I have been asked to deliver a low-voltage disconnect circuit for potential future production. It would drive a 15w load and protect a 12v sealed lead-acid (SLA) battery.


  • Arduino-based ATtiny45v microcontroller
  • preset high/low values at 11.3v and 10.5v
  • An asynchronous banner/Flasher to expose the observed ADC value

See http://hlt.media.mit.edu/?p=1695 for an awesome reference on using an Arduino UNO for the programmer, as well as the drivers for the ATtiny series. http://techhouse.brown.edu/~dmorris/projects/tutorials/transistor.switches.pdf is a great reference for transistors-as-switches.

The circuit is similar to that used in the LVD Project, with some minor variations for this application:

  • red & green LEDs (driven by the microcontroller) to indicate on/off state
  • green LED can be used to signal the actual ADC level (with a jumper)
  • no pushbutton calibration

The sketch is included below (attached), as a single sketch requiring no external libraries. It runs in 2,134 bytes of FLASH (about half the capacity of the ATtiny45).


  • ATtiny45v-20PU
  • Bestar 12v / 2A micro relay
  • LM7805 voltage regulator
  • 3904 transistor
  • Green, Red LEDs
  • precision 1/8W 1% resistors: 10kΩ, 49.9kΩ
  • 1/4W resistors: 10kΩ, 100kΩ, 470Ω

Note that all resistors can be 1/8w.

Contact me if you are interested in a custom application.