555 Timer Trouble

Discussion in 'The Projects Forum' started by pchso, Mar 13, 2009.

  1. pchso

    Thread Starter New Member

    Mar 13, 2009
    Hey guys.

    I'm running a 555 timer as a monostable multivibrator. I've gotten it to light an LED for a specified amount of time, but when I hook it up to a 6V or 3V engine, which is what I want it to run, it doesn't work. More specifically, with the engine, when I press the push button that activates the circuit, the engine runs for as long as the push button is pressed but then stops when I release the button.

    Although I don't think it should matter, the push button I have opens the circuit when compressed and closes it when open, so I hooked it up to a 7404 hex inverter. I doubt it should make any difference, though. Again, the circuit works perfectly with the LED.

    My thought is that the 555 timer's output is high enough voltage to run the LED, but too weak to run the engine. Any ideas on how to make the output stronger? I don't have anything in the control voltage right now.
  2. pchso

    Thread Starter New Member

    Mar 13, 2009
    This is my circuit, in case you wanted to know (minus the LED and 470 resistor and plus an engine).
  3. thingmaker3

    Retired Moderator

    May 16, 2005

    You need to switch a MOSFET or a Darlington transistor with the 555, and use said switch to turn the motor on or off.
  4. pchso

    Thread Starter New Member

    Mar 13, 2009
    Thanks for the quick reply. Any chance you'd teach me how to use said MOSFET or Darlington transistor? Keep in mind I'm completely new to this electronics stuff and that I am trying my best to muddle through this mess.
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Do you know current draw of engine at 9V? With 555 held on it runs so try # 1, add a diode , 1N4002, or similar between pin 3 and motir. If that does not help , as suggested add a transistor, darlington T or MOSFET.Resistor values depend on device. For run of the mill med. power tran. try 1k from 555, 10 mA base current, other R 2.7k
  6. Wendy


    Mar 24, 2008
    You don't need the Base Ground resistor, the 555 switches to ground. Something more like this I think...


    Instead of the relay use a motor.

    Most power transistors will work, unless this motor is much bigger than I think it is. I'm figuring around an amp or so, correct? If so R2 is 470Ω ¼W for R2, and a button for C1.
    Last edited: Mar 14, 2009