Electronic – MOSFET source measures 12V, still nothing will turn on

mosfetpiezotransistors

Please, bear with me, I'm a real rookie at electrical engineering. So please be easy on me. 😉

I've got the following circuit. I have a 5 seconds HIGH / LOW interval on D5 and am seeing the voltage drop to near 0 and rise to 12V when measuring with my multimeter on the terminals of the speaker. Still, my Piezo speaker won't make a sound. When connecting it straight to the 12V source, it works as perfectly. What am I missing? Current at the source of the MOSFET I guess? But, why? Did I connect it wrong?

P.s. also tried connecting a LED with a 470 ohm resistor after the MOSFET, but that does not turn on either. And this also simply works directly on the 12V power source.

Circuit

Best Answer

First, you should start by drawing the schematic in the "standard" way: from left to right. The first item should be the power supply and then the rest of the components.

Also try to isolate the circuits in small functional parts. I mean, one can suppose that the Wemos and the 7805 will work as expected.

So I would draw the schematic for your questions like this:

Your circuit rearranged

Here you will see that the N-MOSFET is not in the lower side so

First problem: You are using an N-MOSFET as a high-side switch

Changing the components, and placing the IRF520 as a low-side switch gives us this circuit

One option

However, the IRF520 if not guaranteed to work with 3.3V of the Wemos so it may or may not turn on. The Vgs of the IRF520 is between 2 and 4 volts, sometimes 2, sometimes 4.

To sum up:

  • Problem 1 Your IRF520 should be used as low-side switch
  • Problem 2 You will have to use a buffer for the IRF520 or use another transistor.
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