Since I in different occasions had this question about how to simulate a
keypress on an existing keyboard, I decided to post the answer to all.


Original Question from andre@compufire.com: "using pic to control a vcr"
>Hi to all engineers in this list.
-snip-
>I decided to drive them
>from switches like pressing buttons. switches are connected together
>in multiplexed method that means each switch when you press it
>doesn't ground the only access i have is switches. my question is
>to press a button electronically should i use optoisolater or can I use
>electronic analog switch like 4066. any recommendations ?
>
>Andre                         thank you very much.>
>==========================================
>= http://www.compufire.com               =
>= mailto:andre@compufire.com             =
>= mailto:mcu-engineering@compufire.com   =
>= Andre Abelian: Engine Electronics, Inc.=
>= Tel 909-589-5485  Fax 909-598-5695     =
>==========================================

Cheapest (i belive) is to use a transistor resistor and a diode for each
button. 

What a coincidence: I actually today built a remote controlled PC keyboard
(15 buttons) with this technology:

The keyboard controller scans the keyboard by asserting one row at a time
ti GND, others to Vdd. The horizontal lines it pulls high.

For each button to be emulated I connected a NPN transistor«s emitter to
the key«s scan row, and the collector via a diode to horizontal line.
(diode kathode to collector) I drive the transistor with logic level from
IR decoder via a 100 kohm R.

>what is the idea of using a diode in each switch?

If, when this transistor«s emitter scan line is high, and another key
sharing the same feedback line as the transistor«s collector is pressed, it
will sink the collector voltage. 

If you are then holding the base high the transistor will lead current from
emitter (now positive) to collector.  As the keys here have some high
resistnace (rubber-type) the transistor will win and the keystroke not
detected. 

Anyway, in my application it would not matter, since only one transistor or
 switch is activated at the same time, but I want to make it reliable,
since I will sell it. (For controlling engine test equipment when sitting
in car.)

Also beware that when your input to the transistor is high, the NPN
transistor will most of the time have a negative Vbe. Almost any transistor
can take -5V, but I don«t know about -Vcb, (which we will have if we don't
use diode)

/Morgan
Morgan Olsson, MORGANS REGLERTEKNIK, Sweden, ph: +46 (0)414 70741; fax 70331
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