A Small Touch-Controlled SOLID STATE RELAY

The demand for modern touch-based control systems is rising quickly, driven by their sleek design, ease of use, and lack of mechanical components. This system introduces an AC touch-controlled solid-state relay (SSR) capable of handling input voltages from 100V to 250V AC. It is designed to switch loads such as bulbs, fans, and other household appliances. At its core is the TTP223 touch sensor IC, which integrates an ADC converter and supports selectable touch modes via a solder jumper (J1). These modes can be configured through TOG (toggle) and AHLB (active high/low at power-up) settings, offering flexibility in operation.

- By ASHWINI KUMAR SINHA

The switching is managed by a BT136S TRIAC, delivering reliable SSR performance without moving parts. By combining touch sensing and relay functions, the system achieves a compact, smart, and silent design within a 4cm footprint. Two terminal inputs provide straightforward connections—one to the AC power line and the other to the appliance, ensuring simple and efficient control.

Fig. 1 shows a compact, modern AC solid-state relay (SSR) module with integrated touch controls. It features two capacitive touchpads (clearly marked with touch icons), driven by the TTP223 IC, allowing connected appliances to be toggled with a simple tap. The large black component is an optically isolated TRIAC-based SSR or relay, designed for switching high-voltage AC loads safely.

The BT136S TRIAC serves as the SSR, capable of switching 100-250V AC loads (up to 4A, ~880W), suitable for bulbs, fans, and appliances. The components required to assemble this system are listed in the Bill of Materials table.

The system is divided into two main circuits: the touch sensing circuit and the solid-state relay (SSR) with a snubber circuit. Both sections are integrated into a final combined system.

Touch sensing circuit

A capacitive touch sensor is used for the touch system. This sensor operates effectively even behind plastic or glass enclosures by detecting changes in capacitance, making it suitable for sleek, sealed interfaces. The TTP223 IC allows selection of various touch behaviours as defined in the datasheet. Fig. 3 shows IC TTP223 pinout details, and Fig. 4 presents the touch sensing circuit.