Simple One-Wire Touch Detector

This simple circuit can be used to activate various devices, such as microcontrollers, relays, secret alarms, or even to turn on LED1, which illuminates as long as a metal plate is touched. The circuit consists of a voltage divider with resistors R1 and R2, a Schmitt trigger/inverter gate from a 40106 IC, a small capacitor to mitigate strong RF interference, and LED1 with a current-limiting resistor R3. The metal plate is connected via a wire to R1. R1 and R2 together form a voltage divider. Given that the current from the human body is very small, R2 is typically set to a high value, around 10 Megohms, to maximize the voltage across R2, allowing it to be detected by input pin 1 of gate IC1. R1 is included to protect the inverter gate input from electrostatic discharge (ESD) energy, which may occur when a person accumulates static electricity, such as from walking on a carpet with rubber soles. The sensitivity of the detector can be increased by experimenting with lower values for R1, such as 1 kΩ, and using a smaller metal plate. The value of the pull-up resistor R3 is calculated to ensure that the current through LED1 remains below its maximum continuous rated value, which is typically 20 mA for standard LEDs. The circuit will still function if LED1 is removed, allowing R3 to be connected directly to output pin 2, with a microcontroller input pin connected to pin 2. It is important to verify that the microcontroller has a weak pull-up to +VDD at its port line.

The circuit operates on the principle of capacitive sensing, where the human body acts as a capacitor. When a person touches the metal plate, a small amount of charge is coupled into the circuit, creating a voltage across the voltage divider formed by R1 and R2. The high resistance of R2 ensures that even a minuscule charge can generate a detectable voltage level at the input of the Schmitt trigger. The Schmitt trigger provides a clean digital output by converting the analog signal from the voltage divider into a sharp transition between high and low states, effectively filtering out any noise that could cause false triggering.

The capacitor included in the design serves to filter out high-frequency noise, thereby enhancing the stability of the circuit's operation. The choice of capacitor value can be adjusted based on the specific application and the levels of RF interference present in the environment.

R1's role as a protective resistor is critical; it limits the amount of current that can flow into the inverter gate, safeguarding it from potential damage due to ESD events. The sensitivity adjustments mentioned allow for flexibility in applications, enabling the circuit to be tailored for different environments or user needs.

When connecting the circuit to a microcontroller, it is essential to ensure that the microcontroller's input pin is configured to handle the output from the Schmitt trigger correctly. The use of a pull-up resistor on the microcontroller's input pin can help maintain a stable high state when the circuit is not activated.

This circuit design is versatile and can be adapted for various applications, including touch-sensitive interfaces, alarm systems, and interactive projects, providing a foundational understanding of capacitive sensing technology in electronics.This simple circuit can be used to activate whatever you like, for example, by connecting it to microcontroller, relays, secret alarms, robot applications or just turn on LED1 which lights up as long as you touch the metal plate. The circuit consists of voltage divider R1 and R2, one Schmitt trigger/inverter gate from a 40106 IC, a small capacitor

to keep strong RF at bay and LED1 with current limiting resistor R3. The metal plate is connected via a wire to R1. R1 and R2 together form a voltage divider. Since the current from your body is very small it`s understood that R2 has a high value like 10 Megohm to maximise the voltage over R2 so it can be detected by input pin 1 of gate IC1. A. R1 has been added to prevent electrostatic discharge (ESD) energy from damaging the inverter gate input.

ESD may occur when you have been charged with an amount of electro static energy by walking on a carpet with rubber soles. You can increase the sensitivity of the detector by experimenting with lower values fore R1 e. g. 1 kO and a smaller metal plate. The value of pull-up resistor R3 is calculated such that the current through LED1 is below its maximum continuous rated value.

Most regular LEDs are 20 mA types. The circuit still works if you remove LED1 and just have the pull-up resistor R3 connected to output pin 2 and then connect a microcontroller input pin directly to pin 2. Do check however that the microcontroller has a weak pull-up (i. e. to +VDD) at its port line. 🔗 External reference