I'm attempting to used a simple inverting MOSFET switch circuit that I have used before in another project successfully. I built it exactly as I did before, using it to turn on the exact same small signal relay, but this time it doesn't work. The first FET, Q1, is turning on and off as it should, but on the second FET, even though 3.5V is present at the gate it will not turn on (the datasheet says it should turn on at that voltage). I've tried using a new part just in case it was defective but not so. I've checked my grounds as well. I can't believe something so simple won't work, especially since it worked fine previously. I'm not a circuit design expert so if anyone out there can tell me why Q2 isn't turning on with Vgs = 3.5V I am all ears.


Thanks in advance!

 

You probably have a dead MOSFET as you forgot the protection diode across the relay coil.

 

Maybe lan0 is right. The MOSFET maybe faulty. Please test the MOSFET separately. Disconnecting it from your circuit. You can check it with a multimeter.

Also make sure that you have connected it in the right polarity. Also check the continuities of your jumper wires. If you wanna know about the different operational modes of P and N-channel MOSFETS elaborately, you can read this:

www.theengineeringprojects.com/2020/08/mosfet-what-the-mosfet-is-and-how-it-works.html

 

The 2N7000 is barely adequate for your application,
by all specifications,
it's probably smoked.

Instead, use something like a VN3205N3-G
https://www.digikey.com/en/products...YmXDNJIKQCZE8qMtUlI5AKyate0yABqAOQDMOgAwWm4oA

And, install a Freewheeling-Diode on the Relay-Coil.
.
.
.

 

The 2.4V relay is being overdriven by 3.5V, so it will take 85mA.
A 2N7000 is good for 350mA, and has a RDSON of 5Ω at 3.5V gate voltage.
As the maximum for Vgsth is 3V, you might just be in trouble with 3.5V for that load, but unlikely on the balance of probabilities. Don’t go making 1000 off based on that design, but for one yOu would be unlucky if it didn’t work - not a design to use in production.
Voltage breakdown due to the relay flyback voltage is the most likely problem.

 

Well, problem solved. The relay pins were shorting on the breadboard. I wasn't paying close enough attention to the wiring. Thanks Ian0 for the diode callout. I've implemented that as well. And thanks to everyone for the great recommendations. Cheers!