Using an NMOS as a switch for a 10V DC source in a 555 timer circuit

Question

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:

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.

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

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}