12V 70Hz 555 oscillator to P-Mosfet.

Discussion in 'General Electronics Chat' started by Rittter, May 19, 2016.

  1. Rittter

    Thread Starter Member

    Dec 5, 2015
    I am using a 555 timer set at 70Hz, pin 3, to the gate of a p-MOSFET. I am using the same 12VDC supply for both the 555 and to source the MOSFET. Is there a way to physically determine what the frequency is discharging the source pin on the MOSFET ?
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    If the mosfet is properly connected to a load, then the MOSFET should be discharging at 70Hz. Put your oscilloscope probes across the load and measure the frequency of on/off cycles.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    If you don't have a scope, you can add a CD4060 signal divider chip and use the Drain of the P-channel mosfet to drive the clock-input to the CD4060. You will be at/near 70Hz if the divide by 2^15 pin swtiches through an on/off cycle in 7 minutes, 48 seconds.
  4. MrSoftware

    Senior Member

    Oct 29, 2013
    If you don't have a scope, check if your multi meter has a frequency counter. If so, put that across the output and see if it gives you what you're expecting.
    GopherT likes this.
  5. Marley


    Apr 4, 2016
    Remember that the output pin of the 555 does not go all the way up to the +12V supply when the output is high. There will be at least a volt drop, possibly more. It is possible that the P-MOS is not properly switching off. I would connect a resistor between the gate and source to ensure the source-gate voltage difference is close to zero when the output is high. About 1k ohm should do it. This will be OK at 70Hz. Not so good at much higher frequencies as the capacitance of the gate is quite high and will take some time to discharge through the resistor.
    GopherT likes this.
  6. ScottWang


    Aug 23, 2012
    If you have the o'scope that is better, otherwise the frequency meter is also good to measuring the frequency, if you don't have o'scope and frequency meter then using the multi-meter and turn to the AC 10V and in series with a 0.1uf/25V ~ 1uF/25V capacitor to test the Vout(pin 3), if there is voltage show up, it means that the Vout has generate the pulse.
  7. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    According to your post, the MOSFET source pin is connected directly to the 12 V power supply. As such, therewill be no signal there to observe. Please post a schematic of your circuit so we can make informed recommendations.