This hack will transform a cheap easily sourced calculator into a versatile counting machine. It can be used as a cheap method to measure distance using a wheel, reed switch and magnet (think bike odometer).
So what else can it do you ask? Well, how many times does your central a/c turn on in a day? How often does that radiator fan in your car kick on? How many times does that refrigerator door open in a day/week/month? And the list goes on and on... in fact, have an idea of something repetitive to count? Do post what and the method to count it ;)
Distance meter: Wheel of known circumference, reed switch, magnet
A/c: Relay on thermostat line
Radiator Fan: Relay on fan circuit
Fridge Door Open: Relay or photo sensor on light or reed switch/magnet
In the spirit of the magnet challenge - this is going to be built as a distance meter for a bike and I'm going to measure how many times certain doors around campus open/close during a specific time interval (just for fun :P)
Step 1: BOM - Bill of Materials
Quick list of materials and tools you may want on hand.
Some form of adhesive (to attach magnet to a spoke -- or make a holder -- etc.)
Calculator using a PCB
Wire (nothing heavy duty - little scraps and leftovers are perfect for this)
Magnet (suitable for activating the reed)
A Zip tie or two