Connect external switch to PC

It would be a switch matrix type arrangement in your keyboard.

These invariably have diodes inserted to prevent "phantom key presses" from 2 keys on the same row or column being pressed together and simulating a 3rd key being pressed. The couple of times I tried this, I used an optocoupler to trigger the key, for safety reasons (optical isolation), and this would have its own internal diode (the phototransistor).

having said this, I have found keyboard hacking to prove unreliable.

In these situations I would consider using those "MAME" - video arcade keyboard emulator modules. They just plug in to a USB or PS2 socket and appear to windows as a keyboard, but they have screw terminals that you simply take to ground using any kind of switch you want to use to "press a key".

They are not just for arcade games (in fact I have never used one on an arcade game:), they will work with any program that can accept input from a keyboard.

That makes sense but I can't see any components. The keyboard just consists of a hard and soft circuit board. The flexible board doesn't have any components unless a diode is built into the track? I would have thought if they were going to use diodes then they would put them on the hard circuit board.

Perhaps they are using a different method of sending out different pulses in different lines and then look for the resulting waveform?

I've had some in service for over 5 years. Although that was in the day when a USB keyboard was over $20. Today's $8 keyboards might not be so reliable. Interestingly the last time I made these I had none of these troubles.

Thanks for that, that is an option. It is more expensive than what I sell my module for now ($60) but it's one of those situations where cost isn't a huge issue.

Michael

This may well be too expensive for your module also Michael, but here is an OZ designed and produced alternative:

Cheers Don...

--
Don McKenzie

Site Map:            http://www.dontronics.com/sitemap
E-Mail Contact Page: http://www.dontronics.com/email
Web Camera Page:     http://www.dontronics.com/webcam
No More Damn Spam:   http://www.dontronics.com/spam

Breakout, Prototype, Development, & Robotics Boards:
http://www.dontronics-shop.com/sparkfun-electronics.html

Would it be less messy to use the handshake lines on a COM port, assuming that one will be available? A small amount of software may be required, but the hardware side would be a heap neater.

"Mickel" schreef in bericht news:%pJJm.53338$ snipped-for-privacy@news-server.bigpond.net.au...

Those keyboard switches are in a matrix and you don't know the matrix layout. Guess wenn you connect both switches separately (so no common wire) your problem may be solved.

petrus bitbyter

The older KB's might have used different methods, the problems I had were indeed with the newer "cheapies" for $9.95 from discount stores - this was in early 2007.. Looking at notes I made at the time, the problems involved keys intermittently not registering when triggered, and even "locking up" the KB. Certain specific keys didn't register at all when triggered - for no reason I could find at the time.

Didnt see the point in hacking up a good quality KB as at the time, they were not much cheaper than some of those "MAME" modules were (about $30) and after all - "we only needed the PC board and the IC" ;) Might have discovered that even the good KB used the same technology as the cheapies and also not be easy to interface ?

I also remember that earlier on, a friend of mine made up a MAME video game system into a home made arcade cabinet for his kids and hacked up a PS2 keyboard to connect up the joystick and buttons. It worked fine, the only problems he had was having to bring 2 wires up to each button, instead of the usual method of every button having a common ground. Was a mess of wires, but he was happy with it. This also would tend to indicate that the older keyboards were less trouble.

One option would be to go to rubbish dump sales or charity shops and buy some old keyboards ?

The only other theory I can think of with your situation is that from memory of working on these cheap keyboards, the "key contacts" are 2 white translucent plastic sheets that are pushed together when a key is pressed.

These may not be a simple switch that closes and shows close to zero ohms, they might have a substantial contact resistance, hence working when you add the resistor.

For all I know It may also work by measuring inductance, or capacitance. If this is the case, then having a length of wire hanging off the key matrix may act to increase capacitance, or pick up mains hum or other interference causing the problem you have with it not working when the wire is attached.

Maybe the diode happens to block interference or limit it to one side of the matrix.

Remembering too, some of those MAME things can be in the $30-40 range if you dont need a lot of inputs like the one shown. The one shown was the only one found on Ebay at the time I searched, just to give a pic of one for you.

I do know that matrix layout because I followed the tracks on the flexible board. AFAIK, I had the wiring hooked up correctly. I checked multiple times for extra circuits that I might have missed but could find nothing. I thought there might be a complete loop on the flexible board all the way back to a different pin on the solid board but there was nothing I could see.

Michael

We used to use serial but the whole idea behind using a USB keyboard was to eliminate the serial port. The serial port had the problem of some false triggering with clients who had some noisy equipment. I could have possibly solved that but some modern PCs don't have serial ports. I could use a USB to serial converter but it might just be easier to make it USB. We also gain the advantage that users don't need to specify which port they are using. Selecting COM1: vs COM2: seems a little old fashioned to me. :_)

Michael

I think I agree with you there. I get a brand new keyboard and throw 95% of the mass of it straight in the bin.

Quite possibly.

That would be an issue for me as I have a lot of double footswitches in the field with only 3 wires. :-)

Bugger that!! :-))

That is true.

This is correct except it is the tracks that have the resistance. From 70ohm to about 30 from a quick test I did (some keys obviously have longer tracks).

When I measured the voltage across the switch (when open) I get -2.5V. When I switched the contacts over I still got -2.5V so something odd is going on there. I thought maybe they send down different frequency pulses so they can detect multiple keypresses.

I will have a look into this and most likely switch to one of these controllers in the future. This is actually what I was looking for but didn't realise existed. Just from a wastage point of view I prefer not to throw out brand new keyboards. Thanks for the tip!

Cheers, Michael

"Mickel" wrote in news:%pJJm.53338$ snipped-for-privacy@news-server.bigpond.net.au:

You could check out devices called 'keyboard wedges'. Such as made by Unique Micro Design in Melb.

I just realised it's not quite what I'm after. I really need 3 wires for my

2 switches, it looks like I would have to use 4 wires. Also I don't want to install any software on the PC and I definately can't use PS2 as this is usually already in use. Bugger, looks like i'm back to hacking appart keyboards.

Michael

The MAME boards that I have seen will work with 3 wires as you require. (All will IIRC)

Each of the switch positions you just take to ground to activate the switch, so you on need one wire for common and one wire for each needed switch.

The other alternative - if you are happy to use a microcontroller, is to make up one of the many PS2 keyboard emulator circuits that are published on the internet (usually using PIC). If you are going to be making a lot of these, this is probably the cheapest and most hassle free way to go.

IF you can't use PS2 into your computer, then you can include a PS2 - USB adaptor, even though it is yet another expense.

Finally, IIRC, some (not all) of the MAME PS2 boards will go in line with an existing PS2 device allowing both to be used.

Ok, my misunderstanding again :-) When it said "no matrix, every switch wire independently" I presumed both wires. Of course it wouldn't make much sense for them to have 40 pins for 20 keys. To have a common ground is actually a better solution for me because when I pull appart a keyboard I'm just lucky that page up and page down keys share a common wire on the matrix, it's quite possible I could find a keyboard that this is not the case.

I'm really keen on a USB solution. From a quick search I just did there are USB options available. I'm in 2 minds, pulling apart a keyboard does work, it's cheap, simple and the board is very small. On the other hand there could be issues for example if someone extends the length of cable to the footswitch. I'll do some research and see if I can find a good USB mame board.

Thanks for all the information.

Cheers, Michaek

You can try using a simple opto coupler to isolate the matrix completely from any long leads to the switches.

if using an optocoupler, pin 1 (A +) and 2 (K -) are the LED, (simply switch +5v to this via a 330 ohm series resistor)

on a 4 pin opto, pin 3 & 4 are the transistor - pin 3 is the emitter and pin 4 is the Collector, wire this directly across the keyboard key you want to trigger. (positive to the collector, though this might be hard to determine on the keyboard matrix, you probably wont damage the opto by experimenting with it in both directions.)

on a 6 pin opto such as 4N26. ' pin 4 & 5 are the transistor - pin 4 is the emitter and pin 5 is the Collector,

There is no guarantee that this will work (it should, if you could get the key to trigger by putting a diode across the pins) , but it only costs a couple of bucks worth of parts to try if it doesn't.