First, the circuit principle

3 wires realize 7 buttons, what is the principle? Directly on the circuit schematic diagram! There will be a little difference in the design of different TV manufacturers, but the basic principle is this:

The schematic diagram is very simple. The main control chip on the “LCD TV control board” has the ADC function, which is judged by the magnitude of the ADC detection voltage. 1. When only the switch SW1 is pressed, since the resistors R1 and R3 divide the 3.3V, the voltage detected by the ADC is 0.073V:

2. When only the switch SW2 is pressed, the voltage detected by the ADC is 0.767V:

3. When only the switch SW3 is pressed, the voltage detected by the ADC is 1.650V:

4. When only the switch SW4 is pressed, the voltage detected by the ADC is 2.222V:

Based on the above principle, only 2 signal lines can realize the functions of 4 buttons, as shown in the figure below: ① the ADC signal line “MCU_ADC1” on the right side of the figure below; ② the ground wire “GND” on the right side of the figure below.

By adding another “MCU_ADC2”, 8 buttons can be realized with 3 wires, which is more than the 7 buttons of this Hisense TV. See the figure below: ①ADC signal line “MCU_ADC1”; ②Ground wire “GND”; ③ADC signal line “MCU_ADC2”.

2. Circuit optimization

In fact, in engineering applications, it is not so simple, and some protective measures need to be added. Take a group of 4 buttons as an example to illustrate. 1. First add anti-static measures. TVS tube can be used for electrostatic protection, see D1 in the figure below (actually used in the figure is a cheaper varistor than TVS tube, part number ICVL0518030FR). Static electricity will be conducted to ground by D1:

2. This is not enough, and then connect the resistor R2 in series to prevent static electricity from being conducted to the main control chip: (due to the high input impedance of the ADC, the effect of R2 on the ADC voltage detection can be ignored)

3. Finally, add a small-capacity capacitor to eliminate the jitter of the keys and filter out high-frequency interference:

The final circuit is shown in the figure below, which is a circuit verified by mass production:

3. Continue to advance

The following key circuit scheme is also a scheme used in batches on TV sets. One ADC can handle 8 keys, and the parameters can be copied:

The neat thing about this circuit is that the keys are prioritized. Assuming that the button SW1 is pressed, then pressing any one of SW2-SW8 will have no effect, that is, the priority of SW1 is the highest, followed by SW2, and so on.

Reviewing Editor: Liu Qing

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