Introduction: Versatile and Low Cost Digital Counter

Picture of Versatile and Low Cost Digital Counter

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

Picture of 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

Picture of 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

Picture of 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!

Picture of 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


Gadget047 (author)2015-01-31

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

crimmer1 (author)2015-01-21

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 (author)2015-01-20

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.

buildingteen (author)2006-11-18

How could you modify this to measure speed on a bike?

binnie (author)buildingteen2007-01-26

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

ancienthart (author)binnie2014-10-11

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.

ancienthart (author)ancienthart2014-10-11

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

handyScrapper (author)binnie2008-06-13

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

Agentfern (author)handyScrapper2012-01-02

That means i'm a genius too :)

Jono1529 (author)binnie2008-03-29

You Could Just Do Distance Over Time

trebuchet03 (author)binnie2007-01-26

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

srilyk (author)trebuchet032007-12-21

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 _

rsgino7 (author)2008-10-29

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?

Agentfern (author)rsgino72012-01-02

Use conductive glue

Agentfern (author)2012-01-02

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?

DIYShared (author)2010-08-28

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.

rocketman221 (author)2011-09-06

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.

aessam1 (author)2011-07-29

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

manicmonday (author)2008-08-23

Oops. Correction. It worked for a few minutes correctly, then the stiff wire started causing the "=" to go off on it's own multiple times. So then I tried another wire that wasn't so stiff, and it worked for a few minutes, then stopped getting connection. So, unless someone knows of a glue that is also a conductor, then that kind of calculator doesn't work for this application.

DIYShared (author)manicmonday2010-12-09

Look at my page how to do it with that kind of calculator:, click on Homemade Measuring Wheel. I have put also instructions on how to do a kind of conductive glue.

beehard44 (author)manicmonday2010-10-23

graphite mixed with glue will do the trick
you can shave pencil leads for the graphite, should be mostly graphite over glue

XOIIO (author)manicmonday2010-04-30
PyroMonger (author)manicmonday2008-12-23

there is a type of conductive glue but thats all I know on the matter. I don't know what its called or where you can get it from, sorry

XOIIO (author)PyroMonger2010-04-30
DeathunT (author)2009-03-12

My problem is that after 5 min, if nothing append, the calculator switch off!!! How can I prevent that?

beehard44 (author)DeathunT2010-10-23

use a different calculator

beehard44 (author)2010-10-23

maybe if i attach that to the NO pin of a relay and attach my input to the coils then maybe i can use it to detect electrical signals.....
i can see a tipping bucket counter here...

DIYShared (author)2010-08-28
To complete my previus post, if you are interested I show in my video how to use a calculator with flex circuit keyboard:

Bob1356 (author)2010-04-22

The counter is an absolute great circuit,  What I need to do is step it up to the next level.  I would like to start with a particular number and count down and when it got to zero provide a change of state output.  After 50 or maybe 100 openings on a door, the change of state output will allow me to lock the door once it got to zero.  I will have to test but I believe that if I start with the number 50 entered and did "-1" then short the leads connected to the equal button will allow the circuit to count down. I would need an output when it read zero.  Thoughts and comments! 

TOCO (author)2009-03-11 is the link for the reed switch at radioshack.

DeathunT (author)2009-03-08

How to you keep your calculator to auto-turn-off? Most calculator turn off after 5 min... PLEASE HELP!

sompie (author)2007-06-13

I had at home a IR sensor so i all put it on my basketball ring and let the calculator on 2 so every time i scored my calculator at 2 points. Sorry for my bad english

Derin (author)sompie2009-02-27

What if you shoot foul shots or 3-pointers?

Herminator (author)sompie2008-12-24

nice idea :)

manicmonday (author)2008-08-22

I bought that exact clear calculator, and it was easier than another than had an actual PCB. I wasn't able to solder the PCB at all, and that was if I was right about the place to solder it anyway. It was very confusing. But the clear calculator was much simpler, and I noticed that there were holes at critical junctures. I placed stiff wire in the 2 holes that lined up with the "=", bent it double, and crimped it down. It was no problem.

yarnspin (author)2006-10-09

Who could start making these to sell with my skein winders. I need a silent counter to count number of revolutions. What would you charge to make 3 to test with?

handyScrapper (author)yarnspin2008-06-13

ur telling me you cant make one yourself?

yarnspin (author)handyScrapper2008-06-16

I just do not have the time. Need someone to make me six at a time or more. Nels Wiberg

yarnspin (author)handyScrapper2008-06-14

Yes, the calculator that I took apart was a printed circuity board and I do not know how to connect the sensor to that. I'm also too busy making spinning wheels to make fibers into yarn. If you would be interested in making them for me let me know. It would help with my Skein Winder sales.

Patented (author)2007-12-09

its a nice idea...i never think to do this!

handyScrapper (author)Patented2008-06-13

wat r u foreign or something? lol jk but to be honest the grammar really sucks.

Patented (author)handyScrapper2008-06-14

Lol im not a foreing but I speak french

frank26080115 (author)2006-11-03

I got an idea to resolve the problem with the different calculators doing different things when you press "=", this uses 2 switches though
You know how it calculates when you press "+" or any other signs? instead of wiring the "=" button to the switch, wire the "1" and the "+", have the magnet or something trigger the "1" and then the "+", shouldn't be a problem to do for a spinning object, might look ugly on doors though.

exellent logic.

Trans_Am (author)2006-09-29


Murf (author)Trans_Am2006-09-29

Excellent Job. Is there a specific type of reed switch that one would use for this? (I live the idea of the fridge counter :P )

handyScrapper (author)Murf2008-06-13

you could make a homemade one (look up here) or buy one from, or whatever. As long as its a true reed switch it should work.

trebuchet03 (author)Murf2006-09-29

I'm using one for a home alarm system... I believe it is a normally closed swtich - but I'd think a normally open would work too - the count would just happen at a different time.

thejrb (author)trebuchet032007-01-20

could u help me with your digi counter i want to know were to solder my wire on the equals wen i run my wire over it it will just keep counting

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Bio: Engineer making renewable energy products for African entrepreneurs.
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