The Third Harmonic has recently created a little gizmo which will sound a buzzer when the light in a room has been on longer than a specified time – in our case, 5 minutes.
It was designed to be used in a room where the only light source is the ceiling-mounted light. The idea came about from the fact that the room’s halogen lighting was occasionally being left on for many minutes after not being needed (this room is only used for no more than a few minutes at a time) but may be left on for over an hour.
An initial design we used involved an arduino being powered from two Li-ion cells (from an old laptop) through the 5v voltage regulator on-board. This method only lasted a day or two at the most before a recharge was required. This, obviously, was not very practical. We tried using a mains transformer (or wall-wart) but due to the wires used, the only ones we had lying around, it looked a bit messy – besides since it was a switch-mode ‘block’, it gave out around ~75v AC on both +ve and -ve wires although it wasn’t able to cause any harm due to such little current provided.
Why the revival? Ever since we ordered some ATMega328p-PU AVRs from the internets, we’ve been thinking of low-power projects and this seemed like an ideal candidate to try it out on. Since the AVRs we’re using can run on as low 1.8 volts, they seemed ideal. What made the project so much more promising is the ability for the microcontroller to ‘sleep’. This allows massive power savings when the MCU (microcontroller) is not being used, i.e. instead of using delay(500) we can use sleep(500), although the sleep function can appear different depending on the approach taken (directly or (what we used) via a library). Using the ‘sleep’ capability can possibly reduce current from ~10.00mA (awake) to ~0.17mA (sleeping) (depending on sleep mode used). This amazing reduction means that our device can theoretically run (not considering self-discharge) for over a year on some rechargeable AAs. We would like to have only 2 AA cells being used, instead of three, but we have yet to work out how to set the fuses to disable brown-out detection to go that low.
What we hope for this little project is to give us experience in the building of electronics, and it seems to have done that as well as given some enjoyment on the way.
Thanks for reading.
We’re mentioned on SCRS Practical Group blog with our gizmo, here.