How can I convert this optocoupler led flasher circuit to switch a mosfet or transistor?

Thread Starter


Joined Feb 8, 2014
This simple circuit makes the LED flash at a "medium rate". I would like to flash the small LED strip you see in the background instead (it pulls 400mA at 5V)

How can I modify this circuit to have a mosfet of transistor flash the LED strip?

Falstad circuit simultation:



Joined May 3, 2013
You replace that ?? with the Emitter-Base terminals of a P-N-P transistor and connect the LED strip between Collector and Ground.


Joined Oct 7, 2019
A Darlington does not turn on well. You will probably only get 4 volts for the LED strip.

Use a "logic level" MOSFET. Most MOSFETs need at least 5V on the Gate to get them turned on well. There are MOSFETs designed to be turned on hard at 3V. You need a MOSFET that can handle 10V or higher and 2 to 10A with a RdsOn of under 0.5 ohms. (and low gate turn on voltage!)

Do not use the opto-isolator.
Last minute thought. Put a 100k to 1meg resistor from Gate to Source. Many computers power up with the output pins in tristate which is not a happy way to leave a Gate. A pull down resistor will cause the transistor to be off in that condition.