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Discussion in 'General Electronics Discussion' started by ronlaj, Dec 2, 2010.

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  1. ronlaj


    Dec 2, 2010
    I want to input a 3 volt 14 sec pulse to a 555 ic and output a 4 sec 12 volt pulse only once. Can it be done? I have experimented with capacitors and transistors somewhat but I need help.:)
  2. carebare47


    Oct 21, 2010
    You could set the 555 in monostable mode:

    Monostable operation

    The timing period is triggered (started) when the trigger input (555 pin 2) is less than 1/3 Vs, this makes the output high (+Vs) and the capacitor C1 starts to charge through resistor R1. Once the time period has started further trigger pulses are ignored.
    The threshold input (555 pin 6) monitors the voltage across C1 and when this reaches 2/3 Vs the time period is over and the output becomes low. At the same time discharge (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger.

    The reset input (555 pin 4) overrides all other inputs and the timing may be cancelled at any time by connecting reset to 0V, this instantly makes the output low and discharges the capacitor. If the reset function is not required the reset pin should be connected to +Vs.

    If this gives a low signal, you could flip it with a NOT gate and then that output could switch 12 volts via a transistor.

    Might be able to help more if you could tell us what this is for =)
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