5 sec delay ON timer

Discussion in 'The Projects Forum' started by YeloBlammo, Nov 13, 2012.

  1. YeloBlammo

    YeloBlammo Thread Starter New Member

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    Nov 8, 2012
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    I've searched and searched and haven't come up with an answer.

    I'm looking to make a delay ON timer. 5 seconds.

    It must behave like so:
    The circuit is unpowered. Power is connected. Timer times for 5 sec, then energizes a relay. The relay stays energized until power is disconnected. It resets when the power is reconnected again ( at no given time)

    Power is ~28Vdc
    Relay coil is 26.5V
    Time until relay is energized: 5 sec

    The timing portion I have working. I can't get the relay to energize AFTER 5 seconds. It energizes immediately at power on, 5 seconds goes by, then it de-energizes.

    My circuit looks like so:

    Attached Files:

    • 555.PNG
      555.PNG
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  2. YeloBlammo

    YeloBlammo Thread Starter New Member

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    Like i said, on my breadboard, the relay turns on when power is connected , then off after 5 seconds.

    I want it to: turn power on, wait 5 seconds (timing), then the relay energizes. It stays on until power is disconnected.

    The relay coil is a 26.5VDC and about 500ohm
  3. Dodgydave

    Dodgydave Well-Known Member

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    You need another npn from pin 3 like this...

    Attached Files:

    YeloBlammo likes this.
  4. elec_mech

    elec_mech Senior Member

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    To add to what Dodgydave said, your timing circuit is correct, but you need to note how the 555 and NPN transistor are working together.

    The 555 outputs a high signal whenever it is timing, effectively on. It outputs a low signal when off or not during a timing cycle. An NPN transistor will be on when the base is high. Hence, when the 555 is timing for five seconds, its output is high and the NPN transistor is on, turning on your relay instead of keeping it off.

    To correct this, you can add a PNP transistor as shown below in the top circuit with a couple of resistors. When the timer is on, the NPN is on and 28V is fed to the base of the PNP transistor keeping it off. When the timer turns off, the NPN is off and the PNP base is connected to GND and turns the relay on.

    A PNP transistor cannot replace the NPN transistor directly because the base of the PNP must receive close to 28V on its base to turn off the relay. The max voltage of the 555 output will be about 8V, not enough to turn off the PNP transistor controlling 28V to the relay.

    Alternately, you can use a second NPN transistor as shown in the bottom-most circuit. When the first NPN is on, the base of the second NPN is tied to GND and effectively off. When the first NPN is off, 28V is fed to the base of the second and the relay will turn on. I'm sure Dodgydave showed this, I just couldn't get the image to open so I thought I'd post one too.

    Hope this helps.

    Attached Files:

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