Build Your Simple One-Wire Touch Detector Circuit

Circuit diagram:

Description

This uncomplicated circuit is suitable for triggering various functions, such as interfacing with microcontrollers, relays, concealed alerts, robotic projects, or simply illuminating LED1 when a metallic surface is touched. The circuit incorporates a voltage divider formed by resistors R1 and R2, a Schmitt trigger/inverter gate derived from a 40106 integrated circuit, a small capacitor to mitigate strong radio frequency interference, and LED1 alongside a current-limiting resistor R3. The metallic plate is connected to R1 via a wire. R1 and R2 constitute the voltage divider.

40106 Integrated Circuit

The 40106 is a versatile Schmitt trigger and inverter IC frequently utilized in digital circuits for implementing comparators, oscillators, and level-shifting circuits. Its operation relies on a Schmitt trigger, which provides hysteresis, meaning the output switches in opposite directions for complementary changes in the input signal. This property allows it to operate reliably even with noisy or unstable inputs, providing a stable switching action. The IC’s inputs and outputs are designed for digital logic levels, making it compatible with various digital systems. Care should be taken when applying power to the 40106 to avoid overdriving the input or output pins, as this can damage the device. The input pin 1 is used to control the switch, while the output pin 2 provides the inverted signal.

Due to the minimal current flowing from your body, R2 is chosen with a high value, such as 10 Megohms, to maximize the voltage across R2, which is then detectable by the input pin 1 of the gate IC1. A. R1 has been implemented to safeguard the inverter gate input from electrostatic discharge (ESD) damage. ESD can arise from accumulated electrostatic energy when walking on carpets with rubber soles. Increasing the sensitivity of the detector can be achieved by employing lower values for R1, for example, 1 kOhm, and a smaller metallic plate.

The value of pull-up resistor R3 is determined to ensure that the current flowing through LED1 remains below its maximum continuous operating rating. Most standard LEDs are 20 mA types. The circuit will function correctly even if LED1 is removed, with R3 remaining connected to output pin 2 and a microcontroller input pin directly connected to pin 2. However, it's crucial to verify that the microcontroller incorporates a weak pull-up (i.e., to +VDD) at its port line to ensure proper operation.

WARNING: This circuit contains electronic components that can be damaged by electrostatic discharge (ESD). Always ground yourself before handling the circuit or its components. Exercise caution when working with electronic circuits. The author assumes no liability for any damage caused by improper use or handling of this circuit. [source: example.comcircuit-diagram-1411]