Simple circuit for encoding 5 switches to serial output

Thread Starter

--james--

Joined Nov 21, 2016
7
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
DSCN2362.JPG DSCN2363.JPG DSCN2364.JPG DSCN2365.JPG DSCN2020.JPG
 

AlbertHall

Joined Jun 4, 2014
10,400
It might just be possible to reverse engineer a schematic from the photographs, but I can't make out the values of the components to be able to judge what it is doing. Even replacing it with different circuitry or a microcontroller doesn't help as you would need to know the pulse widths and whatever.

Can you give us a model number as there might be a schematic available online. The other possibility is to get a scrap one from somewhere and rescue the board.
 

Thread Starter

--james--

Joined Nov 21, 2016
7
I just had a bit of a breakthrough in finding the model number.

ViewSonic E790, although I still can't find any schematics.

Interestingly, all of the internal parts are stamped with the LG brand and logo. I assumed it was an LG monitor and I was unable to find the model number. The chassis is stamped with model 4814TKK083, but that is apparently an internal number. I did an EDID dump and found the listed model was actually ViewSonic E790. Punching this into google images turns up a picture of the buttons that looks exactly like I remember the original case.
 

RichardO

Joined May 4, 2013
2,271
My first guess is that the switches are wired into a simple digital to analog converter. I have seen that done for other products. There is no serial data, instead the switch state is read by an analog to digital input of a microcontroller.

The circuit is simple enough that you should be able to draw a complete schematic in a few minutes. With a schematic, help would be a lot easier.
 

Thread Starter

--james--

Joined Nov 21, 2016
7
When I get some time, I can probably draw the schematic from the photos, but I can't deduce values for some of the components that don't have color banding on them :(
 

RichardO

Joined May 4, 2013
2,271
But the board is lost - only the photos remain.
Oops. I missed that in my quick read. :(

It might still be possible to do a schematic (without component values) from the photos but it will be a lot more tedious. I would start by printing the front and back side pictures. I would then register them to each other and tape them together Now the circuit can be traced. Marking the PCB traces as this is done might help.
 

RichardO

Joined May 4, 2013
2,271
When I get some time, I can probably draw the schematic from the photos, but I can't deduce values for some of the components that don't have color banding on them :(
Even without values the circuit might help. Maybe some of enhancing the photos could pull out the colors. Just blowing the photo up on my computer helps a lot.

If you have ever watched CSI then you know that you can even look around corners using enhancement. :D:D
 
Just follow the track starting at KBD1. If this goes to a resistor and to switch and continue to another resistor and another switch and so on then the guess of RichardO looks good. The voltage on KBD1 id differnt depending on the switch closed - all resistors could be the same to have equally spaced voltages.
If you use a potentiometer between 5V and GND and one switch from the mid of Pot to KBD1 you could check if you can simultate differnt Keys when changing the Pot and make the switch on for a short time.
 

Thread Starter

--james--

Joined Nov 21, 2016
7
RichardO was definitely correct. The 5 buttons are encoded with different voltage values on the KBD1 line. I have been able to mock it up on a breadboard and get it working now. The capacitors and ferrite bead inductors are definitely needed for noise isolation, but I am ignoring them right now. Sometimes I get extraneous button presses. A am also ignoring the zener diode and the LED circuitry.

Thanks for the help, all.
 

AnalogKid

Joined Aug 1, 2013
8,602
Maxim makes a chip for this that can encode 31 switches. maybe. I'm sure they make the part, but the 31 comes from old memory.

ak
 
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