Low-Voltage Remote Mains Switch

This circuit allows a
240V mains appliance to be controlled remotely via low-voltage cabling
and a pushbutton switch. The mains appliance (in this case, a light
bulb) is switched with a suitably-rated relay. All of the electronics is
housed in an ABS box located in proximity to
the appliance. The pushbutton switch and plugpack are located remotely
and can be wired up with 3-core alarm cable or similar. Cable lengths of
20m or more are feasible with this arrangement. When the switch (S1) is
pressed, the input (pin 8) of IC1c is briefly pulled low via the 10mF
capacitor, which is initially discharged.

The output (pin 10) immediately goes high and this is inverted and fed
back to the second input (pin 9) via another gate in the quad NAND
package (IC1d). In conjunction with the 1MW resistor and 470nF
capacitor, IC1d eliminates the effects of contact “bounce” by ensuring
that IC1c’s output remains high for a predetermined period. The output
from IC1c drives the clock input of a 4013 D-type flip-flop (IC2). The
flipflop is wired for a “toggle” function by virtue of the Q-bar
connection back to the D input. A 2.2MW resistor and 100nF capacitor
improve circuit noise immunity. Each time the switch is pressed, the
flipflop output (pin 13) toggles, switching the transistor (Q1) and
relay on or off. Note that all mains wiring must be properly installed
and completely insulated so that there is no possibility of it
contacting the low-voltage side of the circuit.
Author: Bob Hammond – Copyright: Silicon Chip Elecronics

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