Night Light Switch
From
G8MNY@4:902/6 to
WW on Fri Jan 3 21:00:35 2025
By G8MNY (New Jan 16)
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This is one of my simple designs to turn on a low power mains lamp when dark.
The Light Dependant Resistor sensor of course, must not see the echonomy lamp or other lights if it is to work properly.
CIRCUIT
X rated
C1 0u1 4x 1N4001 LÄFuseÄÄÂÄÄ820RÄÄÄ´ÃÄÂÄÄÄÄÂÄÄ´>ÃÄÂÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄ¿ <=
³ Surge | _³_ _³_ ³ 48V³ ³ LDR<=
Lamp C2=== /_\ /_\ ³ ÚÄÄÁÄÄ¿ ³ ³ <=
³ Vadj| ³BRIDGE³ +³ ³Relay³ \³ ³
/ 2 | ³ ³ === ÀÄÄÂÄÄÙ NPNÃÄÄ´
³ Relay ³ ÃÄÄ´>ÃÄ´ C3³100u ³_____e/³ ³
\contacts ³ ³ ³ ³50v _³_ 18K
³ ÃÄÄÄÄ)ÄÄÄÄÄÄÙ ³ Pwr\_/LED ³ NÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÙ
HOW IT WORKS
The small value of C1 a mains X rated Cap is used as high impedance current source (Xc=33kê) to just feed enough current into the bridge rectifier to operate the relay. So C1's value all depens on your relay ratings, (C2 50V can be added to reduce the voltage as an alternative) my relay was a 48V 7mA 7kê coil. The 100uF C3 gives a slow operation of a second or so, & also damps out any mains surges with a flame proof 820R surge R too.
The NPN was a BC337 for my >7mA @ 50V needed to drop the high impedance PSU below the relay hold voltage, but any transistor will do for your V & I & ratings & high current gain. If higher currents are needed, a darlington or pair of transistors may be used for low light sensitivity.
Day light striking the LDR causes it to change from a few 10s of Meg ohms down to less than 10kê. The NPN is then turned on via the sensitivity pot, & the relay supply voltage gets reduced to just the few volts & then the relay releases turning the lamp OFF. There is a large hysterisis to this ON/OFF action, so slight lamp light pickup is not too much problem. The 18K can be changed for different sensitivities.
The power LED lights all the time, brighter when in daylight. Total Power is quite low due to the use of C1 as the "power less dropper".
CONSTRUCTION & TESTING
This is a 230V MAINS circuit! Use an isollated supply for testing, & still keep 1 hand behind your back when you fiddle with it! As drawn most of the circuit is only at +/- 50V AC above Neutral.
It must housed in a proper insulated box Box & connections insulated too, or use an earthed metal Box etc. Extra precautions needed for outdoor use, eg. sealing holes & (or) drilling holes to let water out & box breath etc.
QRM FREE
With no Mains switching Traic, there sould be loaded noise with this unit. (Other than what the Economy lamp makes!)
See my TECH bul on "Christmas Mains Light Chains" & "Fairy Light Flasher".
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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