# Large Rectangular Touch Sensor

I'm looking to make a timer for Rubik's Cube solving, which looks like this:

My design just has some added features and a different output type.

However, I'm not sure how best to replicate the hand sensors. They're just binary, "hand on"/"hand off". The pressure required to trigger the sensor should not be very high.

I have a couple of ideas, but I can't imagine that the actual product does any of these. Which one of these is the best, or is there a better way?

1. Each hand sensor is just a physical array of, say, 10 small pushbuttons laid out in a horizontal line. Each one is wired in parallel between VCC and signal so that if any are pushed, the signal is pulled high. I would have a plastic sheet over the top of the pushbuttons so that pressing anywhere (or everywhere) on the sheet will trigger multiple pushbuttons.

2. Have a strip of conductive metal along the bottom of the hand sensor area, and then another strip on top. Separate the two by some sort of spring, and then when pressed the two will contact. There would also be some sort of plastic sheet over the top just to make it look better.

3. Use capacitive touch sensing. (I'm not sure how cheap this is though, or if it works through a thin plastic sheet.)

The signals (one for each hand) just need to be read by some microprocessor -- I haven't decided on one yet, but probably some simple ATMega.

Or do number 4. Definitely the cheapest and simplest one. Make use of the resistance of your body. The average human got about 5MΩ resistance in their skin. Passing through a small current is absolutely okay. You're just sensing, not electrocuting.

Use a Darlington transistor (make one with two BJT transistors). As you can see, the $V_{low}$ is about 0.5V, in reality it will be closer to 0.7V. If that is too high for your µC(microController) to understand that it is a low, then add another transistor at the output which will work as a buffer.

Here's the link if you want to interact with the schematic.

If the µC can't register that 0.7V is a low input. Then this schematic will guarantee that it will work.

Here's the link if you want to interact with the schematic.

• Thank you! Would the two copper plates both be on the top in this case, but with one on the left half and one on the right half of each hand sensor? I'm trying to get an idea of the physical layout. – Jashaszun Aug 2 '17 at 23:28
• Just two conducting things separated, and the only thing that connects them is your finger. Yes, they would be on the top, and to make sure that the user presses both right and left, they will have to complete the circuit with both of their hands. Anti cheating system. – Harry Svensson Aug 2 '17 at 23:37
• Here's a fun thing, the resistance from your right hand to the left hand is almost the exact same as from your right hand to another part of your right hand. If you don't believe me, grab a multimeter and set it to the mega ohm range and start measuring your skin. I'm measuring right now to make sure I'm not lying. 240kOhm from right hand to left hand with alligator clips. 100kOhm from thumb to same thumb. -- Meh... as long as it's lower than 10Mohm you should be alright. 360kOhm on other thumb – Harry Svensson Aug 2 '17 at 23:42
• Either schematic will probably work ;) Try it out on a breadboard first. And both of the schematics got Darlington :p - It was just a warning for the future ;) – Harry Svensson Aug 3 '17 at 2:12
• So I did actually end up breadboarding it just now, and it works perfectly! Thank you so much for your help. :) – Jashaszun Aug 27 '17 at 7:24