Building an input device to send a specific keystroke (or set of keystrokes) to a computer

Hack the keyboard itself, and put a spst switch in parallel with the "scroll lock" contacts.

Or alternatively, you could completely hack a keyboard into an input device, a la;

and put a smart Y connector to arbitrate.

Good Luck! Rich

Hacking the keyboard is the simplest approach, just a SPST switch and some cable. If you don't want to hack into the computer, Micro Chip and Atmel make microcontrollers that include USB interfaces. This may look like massive overkill for one key, but some of these chips are absurdly inexpensive.

I don't know what it's like with USB keyboards (I've never had one, so I can't look inside) but at least some PS/2 keyboards I've opened have a nice small circuit board, separate from the matrix board for the actual keys. Those then make it really easy to make a custom keyboard or whatever, since it's really easy to extract the small board.

Of course, in the old days, this sort of thing would most easily be done with the use of a parallel input board, or even one of the control lines on a serial board. It does require adapting software to look for the keypress on the serial or parallel board, and these days it seems like serial interfaces are not standard in computers, maybe not even parallel printer ports these days.

If the software can be modified, the most easily accesible thing nowadays may be the mouse, in effect a USB interface that can take a number of on/off inputs.

Michael

My suggestion is not along the lines you are thinking about at all.. Since it's purely a computer DIY solution rather than an electronics one.

Attach an external keyboard to the computer*laptop?) he is using. So you have a separate scroll lock key . No need for an Fn key

Now rip off all the keys leaving just the scroll lock key.

The only disadvantage is it is a small button, but that's no big deal.

So there you have it, your device with a single button.

It would be a relatively big keyboard he would carry around.. just for a tiny little button!!! But maybe with your electronic skills, you could hack it into something smaller! Or with some delicate DIY, maybe hack the button into something bigger!

Another solution may be the ext keyboard, and also to use software to remap every key to scroll lock.. or all Function Keys as in F1-12 To scroll lock. If that's possible to map many keys to one key. But I think the above solution of just a keyboard with scroll lock key, is pretty good.

One of the reasons the clicker idea is a consideration is because it could have a long cable so that staff wouldn't even need to be in front of the computer at all. They could be standing in the window and quickly/easily click something in their hand every time a horse crosses the line.

I want to thank everyone for their assistance so far, by the way. Lots of good suggestions. I popped open a crappy Dell PS/2 keyboard I had laying around and took a look. Maybe it's because I haven't messed with this stuff in a long time, or maybe because it legitimately exceeds my experience, but I'm not really sure how to proceed. I do admit that I got a little confused looking over some of the resources I was referred to. This particular keyboard has the interlocking "C" shaped contacts for the keys, and has a Silitek KMMPA40604 chip on the board. I tried to find info on this chip but had no success. I uploaded a picture so you can see what I'm looking at:

I have a USB keyboard pulled apart too. It definitely looks a lot different. I'm sure neither solution is nearly as complicated as it is appearing to me now, but I'm definitely a little confused as to how this thing works exactly and what I need to do to send the scroll lock key.

Anyhow, I'm not fully certain as to how to test the keyboard because I don't think it's working out like it's supposed to. One thing I referenced was

It mentions testing for a full scale deflection, which is a term I'm not familiar with. I don't have my really nice Snap-On DMM anymore, but the cheap little Radio Shack one I have ought to do fine for this. :) If I'm reading it right, I label the pins on my spreadsheet from 1-8 across the top, and 1-16 down the left. I need to test the keyboard by keeping one probe on each "C" for the key and run the other probe down the pin strip going to the card until I get find continuity between the key's contact and a pin. On the bottom "C" I get a reading on pin

7, and top "C" on pin 9... but they're both on the right most group of pins (the 1-16 range). I thought one should read on the 1-8 group, and the other on the 1-16 group?

Hopefully this makes sense -- I wrote it as I went along and changed it a few times in the process. lol.

at:

By the way I should mention that USB is preferable, but I guess I really don't need to do it that way -- it can be PS/2. I took apart a USB keyboard and it seems to be a lot different so I'm not sure where to even begin testing that one.

I think it's more likely that different keyboards use a different layout than a difference between USB and PS/2 keyboards.

Some keyboards will have one single large board, with the actual circuitry and the keyboard matrix, and those will be less suitable for your project. Others will have a small board off in a corner, that is easy to extract and all that you need to figure out is where to connect the button you want so it is in the right place to be the button you want.

The difference is merely a design decision. At this point, there USB and PS/2 will be basically a single IC or at most a couple of smaller ICs added to a large IC.

Michael

Have Fun! Rich

Get a 12V push-type solenoid, and a bundle of twisted pair. Mount the solenoid over the scroll lock key somehow (I'll leave that to your imagination). Here is a solenoid that may work:

Get a standard fan molex connector, and plug it into one of the fan supplies hanging off the power supply in the PC, which should be 12V. Alternately, use a 12V wall-wart. Connect up the solenoid in series with a normally open push button, and with the supply using the twisted pair. Now, when somebody pushes the normally open push button, it'll activate the solenoid, and push the key on the keyboard.

You can get the wire from a phone store or maybe allelectronics above. Also, the pushbutton can be had from the same guys.

This is fairly easy to make, won't make you hack your keyboard into little bits, and has real 'geek' appeal.

If you want to go with a keyboard, you can get a really cheap USB keyboard at the link above.

Regards, Bob Monsen

Another approach you might consider is to use the sound card input lines with a switch that connects to a 1.5V battery. (Also requires a bleeder for when the switch is open, since the sound card is AC coupled.)

The advantage of this is that it can be accurate to one sample period (20.8 usec at 48000 Hz), whereas anything keyboard related goes through the normal Windows message queue and could have a latency of 10s of msec, depending on what else Windows thinks it needs to be doing at any given instant. (The sound card system is set up to not miss anything.) However, although this approach will have simple hardware needs, it will require much more programming effort to implement.

This approach can easily provide 2 independent inputs, one for each stereo channel. If you need more than that, you could use a simple divider with multiple switches, so each switch applies a different voltage to the input. (Resulting in a different capacitively-coupled spike amplitude at the sound card.)

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

I was thinking of suggesting the parallel port, but it sounds like he has a 'stock' application that he can't really modify. So, he'd need to somehow generate the key input to the application based on some other program running on the PC. That might not be simple (although it might, I don't know).

Regards, Bob Monsen