# 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.

Tools
Soldering Iron
Solder
Some form of adhesive (to attach magnet to a spoke -- or make a holder -- etc.)

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

crimmer17 months ago

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 Lover7 months ago

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.

buildingteen8 years ago
How could you modify this to measure speed on a bike?
8 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
10 months ago

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.

10 months ago

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

7 years ago
Are you the one in your profile picture and is it recent? Because if so you are a frickin genius.
3 years ago
That means i'm a genius too :)
7 years ago
You Could Just Do Distance Over Time
trebuchet03 (author)  binnie8 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
7 years ago
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 _
rsgino76 years ago
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?
3 years ago
Use conductive glue
Agentfern3 years ago
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?
DIYShared5 years ago
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. http://diyshared.com/MeasuringWheel/MeasuringWheel.htm
3 years ago
rocketman2213 years ago
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.
aessam14 years ago
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
manicmonday7 years ago
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.
4 years ago
Look at my page how to do it with that kind of calculator: http://diyshared.com, click on Homemade Measuring Wheel. I have put also instructions on how to do a kind of conductive glue.
4 years ago
graphite mixed with glue will do the trick
you can shave pencil leads for the graphite, should be mostly graphite over glue
5 years ago
6 years ago
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
5 years ago
DeathunT6 years ago
My problem is that after 5 min, if nothing append, the calculator switch off!!! How can I prevent that?
4 years ago
use a different calculator
beehard444 years ago
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...
DIYShared5 years ago
To complete my previus post, if you are interested I show in my video how to use a calculator with flex circuit keyboard:

Bob13565 years ago
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!
TOCO6 years ago
http://www.radioshack.com/product/index.jsp?productId=2419298here is the link for the reed switch at radioshack.

DeathunT6 years ago
How to you keep your calculator to auto-turn-off? Most calculator turn off after 5 min... PLEASE HELP!
sompie8 years ago
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
6 years ago
What if you shoot foul shots or 3-pointers?
6 years ago
nice idea :)
manicmonday7 years ago
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.
yarnspin8 years ago
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?
7 years ago
ur telling me you cant make one yourself?
7 years ago
I just do not have the time. Need someone to make me six at a time or more. Nels Wiberg
7 years ago
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.
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