In the previous design, I was reading the voltage of PIN3 of the PC110 30pin connector which is the original LCD driving voltage. It’s range is about 22V to 30V and is controlled by the contrast keys in I believe 256 steps. I was using two resistors to form a voltage divider to bring 30V down to ~3.3V for the ADC of the PIC. The problem is, it can take quite some time to go from 0% to 100% contrast and even still this only provides a voltage range of ~2.4V to ~3.3V, and the resolution of the ADC is low so the noise is a problem. The intention had been to have more than the standard (3) brightness levels, however in practice this methods is only useful if a high resolution ADC was used or other analog circuity was put in place to better scale the signal.
I have revised the PCB to instead use the original brightness signal located on PIN3 of the inverter wire.
Pinout of 4 pin inverter connector
PIN1 = ~10.5V (Inverter Supply)
PIN2 = GND
PIN3 = Brightness Control (25kohm Low, 10.5kohm medium, 5.4kohm full)
PIN4 = ENABLE (0V off, ~3.4V ON)
I have placed a 10kohm resistor from PIN3 to Vcc (3.3V), and the below measured voltage is as measured at Pin3.
| Intensity | Pin3 Ohms to GND | Measured Volts | ADC | Threshold | PWM% |
| Low | 25k | 2.75 | 213 | <255 | 1% |
| Medium | 10.5k | 1.75 | 135 | <150 | 50% |
| High | 5.4k | 0.75 | 58 | <100 | 100% |
The low and medium brightness is TBD, I will try to select the optimal values to pre-program.
High brightness is only available while connected to A/C. Although the PC110 has no issue providing the current for the LED backlight, it may have been designed to restrict full brightness on A/C because of the higher draw of the CFL backlight inverter.
Either way, the brightness between medium and high will not but massive in terms of light output, so it can be considered with this was you can switch between the lowest brightness, and the ideal brightness.