Hack it before Make it!

So I have written a paper about my experiments and the development of the Gyro’clock project, and it is going to get published in the annual conference proceedings of the American Society for Engineering Education (ASEE), SO EXCITED! I have been preparing for the poster session that’s going to be happening in June.

One of the attractions I am planing to have at my poster session is a counter that shows the number of rotations of the Gyro’clock. For this I need to figure out a way to make a cheap counter. Typically when you are going to make something you have to compromise between the cost and the amount of work. You can buy a LCD shield for you Arduino from Sparkfun at $13 and spend a few minutes writing a short program for it or spend an hour or two building a LED display for less than two dollars. BUT, there is way to have your cake and eat it too with making a counter. That is to hack it from a simple dollar store calculator and a MOSFET (or BJT) switch.

If you open up a cheap dollar store calculator, underneath the buttons you will see a set of traces. When a button is pressed it connects two traces together, depending on which button you pressed. The buttons are literally simple switches. So to make a counter all you have to do is:

1. Solder a piece of wire to each of the two traces of the equal sign button

2. Connect the two wires to a simple MOSFET switch.

3. Turn on the calculator, then press ‘+’ and ‘1’

Now when you close the MOSFET (or BJT) switch, the calculator will first display 1. The second time it will display 2. And the third time it will be 3! You just made, no hacked, the simplest counter in the world. And you can still use the calculator normally as you did before! Yes you can have your cake and eat it too!

So I already had a calculator lying around that could be hacked into a counter. After opening the back cover I found that this one takes cheap to a whole new level. The traces for the buttons are printed on paper in conductive ink! There isn’t even a PCB! There is a tiny PCB for the processor which is sealed with some sort of black goo. So obviously I can’t solder the wires to a paper. Instead I stripped two pieces of stranded wire and taped them into the two traces of the equal sign.

To wire it up with a MOSFET switch, connect one wire to the drain and the other to the source. You may have to switch the two wires around if the calculator doesn’t work (as it normally would) when you turn it on. That’s because there is diode junction between the body and the drain of an n-channel MOSFET, if the source is internally connected to the body of the MOSFET.

Connect the gate to an Arduino output pin. If you are using an n-channel MOSFET, a HIGH on the gate will turn on the switch. Add at least 50 ms delay between switch close and open, otherwise the calculator may not trigger reliably.

That’s it!  a simply hacked calculator counter! I made a video to show you how I am planning to use my counter in the conference. Hope you will find this useful someday,  and remember to Hack it before Make it!

PS: I didn’t invent this by the way. I saw it first in a youtube video while searching for how to program a calculator display.