Power smaller devices with your batteries from cameras, remotes, GPS that won't turn on because they are too weak. This clock/calendar/thermometer uses a 3V watch type battery. It lasted 4 years until the battery drained to 2.44 volts. New batteries come with even more voltage, for example the new AA 1.5 batteries measures 1.59 volts. So the clock will run well within 2.5-3.1 volts.
Savings: $6.27 (source: newark.com)
I measured the soldering iron usage with my Kill-A-Watt. My 30 watt iron used 28 watts for 8 minutes and didn't even register 0.01 kwh so it cost less than 1 cent.
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Step 1: Find a Low Power Device
1. Find a device that takes less power than your camera. The clock only needs to run the lcd and simple electronics. The lcd is very low power because it does not have a backlight, lots of segments, or color. Find something that takes 2 AA batteries or less or even smaller batteries.
Step 2: Check Your Old Batteries
Check your old batteries for ones that have a good voltage with a volt meter. I ended up using two AA batteries at 1.44 volts and 1.39 volts. They won't run a bigger device but in series they have 2.82 measured volts. The clock will run down to 2.5 volts.
Step 3: Solder Batteries
1. Strip wires for attaching to batteries and device. I used extra cat-5 Ethernet cable cut to 6" and stripped at both ends.
2. Tin the wire and battery separately then quickly solder together. Do Not solder watch sized batteries, another website says they explode, you also do not want to heat up the batteries long so make connections quickly.
Step 4: Connect the Wires
1. Twist the battery wires together in parallel and connect to a lead that connects to the device. The clock had tabs for the battery so it was easy to crimp them on with needle-nose pliers. If you solder them, do not overheat the circuit board of the device.
2. Close it up or tape it so the wires don't touch. The clock has a slot for a screwdriver to pry open the door so I just ran the wires through the space.
Step 5: Secure Connections
I just twisted and taped the leads so they don't touch and I can easily replace single batteries later on. You could secure the connections better by soldering and containing them. There is enough room in the clock body pencil holder to hold the batteries but I like the mad scientist look of the wires.