Introduction: Versatile and Low Cost Digital Counter

About: Engineer making renewable energy products for African entrepreneurs.
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.

Soldering Iron
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)
Reed Switch
Magnet (suitable for activating the reed)
A Zip tie or two

Step 2: Testing and Disassembly

First, turn on your calculator and press: "+ 1 =" It should display "1." Now (This is very important), press "=" again. Does it read "2" as the answer? If yes, continue foreward. If not, you need a different calculator that will do this.

The fun part -- take apart your calculator. With any luck, your calculator uses a graphite pad to close the circuit on a printed circuit. Just like most keyboards. You want a calculator that you'll be able to solder onto - so if your calculator printed circuit is printed on a plastic - you're likely to have a lot of trouble (like I did).

Now locate the printed section for the "=" button and fire up the soldering iron.

Step 3: Soldering

Now, note there are two sides of the key switch. You'll want to solder a bit of wire on one side of the switch and another length on the other side of the switch.

Now, again, press "+1" and then short the two wires you have just soldered. If all is well, the answer "1" will be displayed.

Reassemble your calculator but be sure to have the wires come out the side of the casing. A quick touch of the soldering iron should melt a nice little pathway or your two wires.

Step 4: Attach Reed Switch

Solder the two wires to your reed switch. If you have a different method of attachment, go right on ahead ;) Heat shrink or plastic dip at will, but it's not necessary. As you can see, I used alligator clips to test :P

Again, enter "+1" -- swipe your magnet near the reed switch and make sure "1" is displayed as your answer. If not, go back and make sure your reed switch is working and everything is connected properly. Do this a few times and watch it count away.

Now, use 1 or 2 zip ties to secure your reed switch to the front fork of your bike.

Step 5: Collect Data!

My reed switch has a adhesive backing... So go ahead and stick where you want.

I collected from two doors on campus. The first as an entrance to my school's Student Union. I placed it there for one hour during lunch. I dropped off my cargo and grabbed a "Boston Dog" from a nearby restaurant... An hour later - 424. Minus 1 from me opening the door.

So that's 62 minutes and 423 openings of said door. that's about 6.82 openings per lunch minute (12:00-1:00) on monday. Of course, a suitable sample would be to return the next mondays, collect the same data and then calculate a mean, tolerance etc. But hey, I don't have a month to do that - nor do I want to use this to calculate how many times per hour a specific door opens.

Door two is not as exciting.... Same time period - on a teusday...It's a lonely door in the back of the Engineering building. It goes from the atrium to the back service area that leads to the parking lots... A sad 23 in 58 minutes. 2.52 opening/lunch minute. Which makes sense, why go out for food when you can stay at a table in the atrium studying for your next exam you're likely to fail :P