Let me preface this by saying I'm not that much of an EE expert, this is a "hobby" project, I consider myself as knowing enough to be dangerous. 😉 I'd never apply for a full time career as an EE designer, that's for sure.
That said, I saw this: Low Current Battery Monitor Detail Question and this: Low current battery monitoring in regard to battery monitoring, but they don't address the situation I have.
In my case, I have a 6 cell NI-MH battery that is either not attached to the circuit at all, or it's attached and under load. I'm using a battery pack designed for an RC car, and to turn it off, I just disconnect it. However, I want a monitor that will indicate when the voltage drops below about 6V, since that's the point at which I want to consider the battery pack as discharged.
My current design is the following:
Please accept my apologies for the somewhat low quality circuit diagram, there's limits to what I can manage with a generic paint program.
The TI TL7757 is intended to assert /Reset if it's VCC drops below 4.55V, data sheet is here:
http://www.ti.com/lit/ds/symlink/tl7757.pdf
The Bivar 3BC-C-CA-F is a common anode two color LED, data sheet is here:
http://www.farnell.com/datasheets/1756451.pdf
My logic is that the 470 ohm pot will allow me to adjust the voltage on VCC of the TL7757 so that when the battery hits 6V, the TL7757 sees 4.55V.
Above that voltage, /Reset is high, it's an Open Collector output so the Red LED is off, however the leakage through it, the 270 Ohm resistor and the 100K resistor should be enough to turn on the 2N-3904, which in turn illuminates the green LED.
When /Reset drops, current flows though the red LED and the 270 Ohm resistor and the TL7757, illuminating the red LED, the voltage on the base of the 2N-3904 drops, turning it off and with it the green LED.
Have I made any mistakes that might stop this from working as intended?
Best Answer
It looks pretty good, but there're a couple issues I can see.
One downside is you're just about doubling the current by using a 470 ohm pot for the detection part. I don't know how long you expect to have this circuit attached after the battery reaches the threshold for the red to turn on, but this circuit could excessively drain your batteries if you don't unplug it or charge it soon after seeing the red-light.
You should also make sure you're very careful when setting up/tuning your circuit. 1.2V * 6 = 7.2 which is higher than what this chip is rated for. I know you can get slightly higher than that with NiMH's so start at the low side when you're tuning the pot and work your way up. Another option is to add a resistor between the pot and the IC along with a 7V zener diode in parallel with the IC to protect it. Alternately, a higher Vdd IC could be sought.