# How can I amplify a square wave signal with a mosfet?

I have a CCD camera that is triggered by an external TTL square wave. That is each time the camera sees a rising edge on the square wave it takes a picture. The square wave is being output from a custom microcontroller card when in capture mode. So far I have used a oscilloscope to verify that the microcontroller card is indeed in capture mode and outputing a square wave.

I want to create a small circuit where a LED is being controlled by the square wave signal (0->5V). That way I can instantly see if the camera is running or not.

I do not want to draw much current from the signal. I have some 2N700 mosfets. So I want to use one of these to amplify the signal.

This is what I have come up with so far:

simulate this circuit – Schematic created using CircuitLab

Should this work? Is there some easy way (eg. change resistances, I have a wide range of resistances) to improve the design?

BTW the input impedance of my oscilloscope is 200k Ohms. Maybe the input impedance of my circuit should be in this ballpark?

I have tested the circuit with a function generator as input and it appears to work just fine. However when I use it on the real signal it blinks as expected for a while and then get stuck in high state for a while and then blinks again. The stuck in high state is not something I expected. Not sure if this is a problem with my circuit or the software running on the custom microcontroller card.

May it suffice to drive the led directly from the signal:

simulate this circuit

This should draw 5.5 mA from the output of the microcontroller. Would that be a problem?

• The uC can drive the R to LED directly. You do not need anything else. Just choose hi or lo side for power ON and use 1k for low current Jan 13 at 12:42
• No. R to cathode and. +V to anode is low side drive. Low =On Jan 13 at 13:03
• 5V uC have 50 ohm drivers Jan 13 at 13:16
• yes , that's positive logic or high side driver for indicator Jan 13 at 15:03
• 2mA or 10% is recommended minimum Jan 13 at 15:31