What circuit is used to realize the button function of the TV?

1. Circuit Principle

How can 3 wires realize 7 buttons? Here is the circuit schematic diagram! The designs of different TV manufacturers may be slightly different, but the basic principle is the same:

The schematic diagram is very simple. The main control chip on the "LCD TV control board" has an ADC function, and the ADC is used to detect the voltage. 1. When only switch SW1 is pressed, the voltage detected by the ADC is 0.073V due to the voltage division of 3.3V by resistors R1 and R3:

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

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

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

Based on the above principle, only 2 signal lines are needed to realize the functions of 4 buttons, as shown in the figure below: ① ADC signal line "MCU_ADC1" on the right side of the figure below; ② Ground line "GND" on the right side of the figure below.

By adding one more "MCU_ADC2", you can achieve 8 buttons with 3 wires, which is more than the 7 buttons of this Hisense TV, as shown in the figure below: ①ADC signal line "MCU_ADC1"; ②ground line "GND"; ③ADC signal line "MCU_ADC2".

2. Circuit Optimization

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

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

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

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

3. Continue to advance

The following key circuit scheme is also a solution used in large quantities on TVs. One ADC handles 8 keys. The parameters can be copied:

The cleverness of this circuit is that the buttons have priorities. If SW1 is pressed, then pressing any of SW2-SW8 will have no effect, that is, SW1 has the highest priority, followed by SW2, and so on.