How to Connect a Buzzer to Alarm LED...?

Discussion in 'The Projects Forum' started by ScottChi, Jun 12, 2012.

  1. ScottChi

    Thread Starter New Member

    Jun 12, 2012
    6
    0
    Hi Folks,

    I have a burglar alarm panel that has an LED indicator for "Door Open". I also have an elderly relative staying with us who occasionally leaves our garage door open overnight. So I have been experimenting with adding a buzzer that will notify us when the LED is on, so that we'll know to go and check the doors.

    I tried using the LED current to drive a low current buzzer (15 mA) but there is not enough to power it. That lead me to think that a relay, or an equivalent solid state circuit is required.

    I would rather not use an actual relay, but it's an option. I've looked into the circuitry used for solid state relays, but the optical triacs and photovoltiac circuits seem more elaborate than this small project requires.

    I also considered an SCR. I've been out of school too long; after getting ready to order one I discovered that they latch and stay on as long as the anode current is present. I need this circuit to shut off when the LED turns off (i.e. gate current).

    I feel like I'm missing an obvious answer here, but it's just a hunch. Would someone please recommend a good way to add an alarm buzzer to this LED?

    I'm not opposed to building a transistor-based current amplifier if that's the best approach, but I'd have to learn the math for biasing it all over again. The idea of going through that makes a mechanical, corrosion prone relay start looking like a good compromise :p

    Kind thanks for your advice.

    Scott C. in Cary, NC USA
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    Whatever the switching mechanism, you need power for the buzzer. Your choice for the switch depends on the power you are switching. Your system probably has onboard 12V DC? A single transistor will allow the small LED current to control a larger current for your buzzer if it's a 12v DC buzzer.

    You need to determine whether you need a high-side or low-side switch. That is, your LED is turned on either by 1. leaving it connected to +12V and grounding it (low side), or 2. leaving it connected to ground and supplying 12V to it (high side). It will also have a current-limiting resistor in series with it.

    You need a NPN transistor for a low-side switch or a PNP for a high-side switch. Either way, you'll want one rated for at least 2X your system voltage and 2X your buzzer current. (The safety factor is just for long life and to cover boo-boos.)

    If your buzzer needs 120VAC, then a relay is a fine choice. You can power a relay just the same as described above.
     
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