[Karl] needed a programmable real-time clock for one of his projects. He considered adding an RTC chip, LCD screen, and some buttons for use with a microcontroller. That’s not necessarily hard, but it takes time and can be considered a project in itself. Instead, he headed to the hardware store to look for a cheap solution. He was able to get this AC outlet timer for a song. It’s got everything he needs; twenty programmable on/off events, a calendar to track time and day of week, and a user interface made up of a low-power LCD and four buttons. He cracked open the case and patched into the electronics for use with any project.
You can see the solder-tab battery in the middle of the board (green coin-cell). That actually runs the timer circuitry and display. It’s topped off when the unit is plugged into mains, but [Karl] ended up replacing it with a much higher capacity AA rechargeable battery. The device works just like a thermostat, using very little power and driving a relay at the appropriate time. Batteries in thermostats seem to last forever and we can expect the same performance from this device. [Karl] rerouted the trigger signal from the relay to his own 2N2222 transistor. This way the device can switch loads running at voltages other than its own 1.2V operating level.
Stock timers are great. They’re mass-produced which makes them cheap, and you can do some interesting stuff with them. We really enjoyed see this other mechanical version hacked for hydroponic use.
I did something similar to this a couple years ago. had scavenged the timer circuitry out of a old microwave oven which was being thrown out and hooked it up to control a heated bed sheet. (which only had a shut off after 8 hours feature).
To turn it on I need only press power > high > time> 20.00 for 20 minutes of heat.
“Circuitry on the PCB will recharge the battery any time the device is plugged into an outlet.”
I think it need to be made very clear that any chance of mains connection must be removed/disabled in such a hack because there is no isolation between the mains and any connections brought out, just in case anyone has the bright idea of leaving it in place to charge the internal battery.
I used one of these timers about 5 years ago in a time controlled briefcase. My brother had decided only to eat candy on saturdays, so i built him a candy box that only opened on saturdays.