Versatile Activation Circuit
This uncomplicated circuit serves various purposes, allowing activation of different devices such as microcontrollers, relays, secret alarms, or robots. Alternatively, it can simply illuminate LED1 upon touching the metal plate. The components include voltage dividers R1 and R2, a Schmitt trigger/inverter gate from a 40106 IC, a small capacitor for RF interference suppression, and LED1 coupled with current-limiting resistor R3. A wire connects the metal plate to R1, forming a voltage divider along with R2.
Voltage Divider and Sensitivity Adjustment
To detect the small current from the body, R2 possesses a high value, around 10 Megohm, maximizing the voltage over R2 for detection by input pin 1 of gate IC1.A. R1 serves the purpose of averting electrostatic discharge (ESD) energy, safeguarding the inverter gate input. ESD events, resulting from accumulated electrostatic energy, often occur after walking on a carpet with rubber-soled shoes. R1 plays a crucial role in protecting the circuit from potential damage caused by such discharges.
Enhancing Detector Sensitivity
To boost the detector’s sensitivity, one can experiment with lower values for R1, such as 1 kΩ, and use a smaller metal plate. The pull-up resistor R3 is calculated in a way that ensures the current passing through LED1 stays below its maximum continuous rated value. Most typical LEDs are 20 mA types. If desired, LED1 can be removed, and R3 can be directly connected to output pin 2. In this setup, a microcontroller input pin can be linked directly to pin 2. It’s essential, though, to verify that the microcontroller incorporates a weak pull-up (i.e., to +VDD) at its port line.
Functionality of the CD40106B Device
The CD40106B device comprises six Schmitt-Trigger inputs, with each circuit serving as an inverter featuring a Schmitt-Trigger input. The trigger points differ for positive- and negative-going signals, resulting in a hysteresis voltage (VH) between the positive-going voltage (VP) and the negative-going voltage (VN). The device is available in ceramic packaging (J) as well as standard packaging (D, N, NS, PW). All CD40106B devices are operational within an ambient temperature range of –55°C to +125°C.