# Versatile and Low Cost Digital Counter

5 Steps
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)
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## 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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rsgino7 says: Oct 29, 2008. 3:12 PM
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 in reply to rsgino7Jan 2, 2012. 9:44 AM
Use conductive glue
buildingteen says: Nov 18, 2006. 11:11 AM
How could you modify this to measure speed on a bike?
binnie in reply to buildingteenJan 26, 2007. 4:05 AM
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
handyScrapper in reply to binnieJun 13, 2008. 8:54 PM
Are you the one in your profile picture and is it recent? Because if so you are a frickin genius.
Agentfern in reply to handyScrapperJan 2, 2012. 9:41 AM
That means i'm a genius too :)
Jono1529 in reply to binnieMar 29, 2008. 7:10 PM
You Could Just Do Distance Over Time
trebuchet03 (author) in reply to binnieJan 26, 2007. 10:31 PM
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 in reply to trebuchet03Dec 21, 2007. 1:42 AM
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 _
Agentfern says: Jan 2, 2012. 7:11 AM
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 says: Aug 28, 2010. 1:09 PM
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
DIYShared in reply to DIYSharedSep 22, 2011. 5:54 PM
rocketman221 says: Sep 6, 2011. 10:52 AM
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 says: Jul 29, 2011. 9:43 AM
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 says: Aug 23, 2008. 3:40 AM
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 in reply to manicmondayDec 9, 2010. 8:29 AM
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.
beehard44 in reply to manicmondayOct 23, 2010. 3:40 AM
graphite mixed with glue will do the trick
you can shave pencil leads for the graphite, should be mostly graphite over glue
XOIIO in reply to manicmondayApr 30, 2010. 12:25 AM
PyroMonger in reply to manicmondayDec 23, 2008. 2:27 AM
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 in reply to PyroMongerApr 30, 2010. 12:25 AM
DeathunT says: Mar 12, 2009. 8:06 PM
My problem is that after 5 min, if nothing append, the calculator switch off!!! How can I prevent that?
beehard44 in reply to DeathunTOct 23, 2010. 3:46 AM
use a different calculator
beehard44 says: Oct 23, 2010. 3:45 AM
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 says: Aug 28, 2010. 1:30 PM
To complete my previus post, if you are interested I show in my video how to use a calculator with flex circuit keyboard:

Bob1356 says: Apr 22, 2010. 1:37 PM
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 says: Mar 11, 2009. 2:57 PM

DeathunT says: Mar 8, 2009. 5:42 PM
sompie says: Jun 13, 2007. 12:56 PM
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 in reply to sompieFeb 27, 2009. 11:58 AM
What if you shoot foul shots or 3-pointers?
Herminator in reply to sompieDec 24, 2008. 10:24 PM
nice idea :)
manicmonday says: Aug 22, 2008. 4:37 PM
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 says: Oct 9, 2006. 11:06 AM
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 in reply to yarnspinJun 13, 2008. 9:05 PM
ur telling me you cant make one yourself?
yarnspin in reply to handyScrapperJun 16, 2008. 7:00 AM
I just do not have the time. Need someone to make me six at a time or more. Nels Wiberg
yarnspin in reply to handyScrapperJun 14, 2008. 4:29 AM
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 says: Dec 9, 2007. 6:06 PM
its a nice idea...i never think to do this!
handyScrapper in reply to PatentedJun 13, 2008. 8:59 PM
wat r u foreign or something? lol jk but to be honest the grammar really sucks.
Patented in reply to handyScrapperJun 14, 2008. 9:40 AM
Lol im not a foreing but I speak french
frank26080115 says: Nov 3, 2006. 9:00 PM
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.
handyScrapper in reply to frank26080115Jun 13, 2008. 9:04 PM
exellent logic.
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