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Mosfet locking open

  1. Oct 15, 2009 #1
    I have a hobby project that involves controlling a dc motor remotely. I used a pwm circuit to control motor speed. I also have a rf circuit that sends a signal that can be used to turn the motor on/off. Both circuits work when used independently, that is the rf signal will turn a motor on/off (no speed controller involved), the pwm circuit will modulate the speed of the motor (no rf component involved). the problem is I tried to use a mosfet as the switch, controlled by the rf signal. The rf signal turns the motor on but it doesn't turn it off.
    The entire circuit is being powered by a 9 volt battery (small one with the snap on clips).
    Any suggestions as to what I am doing wrong would be welcome. I am a tinkerer not an EE.
    thanks much

    Attached Files:

  2. jcsd
  3. Oct 15, 2009 #2
    I'm having an issue with seeing the text on all of the items. Can you possibly switch to the PNG format? It is a lot better for text/line items when compared to JPEG. Also can you give the resistor values?
  4. Oct 15, 2009 #3
    it's hard to see some of it, but one of the mosfets doesn't have a resistor from gate to ground. perhaps the gate capacitance is high enough to keep the mosfet on for some time?
  5. Oct 15, 2009 #4
    for the rf circuit see http://www.rentron.com/Files/TWSAKIT.pdf [Broken]
    for the pwm circuit see http://www.electronickits.com/kit/complete/motor/ck1400.pdf
    the resistor values for the mosfet that is receiving the signal from the rf are 1K between rf and gate and 100 K between gate and ground.
    The mosfet that doesn't have a ground resistor is within the pwm circuit, that circuit works on its own. Could the lack of the ground resistor cause an issue when coupled with the rf and switching mosfet?
    Last edited by a moderator: May 4, 2017
  6. Oct 15, 2009 #5
    I had the receiver wired to 9 volts, it has a range of 4.5 to 5.5, when I added a 7805 voltage regulator so the receiver was supplied at 5 volts and delivered a 5 volt trigger to the mosfet the circuit worked fine.
    One more hurdle and the project will be complete!
    Thanks to all for help and suggestions
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