Update: We went with Custom Arduino-based LVD solution for this project.

The LVD Project is great, but relatively complex -- overkill for some applications. A more static circuit could be made with a zener diode, a signalling LED, and possibly a relay (for actual switching)

This is a zener-based circuit. An interesting reference is at http://pubpages.unh.edu/~aperkins/pdf/Misc-devices/unijunction.pdf --- highly recommended, but a bit technical.

A subset of an LVD (or a starting point) is just the "detection" (signalling) portion:

A prototype is about the area of a dime, with only 6 components:

This working model lights up right at 10.7v, indicating sufficient voltage for a specific application (SLA batteries with 15w fluorescent load).


743550 10v zener diode, 5240
38359 3904 NPN transistor
333201 small LED (t1)
691340 100k Ohm resistor
691104 10k Ohm resistor
690865 1k Ohm resistor

Wire the LED-side of the transistor over to an appropriately-sized relay and you get the actual protection circuit. For better usability, green and red LEDs can be added on the open/closed size of the relay as well, to indicate low/high voltage conditions.

For lower-power applications I found a Bestar BS-102B relay -- 12V SPDT, switches 2A, and draws less than 200mW.

This email address is being protected from spambots. You need JavaScript enabled to view it. (or comment in G+) if you're interested in this as a kit or working unit. The PCB is annoying to source, but the rest of the parts are $18 for qty 100 via Jameco