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: https://tinyurl.com/yz6hodry

 

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

 

I've successfully used a BC517 Darlington like this:

 

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.

 

Using the AO3400 N-Mosfet produced a similar waveform, the LED strip flashes as intended. In theory a 100K resistor between the gate and source should be present but in practice I saw little change. Anyway, it works and it's adequate to my needs, thank you.