LED string with timer and printed case by Stemer114

Last crawled date: 3 years, 1 month ago

Christmas is coming soon. Take one of these simple battery operated led strings. Mine was from IKEA. You get a nice red battery case, a power switch, a resistor, and a string of 10 red leds with cabling. No way you could source alle these components on your own for less or quicker - unless you are planning to build a 3 digit number of these and break it down.
Anyway, I didn't like the simple on/off switch. Sure, I can switch it on in the evening, but I can't be bothered to remind myself to switch it off before going to bed. A timer was needed.
Nowadays, everything is powered by microcontrollers, why not a simple battery led string as well...
The timer circuit is an ATtiny45 microcontroller, which is cheap, doesn't need many external components and was what I had at hand. See the last picture for the schematic.
The timer circuit does not have an on/off switch. The reset button starts the operation (timer function). After a programmable amount of time (see config.h - anything from seconds to multiple hours, mine is programmed for 6 hours), the led string is switched off and the controller goes to power-down sleep mode (power consumption approx. 0.4uA at 3V). No need for an on/off switch. Pressing the reset button again starts anew. Pressing the second button while the string is active switches to power-down mode immediately. The firmware is in the files.
Because the circuit did not fit into the battery housing, I printed a simple case which houses the strip board with the controller and the two buttons. The size can be adjusted (see params in scad file).
One more thing: You can power this led string easily with used AA batteries. I usually collect used batteries over the year (from toys, alarm clock etc.) - and use them again here. A good re-use for used batteries before giving them into the recycling bin.
The last picture shows a version with 20 leds and 3 AA batteries, where the circuit fits into the battery case.