so far, i will do some tests today on the bench and see what plays best. I have the LDR and leds plus some resitors arriving today so will be able to try the whole setup. Interesting about the 4mA current load to extinguish the leds. Setup tested and works well, i can get a 5k - 40k swing by padding the led with 375R to keep it dim at 4mA, that is plenty enough swing for the application and i have moved on to building the unit now. I'm amazed that it can be done with only a reseistor, an led and an LDR, very useful stuff. Thanks to all who helped, this is a great site. – user3056 Feb 17 '11 at 7:14