# Connecting to circuits on plastic



## geosman (Sep 15, 2007)

I learned about using calculators to make an event counter by soldering connective wires to either side of the "="-key. Want to attach it to my Muon detector and light sensor.
Problem: the only cheapo calculators I can find have circuits printed on plastic (polyester). Ran a hack and found that if I short the opposite sides of the =-key it does increment the count.
How to run wires to the circuit??
1) Use connective paint for rear window defoggers. Cons: $10 for 0.5 Oz. Not very sturdy, i.e. wires to the circuit could twist loose easily.
2) Adhesive conductive metal strips. So far cannot be found locally.
3) Thought of CA-ing the bare wire across the circuit then using a toothpick laying a strip of shim brass wider than circuit across both (to keep pressure onto circuit) and CA-ing that too.
Has anyone had success with any other approach??
Thanks for your input!


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## geosman (Sep 15, 2007)

*[SOLVED]Re: Connecting to circuits on plastic*

Found what seems to me to be a simple solution.
Since the cheapo ($1) calculator's back is built to maintaing pressure on the plastic sheet carrying the circuit where the conductive buttons press against the circuit to make contact, I traced the circuit from the "=" key (both sides, + & -). I cut strips of 5/1000" shim brass which I laid over the traces leading to the button area. I used simple cello tape to hold it in place. If need be one could use the cello tape to insulate such strips from other traces. I let the traces hang outside the calculator case and replaced the screws. The case has a stepped edge which holds the strips well. Then soldered an NPN with the collector to the + lead (check with meter) and emitter to ground. The the signal is applied to ground and base. When an event stimulates the transistor or wire connect is draped across the base to ground connection the calculator increments the count (start by entering 1 and +). It works flawlessly. Sure beats soldering to plastic which at best is iffy.


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