Dusk to dawn switch      

Introduction

This dusk to dawn switch switches on the lamp at night and automatically switches off the lamp in daytime.
The circuit uses an old-fashioned relay as switching element which makes it suitable for all lamp types: not only
incandescent but also TL and compact fluorescent.

Circuit description


When it is light, infrared-diode D1 will be conducting; the voltage on the anode of D1 (see arrow) will be high.
A current will flow through R1 into the base of T1. Because T1 pulls down the base of T2 the relay is switched off.

In the dark, D1 will stop conducting; the voltage on the anode of D1 will become low.
Consequently the base current of T1 will decrease and current will flow into the base of T2 via R3: the relay is switched on.

-The light sensitivity can be adjusted wiith P1; lower resistance = more light necessary to switch off the relay.
-R2 creates some hysteresis to prevent fllickering at the circuit's trip level.
-C1 creates a low pass filter with R1, allso to prevent flickering and to decrease sensitivity to 50Hz hum.
-The trip-voltage on the anode of D1 (seee arrow) is about 1 Volt; below 1V the relay will switch on; above it will switch off.
-I used a LED-shaped IR diode; very conveenient for waterproof mounting in the lamp fixture.
-The circuit will also work with other liight sensors as photo diodes/transistors and LDR's. IR diodes have the advantage that
they are not very sensitive to light from TL or other fluorescent lamps which emit very little infrared !


Dusk to dawn switch schematic & picture of assembled circuit:


(who needs circuit boards anyway?)


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