Need help with silencing a beeping (square wave) alarm

Discussion in 'The Projects Forum' started by Kibosh, Feb 19, 2012.

  1. Kibosh

    Thread Starter New Member

    Feb 19, 2012
    Hi folks.
    I am trying to figure out a circuit using logic gates or maybe some kind of flip flop. I am installing a remote beeper with silencing switch. The existing alarm signal I have to work with, and must connect to, for this remote beeper is a 24VDC square wave at 0.5 Hz (meaning the beeper is beeping once every 2 seconds, ON for 1 sec, OFF for one second) In the off state, the signal line goes to ground.
    To silence the beeper at this remote location, I want to use a momentary switch which will switch off the beeper and keep it off indefinitely. The alarm signal coming in will still be active however. The circuit should reset itself when the alarm signal disappears, such that the next time the alarm signal starts, the beeper will be audible again.
    I can connect the output of the flip flop/latch to a transistor which would energize the beeper. This part of the circuit is not a problem, I can figure that out, I just can't figure out the latching or what kind of gates to use.
    One option I would consider is to use a smaller beeper which can be driven directly by the circuit, eliminating the transistor. I would guess the beeper current would have to be below 5 mA or so.
    Another option I have considered is to use the incoming square wave 24VDC alarm signal, (there is some power on tap here, the signal is coming from a 3 amp set of relay contacts in a PLC output card, which is wired to the local beeper next to the machine) and run it through a diode and to a large cap for rectification, (think charge pump) and use it to set up a constant "logic Hi" acting as an input to the gates, which would of course slowly bleed off to zero (ground) a few seconds after the alarm signal turns off.
    It looks like a NOT SR NAND latch might work, but I am worried about the input states being both hi or both low at times, which apparently is a no-no. Help! Logic gates are a real brain-stumper for me. Thanks very much for your brain-power !!

    PS... Available DC power for the circuit is 24 VDC
    Last edited: Feb 19, 2012
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Hi, still looking for a solution?

    What's the buzzer current?

    Can be done with a CD4093 and a couple of resistors, capacitors and transistors.

    In general you need a retriggerable monostable multivibrator whose output enables the buzzer. And a flipflop function with the remote switch that disables the monostable as long as the buzzer input signal is changing.