How can I make a circuit in which an LED turns on at a certain voltage?

Question

I made a little battery charging circuit and it works pretty well. It's slow, although it is safe. But to check if the batteries are charged I have to take them out of the holder and test them with a voltmeter and yada yada ya. Is it possible to add some kind of timing circuit to my existing battery charging circuit, in which an led or buzzer or something goes off at a certain voltage? It'd make things really easy and I wouldn't be so nervous about blowing up the house or something.

And NO, I don't just want to put an old baking timer or stopwatch next to the charger. That's no fun, eh?

I'm open to all ideas! THANKS!

Answer 1

Simplest solution is to get an op-amp, powered from the battery charger itself.  Hook it up like this:

1. Get a zener diode, and establish a reference voltage, less than the voltage you will be measuring.  Put that reference voltage on the - pin of the op-amp input.

2. Make a voltage divider with a resistor and a potentiometer that measures the voltage of the batteries you are charging.  This voltage goes on the + pin of the op-amp input.  Adjust the potentiometer so that when the batteries are fully charged, the voltage is just above the reference voltage you established in step one.

3. Connect the output of the op-amp to a LED, using an appropriate current-limiting resistor to keep the current at 15mA.  This will give you the indication you are looking for.

Simple, eh?

Answer 2

sure. It depends on the battery type and if you have any other voltage available besides the battery voltage.

you can use a zener diode, voltage regulator, lots of circuits. You can also shut off the charger automatically instead of lighting a light which you may not see.

best, if you have the voltage available, is a LM339 comparator. Connect one input (-)  to the battery (via a resistor and some diodes for protection) and the other (+) to a reference voltage. In the output of the comparator, put a LED and a resistor to some + supply.

When the - input goes above the reference, the output will swing low, turning on the LED.

Reference you can derive from a resistor divider from any + voltage if it is stable enough, or via a zener. It should be equal to the voltage at which you want to light the LED.

I'd also add some hysteresis via a high value resistor from the output to the + input to prevent the output from oscillating when the inputs are close to equal.