Wanted: Simple voltage-actuated switch

Hi All,

I'm pretty sure this is easy but I'm not experienced enough to actually know how to do it:

I want to remotely monitor the charging of a 12V battery using a digital output that I have available. My idea is when the battery is charging it will see >13V and when it is not charging it will see <13V. So, I'd like the digital output to go high at >13V and low at <13V. This circuit will be powered by the same 12V battery it's measuring so current use should be minimal, particularly when the battery's not charging.

If my battery is charging, I would expect to see the digital signal go high and if it is not, I would expect to see low. (I imagine ~5V would be sufficient to trigger the digital high; ~12V would be good.)

I've been reading about crowbars using SCRs which sound OK (using a zener diode to activate the SCR and a bunch of resistors to keep the current reasonable) but the problem seems to be the SCR would only activate once, then stay on! Not quite what I had in mind...

Thoughts? Suggestions?

Many thanks.

A resistor and 5V Zener diode as a reference.
A pair of voltage divider resistors to scale the battery voltage.
A comparator IC like an LM311.
A resistor and 5V Zener on the comparator output.

Ken

Simple Low Voltage Monitor based on the LM431/TL431

Even Simpler?

Thank you both for the circuits. I like that Simple Low Voltage Monitor - looks especially easy to build.

Ideally the current through this circuit would be minimal - well less than 10mA, so I don't discharge my battery while I'm measuring it!

I have realized that I may be overthinking: First of all, the circuit does not necessarily need to go high when the voltage is above 13 -- it could go the other way.

Any reason I couldn't wire from +batt, through a suitable resistor to give me, say, 2mA of current, to the digital + input, then to ground through a 13V zener? I think the circuit would be digitally high until the breakdown of the zener, then would go low above the 13 volts. Is that right?

Just to close the loop on this one: I built a circuit similar to the one shown in the "Simple Low Voltage Monitor" thread. I tried the zener diode thing but it didn't work for my low-current application. It might work for a higher current one... Anyway, I used a 5k potentiometer for a 1k for R6, no R3 or R4, and a 2.?k for R1, across which I measure the voltage. I did not include Q1, R5, or D2. Seems to work OK. My current draw is ~3mA below 12.9V and ~6mA above (which is fine for my needs). It was easy to build: two resistors, one pot, the LM431/TL431 and some perforated board.