This document describes how to modify the top panel of the 3278 A02 / 3279 2C keyboard shown below:
Note: for more details about the original state of the board, please see here, and here.
The original led board was designed to power the leds at 24 volts, so using the original current limiting resistors at 5V, which is what our controller is running at, would make the leds too dim. In order to have the leds light at a similar brightness as the original working condition of the keyboard, the resistors have to be replaced.
The following values have been calculated as recommended replacements. If you want to use these values directly, please make sure that the board you're modifying had the same original resistor values as mentioned in this table. Resistors are listed left-to-right:
| Led color | Index on PCB | Original resistor (@24V) | Recommended replacement (@5V) |
|---|---|---|---|
| Red | 5 | 2.2 kohm | 270 ohm |
| Green | 4 | 1.1 kohm | 120 ohm |
| Yellow | 3 | 2.2 kohm | 270 ohm |
| Yellow | 2 | 2.2 kohm | 270 ohm |
| Green | 1 | 1.1 kohm | 120 ohm |
Note: if you would like to see the raw spreadsheet we used to calculate the resistor values please see here(.ods) or here(.pdf)
- 10-pin ribbon cable with 1.27mm pin pitch. We recommend using the single-color kind, that has only one edge wire with a different color. For example all-grey with pin "1" marked with red.
- Example sources:
- tme
- ebay, search "10 pin ribbon cable 1.27mm"
- Example sources:
- 2x5-pin female IDC connector with 2.54mm pin pitch, and 1.27mm ribbon cable pitch:
- Example sources:
- tme
- ebay, search "2x5 10 pin idc female"
- Example sources:
- You also have the option to buy pre-crimped ribbon cables, and cut off one end, however you have less control over the length of the resulting cable harness.
Note: when crimping the IDC connector onto the ribbon cable, make sure to respect the pin 1 marker, which is usually a triangle/arrow showing which side pin 1 is on.
| Ribbon pin index | Ribbon wire color | Controller net name | GPIO | Connect to |
|---|---|---|---|---|
| 1 | Red | +5VD | "+24V" pin on led board, which is the common anode | |
| 2 | Grey | LED1 | PF5 | "5" on PCB, The left-most led cathode |
| 3 | Grey | LED2 | PB4 | "4" on PCB, The second led cathode from the left |
| 4 | Grey | LED3 | PB5 | "3" on PCB |
| 5 | Grey | HEADER2_LED4 | PE6 | "2" on PCB |
| 6 | Grey | HEADER1_LED5 | PD7 | "1" on PCB, The right-most led cathode |
| 7 | Grey | BUTTON1 | PB6 | The left-most push buttons's middle-left terminal |
| 8 | Grey | BUTTON2 | PB3 | The middle push buttons's middle-left terminal |
| 9 | Grey | BUTTON3 | PF4 | The right-most push buttons's middle-left terminal |
| 10 | Grey | GND | The bottom-left pins of all 3 push-buttons |
Here is a futher illustrated image, explaining how to connecto the wires:
Here is an example of how to provide some strain relief to the ribbon cable using some heatshrink tubing, and a cable tie:
Always make sure the pin "1" of the ribbon cable (the side marked with red) is lined up with pin "1" on controller. Note: pin 1 on the controller is the only pad that is square, and it's also marked differently on the silkscreen.
