Hacking a simple calculator membrane keypad

[lefam]

Hello guys!

I want to directly hook to the printed circuit board of a simple calculator machine. The calculator has a membrane switch (keyboard).

How can I switch on the calculator without using the membrane switch (ie, working directly with the PCB)? Touching directly with a finger on the spot of the PCB doesn't work.

My plan is to interface the calculator with a computer using the parallel port.

Thanks in advance!

 

[LTX71CM]

You need to scrape off the conductive button coating and solder to the button contacts. You can save time and effort by tracing which connections are shared and working out the matrix. A 4066 can be used to "close" the switches via the parallel port.

 

[(*steve*)]

Another option is an optocoupler. That frees you from having to have an electrical connection between the two devices.

Assuming you can have a common ground and that the voltages are compatible, a 4066 seems like a good option though.