I used NI multisim and created a circuit with a 555 timer that will allow a diode to flash at a frequency of about 4HZ. This is the circuit:

enter image description here

As you can see in the above image, the oscilloscope graph is a square wave constantly changing from high to low which means the light is flashing on and off. Nothing wrong with this.

After that, I tried to make a simple modification to the circuit by adding in an NMOS as a switch for the 10V DC source. In the image below, I am pretty sure that I set up the NMOS correctly. I'm feeding the gate of the NMOS a voltage of 0V. You'd expect the output of the oscilloscope to be a flat horizontal line at y=0 which means that no current/voltage is reaching the diode. However, this does not happen. I see the same square wave from before.

enter image description here

I hope the images are clear enough. I am unsure what I have done wrong. Any help would be greatly appreciated. Thank you!

  • 1
    \$\begingroup\$ Hmm, it looks to me like your NFET will just short the power supply and blow up if you turn the gate on. Otherwise it makes no difference to the circuit. It should be in series with the 10V supply, and then you will need a voltage somewhat higher than 10V to turn it fully on. \$\endgroup\$
    – John D
    Nov 11, 2020 at 0:41
  • \$\begingroup\$ So you're saying that the NFET/NMOS should be in series with the 10V supply? I thought this was already the case in my circuit. Is it possible that you could show me a diagram so I can see what you mean. As in, how it would look like once it has been implemented. Thanks. \$\endgroup\$
    – mstar42
    Nov 11, 2020 at 1:11
  • \$\begingroup\$ Could you describe how you see the NMOS working in your circuit? What exactly do you intend for it to do and how? \$\endgroup\$
    – brhans
    Nov 11, 2020 at 1:11
  • \$\begingroup\$ Hi brhans. The idea is that if I'm giving the NMOS a gate voltage of 0V, then it should act as an open circuit to the 10V source and current is not allowed to flow from it. Is that not how an NMOS works? It acts as an open circuit for a low gate voltage and acts as a closed circuit for a high gate voltage. \$\endgroup\$
    – mstar42
    Nov 11, 2020 at 1:15
  • 2
    \$\begingroup\$ R3, R4 and Q1 are not connected to the 555 timer circuit ... they only share a power supply \$\endgroup\$
    – jsotola
    Nov 11, 2020 at 2:07

1 Answer 1


I didn't have time to answer before, but as I said in my comment the FET must go in series with the supply and the load (as with any switch). This is shown below.

In the diagram on the left it shows an N-FET switch, but note the need to drive the gate well above the 10V supply to turn it on. (You want the source at 10V, so the gate to source voltage has to be well above the threshold voltage, by enough to achieve the desired RDSon. The diagram shows 15V as an example, but that may change depending on the FET.

If you use a P-FET as in the diagram on the right, you can turn it on by applying 0V to the gate, and off with 10V. Therefore you don't need the additional supply voltage. You have to check to be sure that the max Vgs is less than -10V.


simulate this circuit – Schematic created using CircuitLab


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.