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 NXP Semiconductors


  • Simple Touch Light Switch

  • Created: Sep 28, 2014

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Summary

This project is a type of switch that is sensitive to human touch. It uses 74AHCT00BQ NAND gate chip, which is a high speed CMOS device that provides quad 2-input NAND function and a touch sensor to basically turn ON the lights when touched. Evidently, this device is more convenient to use than conventional light switch since it does not require an extra force to turn the lights ON/OFF.


The brain of the circuit is the 74AHCT00BQ NAND gate chip that requires only a very low current to its gates to control the system. In the circuit, the capacitor stores the switching state at a certain time and is charged or discharged according to the output signal of the first gate (1Y) of 74AHCT00BQ chip. When the sensor is touched, the state is returned to the input of the first gate of 74AHCT00BQ chip that changes the switching to ON state. The high sensitivity of the sensor makes the operation possible. In addition, the two gates are held at logic state 1 continuously by means of resistors R1 and R3. When the switch is touched, the skin resistance closes the circuit between the corresponding gate and the negative supply. The skin resistance for small areas of the skin is normally much lower than 10MΩ and the gate is effectively brought to logic condition 0, causing the state to change. Any contacts with the sensor will cause the circuit to reverse its state of balance and in effect toggle the switch as it uses a relay, which is driven by a transistor.


Touch light switch has various applications, which is more practical and convenient to use in business offices, industries, and at home. This project can be integrated in ceiling lights, table lamps, and bedroom lights. Furthermore, the device can be an electronic buzzer that only needs a speaker with a little alteration of the circuit.

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