Electronics/Computer Keyboard question

I am trying to “modify” a computer keyboard (PC) to add external jacks that will allow me to close a switch with a key-press.

I purchased two different keyboards and then got one from our IT guys (at a University) but all three have a thin plastic membrane with a circuit board embedded.

I need an old-fashioned keyboard with actual buttons that close a switch when pressed. How far back in the past do I need to go?

You don’t. Look for one with separate keyswitches:

Keyboards with mechanical keys are still made. The Cherry G80 has mechanical keyswitches. The Matias Tactile Pro is made for a Macintosh, but it is USB so it will work with Windows XP with only a few differences.

Come to think about it, even with the dimply-membrane type of keyboard, there are still keyswitches of some sort. Typically, pressing a key dimples the membrane and presses a conductive pad across a pair of electrical contacts. (Smooth and flat keyboards, that’s different.)

Ideally you would have the electrical schematic of the keyboard. You could solder two wires to the contacts (or their connecting traces) which are closed by the key you want to use, and run those outside the case to your other hardware.

You need to know just what the key contacts are connected to. Often one contact is “grounded” (connected to zero volts) and the other is “pulled high” (connected to a steady DC voltage of a few volts) through a resistor. The voltage at a point between the resistor and the contact will be high when the keyswitch is open, and low when it is closed. This difference is sensed by the rest of the keyboard circuitry and software, and translated into input characters.

If you only want to connect another keyswitch in parallel with the existing one (so that you can push the O key from two different locations, say), a simple external contact will do.

If you want the existing keyswitch to not only be a key input to the computer, but to also control some other hardware independently, you will need to provide some electrical isolation so that you other hardware does not affect the operation of the existing keyboard circuitry. A transistor or relay with a high-impedance input will do; it will react to the changing voltage from your keyswitch, without loading the keyboard, and use its own externally-supplied power to drive your other hardware.

thanks… I had thought about soldering to the membrane-style keyboard, but the membranes are very sensitive and I would surely melt it or otherwise wrinkle it, and it needs to lay very flat.

I have a friend who just e-mailed me and had an old keyboard (12 years) that he is giving me… if that does not work, I will go with the mechanical one that was linked.

I am not sure what the DAS-Keyboard will do, as I still would need to solder to the keys to make an external switch.

Also, I just discovered this schematic if I want to build a custom input/output device.


Does your keyboard not have a circuit board in it? The ones I’m thinking of have either a) a flexible elastomer membrane with rounded keys and conductive pads and a circuit board with contacts that the pads bear on when the keys are pressed, or b), mechanical keyswitches soldered to the circuit board.

I just used that one as an example of a keyboard with mechanical switches. But you don’t actually need mechanical switches to do this. Or even a circuit board. But soldering to traces on the circuit board is a lot easier than finding and soldering to wires or traces embedded in plastic.

That schematic is for the encoded keyboard output, after the raw keypresses have been encoded into binary data and sent to the serial link to the computer. If all you want is a simple on/off switch, you don’t need to deal with that level of complexity.

I have soldered onto a membrane keyboard to jumper where the trace was broken. I had to use hair thin wire, and the gap got a lot wider as the attemps wher made. Don’t ruin a good board trying to solder to a membrane, you need a different type.

I know that there are circuit repair pens for trace repairs, but for one keyboard it’s to much an investment.