Here is a simple capacitive touch switch circuit for electronics hobbyists. Even though the circuit uses smd components, construction of the touch sensor switch is not very hard because the whole circuit can be easily fabricated on a piece of smd prototyping board. At the heart of the circuit is an inexpensive (`20) smd chip TTP223 commonly available in BA-6 package. The TTP223 is a touch pad detector IC which offers one touch key, designed for replacing traditional direct key button switch with diverse pad size. Low power consumption and wide operating voltage are the contact key features for dc or ac application.
The circuit can be interfaced to any microcontroller through the 3-pin header J1. Recommended VCC is regulated 5V, and the available output (OUT) is active-high (H) in case of a finger touch at the touch pad (TP). LED 1 (green) is the power (PWR) indicator, and LED 2 (red) is an optional output status (OUT) indicator. Just bridge the solder jumper JP1 to enable the output status indicator, if necessary.
Prototype built on a general purpose circuit board using traditional (leaded) components with TTP223 mounted on an smd adaptor pcb (SOT-23 to DIP adaptor) was successfully tested using a small (10mm x10mm) copper clad board as the touch pad. If you are using this method, try to keep the length of interconnection wire from touch pad to pin 1 of TTP223 as short as possible.
In some modules, a 1K resistor is inserted between the touch pad and pin 3 of TTP223, but the same is not tested at this time. Note that the total loading of touch pad size and capacitance of connecting line on circuit board can affect the sensitivity. So the sensitivity adjustment must according to the practical application (larger and/or thinner touch pad will increase the overall sensitivity).
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