555 Monostable times out...

Discussion in 'The Projects Forum' started by mark.w, Oct 16, 2013.

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  1. mark.w

    Thread Starter New Member

    Oct 16, 2013
    Please can somebody advise for the following problem.
    I'm fitting a 555 Monostable cct into my car so that it triggers upon locking to give me 5 seconds of delay. It all works fine....lock car, cct triggers the 555....it operates relays and gives power to the car's circuits I want.
    However, if the battery voltage drops to about 12.3v the timer triggers ok but does not reach the point where it would normally drop out.
    I've measured the rising voltage on pin 6, the 555's threshold pin, and it doesn't get past about 8v when in this fault condition.
    I'm using this circuit (http://www.maplin.co.uk/media/pdfs/N32FL.pdf) but have replaced the original timing electrolytic capacitor (1000uF) with a 10uF to make the adjustment range more suitable. It was 1sec to 15mins......it's now virtually 0 secs to 11 secs. The timing components are 1x1k resistor@1x1M pot & 1x10uF Electrolytic.
    The on board relay has been 'turned around' (electrically) so that the 555 is sourcing the load not sinking it too, as that would place unnacceptable drain on the battery.

    As I say it all works fine until car battery voltage approaches 12.3 v and then it won't get out of triggered mode......
    Last edited: Oct 16, 2013
  2. bertus


    Apr 5, 2008

    I am closing this thread as it violates AAC policy and/or safety issues.

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