Trying to wire a led circuit to an alarm clock.

Discussion in 'General Electronics Chat' started by Beary, Nov 19, 2016.

  1. Beary

    Thread Starter New Member

    Nov 19, 2016
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    So I have a breadboard set up to where I'm using it to power two led lights with a 9 volt battery, and I want to connect it to a cheap alarm clock that I have. I opened the alarm clock and I cut the wires that are going to the speaker, and hooked them up to my circuit. What I'm wanting to happen is for the alarm to be set and then when it goes off it will send power to the circuit. But instead, the power just went straight through the alarm clocks circuit board and the circuit turned on. So I'm wondering if I need like a switch to turn on or something. The alarm clock takes one AA battery, and i was thinking of just plugging the 9v in its place but that made me thing that maybe it would just fry the entire alarm clock. I'm kinda stumped. Help me if you can. :)
     
  2. ian field

    AAC Fanatic!

    Oct 27, 2012
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    1,198
    Is it a speaker with a coil, or a piezo sounder?

    Most cheapo alarm clocks use piezo - they form a partially mechanical resonant system and often have a peaking inductor.

    There's usually big enough voltage spikes to fire an SCR - a TO92 type like the 2N5061 should be more than sensitive enough - you might get away with a low end TO220 device.

    If there's an inductor in parallel with the piezo; you can just connect the gate/cathode across it - the DC resistance of the inductor will shunt against false triggering. Otherwise you may have to couple with a capacitor and shunt the gate with a resistor - wouldn't go lower than 1k, or higher than 10k.
     
  3. Beary

    Thread Starter New Member

    Nov 19, 2016
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    the speaker was really tiny and annoying
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
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    Design 938 alarm clock modification.PNG

    What is suggested is something like this. This makes a big assumption about what the output of your alarm clock looks like. The clock runs on 1.5 V. It only takes about 0.8 V to trigger the SCR. Once triggered it stays on until you push the normally closed switch and interrupt power. You said two LEDs running from 9 V?.I assumed 30 mA. That could differ.
     
  5. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Sometimes the output for a piezo sounder if it has an inductor will just about drive a LL MOSFET. that can dump a decent amount of current through a much better speaker. Another reason for wanting the inductor, is to shunt the MOSFET gate when there's no signal.
     
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