Mechanical Engineering student here.

I am attempting to build my own positive air pressure respirator using a vacuum motor.

I have a little CPU fan to put at the exit vent of the respirator to detect when I am breathing out, and subsequently turn the vacuum motor (which is supplying air when I'm breathing in) off for the period of exhalation.

I thought I bought everything I needed for making a NOT gate to control a motor like this, but the MOSFET is behaving in a way I didn't expect. It causes my 3.6 V vacuum motor to draw something like 6 V when I run it off the source. I have also tried putting the MOSFET after the motor and running it into the drain. This is beyond the capacity of the battery I'm planning to use.

Is the behavior expected, and should I get a 6 V battery? Is there some other component I don't know about that would solve this problem? Below are my current components with the specifications that I know of, and a picture of my planned circuit diagram.

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I have had limited success using this NOT gate configuration to slightly de-power an red LED in place of the vacuum motor, and with a transistor in place of the MOSFET.

  • 2
    \$\begingroup\$ Seeing that rug through the plastic container is going to give those poor MOSFETs nightmares for life... \$\endgroup\$
    – TypeIA
    Aug 6, 2022 at 18:37
  • \$\begingroup\$ I fear no static, lol \$\endgroup\$ Aug 6, 2022 at 18:42
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    \$\begingroup\$ A CPU fan is not a plain motor but a chip that turns windings on and off to create a moving magnetic field. It's not meant to be used the other way 'round as a generator. And that schematic is a plain short circuit should the BJT on the right ever turn on. \$\endgroup\$ Aug 6, 2022 at 18:47
  • 7
    \$\begingroup\$ A short is a short. As in unlimited current, dead transistor, overloaded supply... \$\endgroup\$ Aug 6, 2022 at 18:53
  • 1
    \$\begingroup\$ You want to put a switch (or switch-like device) in series with the motor to turn it off - just as if you were controlling the motor with a manual switch. \$\endgroup\$ Aug 6, 2022 at 18:57

1 Answer 1


Give this a run. The 0.8 V from the fan may just be enough to turn Q1 on, which will then turn off M1.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Oh shoot a whole diagram. Thank you Jens, ill give it a try and report back the results! \$\endgroup\$ Aug 8, 2022 at 14:05
  • \$\begingroup\$ It works! The CPU fan turns off the vacuum motor. It still wants to draw 6 V from the power source to run the motor at full strength, but I think I can work around that with the help of a bigger battery. I think I'm starting to piece together how transistor logic works now, thank you so much for the help Jens. \$\endgroup\$ Aug 8, 2022 at 14:23
  • \$\begingroup\$ @LoganKlingler Nice! You may try as well another MOSFET with lower gate threshold voltage. The RFP12N10L is not perfectly conducting at only 3.6 V gate voltage. \$\endgroup\$
    – Jens
    Aug 8, 2022 at 14:30
  • \$\begingroup\$ I see, so the mosfets I have are a little to high spec for the motor I'm using. Ill do some testing with other ones as soon as I get a chance to skim Digi key. \$\endgroup\$ Aug 8, 2022 at 19:01

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