Flip-Flop Combo lock reset

Discussion in 'The Projects Forum' started by holabr, Jan 1, 2012.

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  1. holabr

    Thread Starter New Member

    Feb 8, 2010
    I am trying to build a simple circuit using CD4013 flip-flops that use a combination of two 12vdc pulses within a set period of time to close a relay. I'm thinking of 4 pulses in a unique pattern such as P1-P1-P2-P1. Logically, when the first pulse comes in, it would trigger the first flip-flop and start a timer. If the following pulses arrive within the time period in the proper order, the relay would energize. If not, the circuit would reset and wait for the next P1 pulse. I found a schematic as a starting point and revised it to come up with the attached. It doesn't guarantee the exact combination (P1-P1-P2-P1) but it does force two P1 pulses only before the P2 pulse. I am using modeling software to test the circuit but have a problem with the auto-reset after the solenoid is activated. According to the original lock circuit, the R12/C2 timer circuit is supposed to reset the lock through diode D1. If I close SW4, the circuit doesn't work at all. If I leave it open, the circuit works but does not auto-reset. Any ideas?

    Also, what would be the easiest and/or best way to put a timer on the overall circuit that would require the proper input (P1-P1-P2-P1) within 5 seconds or the circuit resets?
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    1. The SET inputs on the FF's are floating when the diodes do not conduct. That's the case with SW3 and SW4 open and |Q of IC2b LOW.
    Connect pull down resistors to them. You will need to remove or change R6.

    2. Right now R12 isn't doing anything. If it's purpose was to create a delay when Q1 turns ON it doesn't do it because R12 is not in series with C2 when charging.
    If it was supposed to create a delay when Q1 opens it doesn't do it neither because the capacitor discharges into the solenoid through the diode D3.

    3. SW1 and SW2 need to be debounced.

    How is P1 and P2 related to your circuit? Is P1 = SW1 and P2 = SW2?
  3. holabr

    Thread Starter New Member

    Feb 8, 2010
    Thanks for your input. Yes, SW1 = P1 and SW2 = P2. I've attached the original circuit that I used as a basis. According to the original, R12 was to work as a time delay reset after the solenoid is activated. Did I somehow configure it wrong in my circuit? If I remove D4-D8, will R6 then pull down the SET inputs on the FF's as you recommended?
  4. bertus


    Apr 5, 2008
  5. Wendy


    Mar 24, 2008
    You have caught the attention of the entire moderating staff here at AAC. We did not make these rules, but we do enforce them.

    I strongly suggest you read the TOS, as it has interesting things to say about this contingency.

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