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



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    86 Discussions


    Question 6 weeks ago on Step 5

    The only issue is calculators shut off after a period off non use. Do you know of a calculator that won't shut off?


    3 years ago on Step 5

    Just what I was looking for. I'm going to use it to count turns on a coil winder, thanks.


    3 years ago on Introduction

    I used this same concept with a switch to count how many times a machine performed a cycle, this gave me the ability to walk away and have an accurate count of how much of my item was produced so that I didnt have to go back and hand count each one. Awsome Idea.

    Trike Lover

    3 years ago on Introduction

    My version of this idea used a similar cheap calculator. I wanted to make a turns counter for a guitar pickup coil winder. I found it easy enough to have the counter increment by "+1" for each revolution. However, I then turned my thoughts to being able to have the calculator shut off the winder when a certain number of turns were reached. To simplify matters for my first attempt, I set up the calculator so that I would punch in the number of turns of wire I wanted (e.g. 6000), and then had it subtract "1" each revolution by starting with "- 1" and triggering the "=" contacts. Sure enough, the calculator counted down to zero just fine, once I had debounced the magnetic reed switch. However, I have not come up with any way to implement a "change of state" output that could be tied into a circuit that would cut power to the winder's 12 VDC motor. I have done a good deal of head-scratching over this, but so far I have not found an answer that will make any of my cheap calculators give me a "change of state" when a specific number is reached.

    Of course, counters that have this function built in are available for $40-$50 on EBay and elsewhere. But, being on a disability pension, I'm trying to substitute ingenuity for dollars.

    It is easy enough to set the calculator to count up (or down) for a specific # of turns and manually shut off the winder. But it would be "elegant" to be able to make a Dollar Store calculator do this trick. I have also used a give-away pedometer as a counter. It worked fine, but it has no built-in function except resetting the counter to zero. I went back to the calculator because the number display was much larger.

    I'm sure there must be a way to hack this without spending additional $ on different or purpose-built counters.


    Reply 11 years ago

    not speed distance could be possible multiply the circumfrence of the wheel by the number on the calculator and that should tell you in the same units ( metric / imperial / anything u invent ) that you used for the circumfrenecr of the wheels Speed works on a completely different set up you would need to get rid of the calculator all together as you are couning the certain number of pulses per min or so many secs


    Reply 4 years ago on Introduction

    You could calculate the diameter of the wheel and enter that into the calculator instead of +1.

    E.g. +1.85 would add 1.85 metres to the total for every flick of the wheel.


    Additionally some calculators have an ANS button. Could enter some quite complex calculations that way.


    Reply 10 years ago on Introduction

    Are you the one in your profile picture and is it recent? Because if so you are a frickin genius.


    Reply 11 years ago

    Yep -- you'd need a timer... You can adapt the concept to do so (I think someone has a speed sensor in the magnet challenge contest ;)). But, it can do average speed if you keep track of time :P


    Reply 11 years ago on Introduction

    I wonder if you could do that with just a reed switch, calculator, and stop watch...

    of course then again you could just spend $10 on a cheapo bike calc at walmart _


    10 years ago on Introduction

    Hey, this is a really cool, yet so simple idea. Great Job! I do have problem with the soldering. I do have PCB however the solder does not stick to the board. Any tips?

    1 reply

    7 years ago on Introduction

    This is a great idea, but the calculator shutting off after a while could be a problem (I want to measure how far my pet hedgehog runs on his wheel in one night). Do you have any idea how to make sure the calculator stays on?


    8 years ago on Step 5

    I have used the concept to make a Measuring Wheel. As a matter of fact I didn't know your project before. True! Anyway you did it first.

    1 reply

    7 years ago on Introduction

    Works great, but mine had a plastic flexible circuit board. I had to use some conductive glue to make the connections then some super glue for strength.


    7 years ago on Step 5

    there is a problem in most calculators
    Most of them automatically close after a while to save the battery
    so these calculators can only be used in fast counting