Combining 2 circuits: transistor as switch

Discussion in 'The Projects Forum' started by Wil2k, Apr 13, 2011.

  1. Wil2k

    Thread Starter New Member

    Apr 13, 2011
    Hi everyone. I'm new and I don't have an in depth knowledge of electronics. Will you help me out?

    I am trying to modify a low voltage monitor (kit- Jaycar KA1778) that uses a single 2 colour LED to show when the voltage drops below a preset point. Eg LED changes from green to red when voltage drops to 11.9v. I'm using this in a vehicle so it's running on around 12v. Here's the circuit:


    What I'd like to do is: When the voltage reaches the low alarm point and the LED changes to RED, also trigger a buzzer circuit like the one below. I've played around with this trying to use a transistor to engage the second circuit, but haven't really had much luck as I don't understand the wiring of transistors when used as a switch.


    My questions:
    1) Can I combine these 2 circuits using a transistor triggered by the RED state of the LED.
    2) Do I need to bother using a relay for the buzzer? I'd rather not just for space. The buzzer is a piezo 12v.
    3) Can you please help me with the connections, I'll need it spelled out fairly clearly as I'm a bit of a newbie. :)



    PS: The multivibrator circuit can be substituted for something different / better / more appropriate. All I want is the buzzer to beep / chime repeatedly to get attention.
    Last edited: Apr 13, 2011
  2. wayneh


    Sep 9, 2010
    IMHO, you don't need the relay or the vibrator circuit, unless you like the pulsing it produces. The voltage across the indicator LEDs provides a "logic" signal which you can use to switch a transistor from off to on, with that transistor controlling current through the piezo buzzer. I'd use a MOSFET, with source to ground, drain to the low voltage side of the buzzer, and gate to the LEDs. A traditional BJT would also do a fine job, with base to the LEDs (like the gate), emitter to ground (like the source) and the collector to the buzzer (like the drain).