# Simple MOSFET circuit doesn't seem to be fully turning on

One of my current home projects is redoing the doorbell by retro-fitting a pushbutton and LED inside an older button housing which I already had and looks nice, but didn't come with a very good switch mechanism or light of any kind.

Due to the small size of the interior of the button housing, I had to go with a small through-hole pushbutton, which has a contact resistance of around 60Ω. Due to this, the button itself cannot pass the ~1Amp at 9VDC required to pull in the solenoid in the bell unit and heats up trying to do so.

To get around this, I've been experimenting with using a BUZ71 MOSFET(to which I'm very new) according to the below diagram, the theory being that when the button is not pushed, the ~10mA through the LED will not cause a high enough voltage at the gate to turn on the MOSFET, and when the button is pushed, there will be enough voltage on the gate to turn on the MOSFET, thus allowing enough current to flow to ring the bell.

I started with a smaller value for R1, but the bell never rang, so I've been working up the values to create a higher voltage on the gate. With 120Ω for R1, $$\V_{(GS)}\$$ reaches 4.6V when the button is pressed, and the solenoid tries to pull in but not enough to ring.

Additionally, with any resistor value I've tried, the MOSFET gets quite warm if the button is held in for more than a few seconds. Avoiding this was the main reason I gave up experimenting with BJTs and moved to MOSFETs, as the BUZ71's $$\r_{DS(ON)}\$$ of 0.1Ω should mean it only dissipates 0.225W at 1.5A(max current that would flow through the 6Ω solenoid), right?

I'm guessing there's something obvious that I'm doing wrong, because most of my guidance in the circuit design has been from reading and adapting tutorials online. I might be able to get the bell to function properly if I keep increasing R1, but will this solve the heat issue? Do I need a different MOSFET? I chose the BUZ71 as it was at my local hobby shop and seemed to be able to handle the current I need it to.

The problem is as you turn on the MOSFET, current flows through the solenoid and the MOSFET source voltage rises and tries to turn off the MOSFET. This is negative feedback and will force the MOSFET to behave linearly rather than act as a switch. It would be better if you moved the solenoid to be in series with the drain connection on the MOSFET: -

If you want the LED to extinguish when the button is pressed move the LED and series resistor to connect between drain node and source node of the MOSFET.

On the other hand if you want the LED to light when the button is pressed, then reconnect the resistor and LED across the 10 kohm resistor.

Breaking news: -

Unfortunately, the LED and button in the button housing outside are connected to the bell, power supply and mosfet by only a 2-core cable

Then you will have to use a low value of resistor instead of the 10 kohm in the diagram above.

• Yes, I was just about to say that the 10k won’t light the LED. Unfortunately the button and led outside are only connected to inside with a 2-core cable and I’d like to modify that portion as little as possible as I’ve already mounted it. Is it possible to use the smaller value resistor instead of 10k? Why does it need to be so high? Dec 13, 2019 at 12:45
• See my added section - use a low value resistor (in short). Dec 13, 2019 at 12:46
• Haha yeah sorry I didn’t mention that. I’d originally wired the bell that way because that was easy the way the contacts were arranged, but it shouldn’t be hard to change it. I’ll give it a try tomorrow morning and see how it goes. Dec 13, 2019 at 12:49
• Works perfectly, thank you! Dec 14, 2019 at 0:45
• Congratulations! Dec 14, 2019 at 10:43

Your main problem appears to be that the BUZ71 is operating as a source follower. Try the arrangement shown. This also powers the LED directly from the 9V supply, which keeps the MOSFET gate at zero volts until the button is pressed.

simulate this circuit – Schematic created using CircuitLab

• Unfortunately, the LED and button in the button housing outside are connected to the bell, power supply and mosfet by only a 2-core cable. Also, I don’t think I could fit in the extra button resistor with my current layout inside the button housing. Dec 13, 2019 at 12:39