Capacitive Sensor Circuit: Activate Shop Windows with This Design

Description

The objective of this circuit is to illuminate shop displays utilizing a capacitive sensor positioned behind a postcard-style banner. The banner is placed against the glass within the retail space, and a visitor can trigger the relay by placing their hand on the card, from outside. This design is particularly well-suited for toy stores, enabling the activation of model trains, miniature electric racing cars, and illuminated lights, among other possibilities. Further applications remain open to creative implementation. Adapt this system to enhance the visual impact of your shop window during the upcoming holiday season!

Q1, Q2 & Q3 constitute a high-impedance super-Darlington arrangement, driving the relay and amplifying the 50 or 60 Hz alternating current drawn from the mains supply by the sensor, induced by human contact. C1, D2 & D3 ensure a reliable and clean relay switching operation. A suitable power source is any standard wall-mounted transformer adapter incorporating a rectifier and smoothing capacitor, capable of providing the necessary voltage and current for the relay being utilized.

Circuit diagram:

Parts:

Note:

To ensure proper functioning, the circuit’s ground must be connected via a small-value, high-voltage-rated capacitor to one side of the mains supply socket. The "Live" side is the correct one.

More about components:

The BC337 transistors form a critical element of this circuit. These are NPN bipolar junction transistors (BJTs) commonly used in applications requiring moderate gain and current handling capability. The BC337 provides a stable base current for amplification within the super-Darlington configuration, driving the relay effectively. With a collector current rating of 800mA at 45V, the BC337 can handle the current demands placed upon it by the relay circuitry. These transistors are ideal for this application due to their readily available supply and well-established performance characteristics.