Issue with low voltage for mosfet gate

by orange orange   Last Updated May 22, 2020 22:25 PM

I am a beginner when it comes to electronics so I would appreciate your help with a simple project I am working on and have been struggling with for several days.

The idea is that I have a PIR Sensor outputting to GPIO 24 on my Pi. Via a python script, I would like to switch on a 12-volt LED strip with its own external power supply. I am able to see when motion is detected via the script, so that is working fine. The issue however is that I am not able to properly use my mosfets to act as a switch. In order to take it step by step, for now I am trying to output signal to my mosfet Gate via GPIO 24, and manually alter the PWM via terminal (instead of using the PIR output).

My scheme is similar to this: - however not using the battery, and using the GPIO 24 output instead of PIR.

I have tried this with a IRLB8721 mosfet, but realized it needs a higher voltage than what the GPIO can provide. The most suitable one I could find then was the IRF3205, seeing that it is used more frequently with Raspberry PI/Arduino applications, but unfortunately this is also not working. It seems like my GPIO voltage is still too low.

Alternatively I have a couple of 2N3904 transistors available but I am not entirely sure how I can use these without frying them.

I have noticed that when using the IRLB8721, my LEDs are always on, and I am not able to switch them off. I did make sure not to mix up the Drain and Source, yet still not working.

Could you please guide me in the right direction? I am not able to get any other components for another week, so I am stuck with a diode, couple of 1kOhm resistors, and these 3 types of transistors/mosfets/ Is there anything I can do to fix this?

I have looked this up extensively and tried it on my breadboards but with no success. Only lead I have is pointing to a fairchild mosfet which I am not able to get to for weeks.

Thank you.

Answers 1

What made you think the IRF3205 was suitable? It is not a logic level MOSFET. For use as a switch, ignore the gate threshold voltage. This is the gate-source voltage where the MOSFET just barely starts to conduct. You don't care about that. You care about the MOSFET conducting as much as possible so instead of gate threshold voltage, look at the gate-source voltage used to achieve the desired RDson.

You need a 3.3V logic level MOSFET so the 3.3V on the RPI GPIO can actually turn it on and off.

May 22, 2020 21:52 PM

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