Voltage monitor to trigger reset

[Matthew Murrian]

I have need for a circuit that will perform the following basic function:

- Continuously monitor an external (battery) voltage and when battery voltage exceeds a certain threshold, _momentarily_ short a pin to GND.

I'm rather certain that `momentarily' in this context means approximately how long you would push the reset button on a computer for (since that's the end-user here) ... and not `momentarily' as in a single cycle of a 10MHz clock.

The overall application is a solar powered, remote (as in, left alone and inaccessible for a year or so) scientific measurement station. The controlling device is a Raspberry Pi and if I issue a HALT command to remove power draw (consider a cloudy day and my battery is nearly depleted) then I need an external, low-draw circuit, that can `watchdog' and short the Raspberry Pi RESET pin (to turn it back on) when I regain battery/solar panel voltage.

Can anyone steer me in the right direction? I don't have much experience with circuit design but have always been able to research and hack things together for my needs ... in this case, I'm hoping someone has a graceful and `correct' solution to propose.

 

[(*steve*)]

what voltage do you want the circuit to trip at?

 

[Matthew Murrian]

I had intended to put all that ... sorry about it. The supply voltage for the circuit will be +5V. The battery is a 12V ... so my trigger voltage would probably want to be 11.8-12V.

I'm also concerned about spurious resets ... for example, I don't plan on HALTing the Raspberry Pi when voltage goes below 11.8V. My HALT setpoint (software initiated, I have voltage monitoring w/ a voltage divider/ADC setup) probably won't be until < ~11.3-11.5V so there is every possibility that my Raspberry Pi will be running already when voltage goes >11.8-12V. So, I don't want this circuit to trigger a reset when I'm already up and running...

For that, I was thinking maybe the circuit could include a DISABLE pin? I could connect it to one of the GPIO pins in the Raspberry Pi and latch it HIGH (or LOW) continuously when the RPi is running. Of course, when the RPi is HALTed that pin wouldn't be latched to anything and should default the circuit to enabled.

 

[(*steve*)]

A possible solution is to have a comparator set so that the output goes high when the voltage exceeds (say) 11.9V. This can be fairly simply changed to a pulse to reset your microcontroller.

If the circuit is given hysteresis, a pulse will only be generates as the voltage rises through the set voltage after first falling below some other lower voltage, thus the circuit will not be reset unless the battery discharges significantly.

Is this what you're after?

 

[cjdelphi]

Your 12v battery being? A small lead acid requires a float charge of 13.8 - 14.2v

For a pulse you could use a 555, to detect the voltage, a voltage comparitor

But then it might be simpler to use a small 5v 8bit micro controller / arduino which will barely consume power even from solar directly! It can do both jobs then and more