Hi, I am trying to understand the circuit I have posted in the pictures. It contains 5 button style switches. The purpose of the the board is to encode the 5 buttons and send that information to the main device (a CRT monitor). There are no ICs, the encoding is all done with passive components. Basically, it is just a way for the buttons to be mounted away from the main circuit board.
My issue is that I have lost this board, and I need to recreate its functionality. I took the pictures quite a while ago when I was modifying the enclosure. Unfortunately, there is a bit of glare that makes it difficult to read some of the printing. Fortunately, the board is very well labeled on both sides, but I do apologize for the quality of the pics.
Here is a description of the original functionality of the circuit:
The 5 buttons were Power, Up, Down, 1, and 2. The power button just cycled power between on, standby, and off. The 1 button brought up an onscreen menu. The up and down buttons could navigate through the menu, then the 1 and 2 buttons were used for enter/cancel. There was also a two-color LED that showed power status (off/standby/on). The onscreen menus could control monitor adjustments like color temp, geometry, degaussing, etc.
Here is what I have figured out so far. Or at least, this is what I think that I have figured out. I am a mechanical engineer and this is a hobby project, so I am no expert.
* There are 6 lines: 2 are ground, 1 is +5V, 2 are for the LEDs, and the final one is labeled "KBD1." I assume this is a uni-directional serial data line. The monitor sends the LED commands on two dedicated wires, but the control board sends commands to the monitor via the KBD1 line only.
* The two transistors seem to be only involved in powering the LED.
* As far as I can tell, the switch encoding is performed only by passive components. I believe that the KDB1 line is normally held high at +5V, then when a button is pressed it goes low for some length of time associated with the particular configuration of the resulting RLC circuit. It looks like the capacitors and inductors are mostly shared, then each switch forms a path to ground through one or more resistors. If I am correct, this means the length of time that the KBD1 line is held low shows which button was pressed. I have tried to look up "pulse length encoding" and other terms that come to mind, but I have not found any particular information. Knowing the proper name of this type of encoding would probably help.
* There is one diode, but I have no idea what purpose it serves. I imagine it could serve a protective feature, or it could serve to isolate the buttons from each other to solve problems if multiple buttons are pressed.
My ultimate goal would be to hook up the KBD1 line to a serial port. I am hoping to use software to send the proper signals to emulate button presses. I have also considered just trying to re-create the old circuit board. It would be manageable if I could identify the proper values for all of the components. The resistors at least are marked, but the inductors are a mystery to me. I have also considered hooking 5 buttons up to a small AVR microcontroller like an ATtiny24. This is just brainstorming. I am just looking for a workable solution to allow me to turn the monitor on (most importantly) and adjust it.
If anyone has any insight or advice, I would greatly appreciate it.
Regards,
James
My issue is that I have lost this board, and I need to recreate its functionality. I took the pictures quite a while ago when I was modifying the enclosure. Unfortunately, there is a bit of glare that makes it difficult to read some of the printing. Fortunately, the board is very well labeled on both sides, but I do apologize for the quality of the pics.
Here is a description of the original functionality of the circuit:
The 5 buttons were Power, Up, Down, 1, and 2. The power button just cycled power between on, standby, and off. The 1 button brought up an onscreen menu. The up and down buttons could navigate through the menu, then the 1 and 2 buttons were used for enter/cancel. There was also a two-color LED that showed power status (off/standby/on). The onscreen menus could control monitor adjustments like color temp, geometry, degaussing, etc.
Here is what I have figured out so far. Or at least, this is what I think that I have figured out. I am a mechanical engineer and this is a hobby project, so I am no expert.
* There are 6 lines: 2 are ground, 1 is +5V, 2 are for the LEDs, and the final one is labeled "KBD1." I assume this is a uni-directional serial data line. The monitor sends the LED commands on two dedicated wires, but the control board sends commands to the monitor via the KBD1 line only.
* The two transistors seem to be only involved in powering the LED.
* As far as I can tell, the switch encoding is performed only by passive components. I believe that the KDB1 line is normally held high at +5V, then when a button is pressed it goes low for some length of time associated with the particular configuration of the resulting RLC circuit. It looks like the capacitors and inductors are mostly shared, then each switch forms a path to ground through one or more resistors. If I am correct, this means the length of time that the KBD1 line is held low shows which button was pressed. I have tried to look up "pulse length encoding" and other terms that come to mind, but I have not found any particular information. Knowing the proper name of this type of encoding would probably help.
* There is one diode, but I have no idea what purpose it serves. I imagine it could serve a protective feature, or it could serve to isolate the buttons from each other to solve problems if multiple buttons are pressed.
My ultimate goal would be to hook up the KBD1 line to a serial port. I am hoping to use software to send the proper signals to emulate button presses. I have also considered just trying to re-create the old circuit board. It would be manageable if I could identify the proper values for all of the components. The resistors at least are marked, but the inductors are a mystery to me. I have also considered hooking 5 buttons up to a small AVR microcontroller like an ATtiny24. This is just brainstorming. I am just looking for a workable solution to allow me to turn the monitor on (most importantly) and adjust it.
If anyone has any insight or advice, I would greatly appreciate it.
Regards,
James