I am building a small circuit that will have a supply voltage of 9V from a battery. I would like to detect when the battery is wearing out, using a led that lights up when the voltage passes under a certain level. Does anybody have an idea for a simple circuit that will achieve this?
Hi, Frederik. Look at the National Semiconductor LM10, a low power op amp with built-in reference. If you look at the circuit titled "Battery Threshold Indicator" in the upper right hand corner of p. 14 of the datasheet, you'll have exactly what you need, as long as your threshold is 6V. You'll have to tweak resistor values a little for higher voltage, but the circuit will work as-is with a 9V battery. It uses less tha 0.25mA when V is above Vth, and a little over 5mA when the LED is on (use a high-intensity LED).
In the case of alkaline 9V (which is probably what this OP was asking about), it probably makes sense. Alkaline's have a drooping V as they discharge and a 9V battery stacks six of these, so the slope is fairly pronounced. So, I'd guess a voltage threshold would be reasonable, in this case.
Nicad discharge curves are flatter, I believe. Their voltage drops very rapidly right at first to about 1.2V per cell and then stays close to that until near dead. A voltage threshold would probably be less useful here. There are a number of Coulomb-counting chips now that try and keep a kind of "fuel gauge" for Nicad and NiMH, but in the case of a 9V __alkaline__ this probably isn't needed.
You probably need to specify the battery type. Some battery types are easier than others to monitor.
You also don't specify how much current draw for the circuit can be tolerated. A lot of this will depend on just how long you expect to be using the battery, whether or not you expect this detection circuit to operate all the time or just when your own small circuit is running, how much current draw your circuit takes and how often it's running -- questions like that.
One problem is that when your battery gets very, very low, it won't have enough voltage to keep the detection circuit operating as designed. So while it may work okay when the battery gets lower than some threshold voltage, there will be yet another point below which it will cease to operate altogether.
Another issue is that running an LED continuously when the battery is low is probably a poorer choice, as it takes away the maximum possible energy from the battery at a time when the battery is at its weakest. A possible solution to this is to blink the LED periodically. This reduces the average consumption at this more difficult time. Pulsing the LED may reduce the average by one or two orders -- which can be helpful.
And, of course, there is the issue of simple vs complex and how to trade off all these issues. You might look at Maxim's voltage monitor ICs -- some draw only a few microamps (thinking perhaps of the MAX6459.)
I've been following this post and I have a question regarding low voltage indicator circuits.
How could you "buffer" the indicator circuit to take any sudden but temporary drops in voltage from the battery powered circuit it's monitoring into account? A circuit that occasionally activates a motor or solenoid for example.
I'm thinking of using a low voltage detecting circuit to trigger a 555 to run a 3VDC motor for approx 15 seconds. It's also remotely triggered and once the motor starts turning I think that it'll just keep on being re-triggered by a low voltage circuit until the battery dies.
The battery was to be a 9V alkaline but I read that they actually supply only 50mA before a dramatic drop in voltage occurs. True or not I'm not sure.