555 wRelay not functioning

Discussion in 'The Projects Forum' started by ddaytona1, Mar 2, 2011.

  1. ddaytona1

    Thread Starter Member

    Nov 3, 2008
    I've put together a 555 monostable timer shown below. Running the timer using a +5V supply, without the Relay in place and mounting a LED after R1, The LED stays on for the required 30sec.

    When I add the 12V relay to the circuit along with D1, Q1, R1 and relocate the LED with a 470 OHM resistor - and supplying +12V to the relay portion of the cirucut while still providing the 555 with +5V, The LED will turn on for 30 sec, but the relay will not function. If I take the relay out of the circuit, and add +12V to it while it sits on my desk, the relay functions fine.

    Would the current limiting of the 470ohm resistor preventing proper function of the relay in circuit? My next step would be to jumper around the resistor & LED to see of the Relay operates. All I'm using the LED for is to provide some visual confirmation the the timer is staying on for 30sec.

    Thanks for any suggestions !
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Yes, very much so. Take out the resistor and LED and it should work.
  3. Wendy


    Mar 24, 2008
    You can also move the resistor/LED between the + power supply and the collector of the transistor. It will work in parallel with the relay.

    Location is everything.
  4. ddaytona1

    Thread Starter Member

    Nov 3, 2008
    Thanks for the comments !