Kevin Darrah recently purchase an inexpensive soap dispenser with the idea that he could use it instead for hand sanitizer. What he came up with so far is very much in prototype form, with a breadboard, wiring, and a custom ATmega328 PCB hanging off of the side. To get it to spray this new substance, he took the nozzle off of a bottle of “manual” hand sanitizer, allowing it to push liquid out with the motor from the original dispenser.
The motor is actuated by an FQD13N06LTM MOSFET, with a flyback diode. Hand detection uses the original IR emitter/receiver that came with the dispenser, along with additional resistors needed for proper reading and power. Code and other info is available on GitHub.
It’s an interesting enough hack by itself, and could be quite useful once he gets things stuffed into the original housing. The bulk of the video, focuses on how the new device works, with an emphasis on low-power design. Techniques implemented include putting the unit to sleep for one second between readings, as well as experimentation with the amount of light emitted from the IR LED via the input resistance value. His exploration enabled him to (so far) get the average current down from 270µA to just 41µA for dispenser, which will have a dramatic effect on battery life.