Tiny Soap Timer

My wife is a teacher in an elementary school in Germany. Due to Corona the pupils must wash their hands, when entering the classroom. And they shall wash their hands with soap for at least 30 seconds, then rinse and dry them.
So my wife asked me, if I could build a device, which could be used as a timer.
The requirements:

  • touchless activation
  • there is no power socket near the sink

I added a few requirements from my side (just because I had the parts at home):

  • flashing dual-color LED
  • buzzer
  • battery-charger
  • easy permanent switch off

The idea:
Touchless activation via an LDR (light dependent resistor). Timer starts and flashes the LED red in the first 25 seconds. Then 5 seconds flashing the LED green. After the 30 seconds expires, a sound is played and the green LED blinks ‘friendly.
I used an Arduino Nano on a breadboard for development. The target processor is an ATTiny 85, which is supplied via a small Lithium-battery. The power supply is switched on and off by the ‘misuse’ of a kind of knock sensor.


  • The first core I used for the ATTiny 85 did not provide support for Arduino’s Tone-functions.
  • The knock sensor did not give enough contact, so I used three in parallel.
  • When glueing the first buzzer, it did not give any sound.

Schematics will follow soon

TinySoapTimer.ino (7.39 KB)

Some pictures of it:

… and a short movie

TinySoapTimer.mpg (1.91 MB)

... and finally the schematics :slight_smile: