The objective of the circuit is to create touch switch control that is sensitive enough to detect the input to perform the desired output.

Terminology

  • 4011 – a quad 2-input NAND gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption
  • BD679 – a monolithic NPN Silicon epibase power Darlington transistor with resistors and diode in a TO 126 plastic package and is typically used for AF applications due to its high current gain

Circuit Explanation

The circuit’s operation provides the possibility of closing and opening the contacts of 12 V relay RL1. It can perform the desired functions by adding a conductive surface across the sensor plate to turn ON, for example, an equipment or appliance and be able to maintain power until the sensor plate is touched a second time. Other touch-sensitive switch is constructed on a copper-clad glass, epoxy-printed circuit board using components that are available at most electronic specialty stores.

A metal surface was used in this circuit as the part where it needs to be touched. The indication of the action and triggering of the relay RL1 will be shown using the red LED D3 as the indicator. The design of the circuit is that it should be properly grounded and isolated from the AC power line. The metal surface of the touch sensor should be should be conductive enough to trigger the relay.

The circuit is being supplied by a 12 V source with the IC1 being supplied with 3.9 V of voltage drop. This is made possible by the combination of Zener diode D2 and resistor R5. The value of R1 can be changed to provide a better return of the signal.

Part List

R1=10M 1/4W 5%  [See Text]
R2-3=1M 1/4W 5%
R4=33K 1/4W 5%
R5=1.8K 1/2W 5%		 R6=1K 1/4W 5%
C1-2=470nF 63V MKT
C3=47nF 63V MKT
C4=22uF 25V
D1=1N4148		 D2=3.9V 0.4W ZENER
D3=LED RED
Q1=BD679
IC1=4011
RL1=12V Relay

Application

The theory behind this touch control switch is widely used not only for wheelchair-bound patients, like those with spinal cord injuries, but also for some persons with minimal motor strength and control. It can be used to provide control over conventional light fixtures which could utilize touch-sensitive relay and control all light fixtures and other common household appliances