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.

Cheers

Wil

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.

 

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).