Need help with a delay on timer circuit for a motorcycle

Discussion in 'General Electronics Chat' started by linuxglobal, Jan 4, 2008.

  1. linuxglobal

    Thread Starter New Member

    Jan 4, 2008
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    can anyone help me with a similar circuit?

    I need to build a delay timer for about 20 seconds that controls a 12V DC power source activating a relay. This system will be activating the light harness on a motorcycle so voltage may peak to 13.8V at times. ITs only an issue at startup, hence the 20 second delay, power must be uninterrupted from that point on.

    Basically the circuit will get power of 12VDC at time 0sec and then 20 or 30 seconds later it needs to activate a relay and keep it on as long as the power source is active. When the power source is removed, then it needs to reset and do the same thing all over again when the power returns.

    Thanks in advance.
    Ciprian
     
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    I'm not sure if there is a more elegant way, but I think this might work. Basically, you just setup an RC time constant with a bleed resistor and compare its voltage to a reference. Then, it should be able to drive a FET, then your relay. A realistic circuit might include a cap across the opamp supply rail along with one for the voltage reference (potentiometer divider).

    It will require a bit of discharge time before its next delay. I'm sure someone will have a better solution, perhaps time to uproot the 555 timer? :p

    Steve
     
  3. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Or maybe there is no need for a bleed resistor, and you can use a diode going back to the main 12V supply. I gotta check that though, it's late and I might be completely wrong :rolleyes:
     
  4. linuxglobal

    Thread Starter New Member

    Jan 4, 2008
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    i think i thought this would be easier LOL. Is there a DIY kit i can buy somewhere for this stuff Steve?

    Thanks in advance.
    CM
     
  5. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    There are relays with built-in timers -- can be expensive though.

    As far as DIY goes you can use a small microcontroller that drives a latching relay. I had to cycle power to a circuit continuously -- 1 sec ON and 30 sec OFF for 10K cycles for a test awhile back. I used a 555 timer the first time and then switched to a micro when they kept changing the duty cycle -- so it was easy to change the timing. It used about half a dozen parts. I even used the 555 as the relay driver.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Why is the 20 second delay even necessary?

    Is this an attempt to compensate for a worn battery during starting?

    Keep in mind that if you DO put this delay into play, if there happens to be a momentary interrupton of power for some reason, you'll lose your headlight for 20 seconds - possibly while riding at night! :eek: The outcome may not be good nor pretty.

    Rather than a mandatory "lights-out" time period, I would opt for a switch.

    Otherwise it might be lights out for the rider.
     
  7. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    Yeah, it isn't really an elegant solution, but it was all I could think of. If I had a task like this, I would just use a microcontroller as suggested above. It is really easy to write code for something like this :)

    I can simulate another idea later, see if I can make it work a bit better. But Wookie is right, you don't want to comprimise safety from a potential fault in the circuit.

    Steve
     
  8. NIDOS

    New Member

    Jan 5, 2008
    2
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    HI ALL! im looking for a simple timer circuit operated with a momentary push button with delay of 20 sec.when the button got pushed the timer circuit lits up an led after 20 sec.
    Its very suitable for me, if the IC used will be 555.
    THANKS IN ADVANCE...NIDOS
     
  9. NIDOS

    New Member

    Jan 5, 2008
    2
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    Plz Yara...!!! Its Urgent! Guide Me... Wanna Be Your Thankful:)
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    See the attached for a 20-second timer circuit. This circuit was designed to run on 12V. Timing will change if other Vcc are used.

    In addition to what is shown on the schematic, connect a large-value resistor, such as 100K, from pin 2 to Vcc, and your normally-open pushbutton from pin 2 to ground. To start the 20-second time delay, push and release the pushbutton.

    For your LED, connect the negative (short) lead to the output, and use a current-limiting resistor from the other lead to Vcc. If you are using a standard red LED, 2.5V @15mA, use a 680 Ohm resistor, which will allow about 14mA with a Vcc of 12v.
     
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