hi,i want to build a LED voltage indicator. when i get to 12V i want the led to turn green. when its lower than 12V i need the led to turn red.
i am using a hand crank generator and need to make the change in 12V.
what part do i need for this?

thanks in advance.

 

Probably not the best way, but a unpowered approach can be found in my article, LEDs, 555s, Flashers, and Light Chasers Chapter 3, The LED / Resistor Only Bargraph .

 

It's going to depend on how exact you want the transition, and where you want the circuit to turn on the red LED. Geeze, it also depends on how high the voltage can go. Lots of little details...

Here's one crude cheap scheme that may work for you... you will have to play with the values:

For low voltages Q1 is off, Q2 is on, so the RED led is on. When the voltage gets up to around 11-12 volts Q1 turns on, turning on the green LED and turning off Q2 which turns off the red LED.

P1 lets you adjust the voltage where this turns on... I don't expect it to be too exact but it's something simple for an indication without using a voltmeter.

 

I like ErnieM's circuit, but I think there's a problem. When Q1 is off there is still a current path through the green LED, D2. It might cause the LED to be slightly on.

Also, I would put a resistor from the base of Q2 to ground to ensure it can turn off. Something like 100K or 200K should be good.

 

I like ErnieM's circuit, but I think there's a problem. When Q1 is off there is still a current path through the green LED, D2. It might cause the LED to be slightly on.

Also, I would put a resistor from the base of Q2 to ground to ensure it can turn off. Something like 100K or 200K should be good.

Yep... needs a diode between the two stages. Good catch.

But no need for the resistor on the Q2 base: Q1 pulls that base to ground so it never floats when off.