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G8MNY  > TECH     10.07.18 11:03l 68 Lines 2981 Bytes #999 (0) @ WW
BID : 28452_GB7CIP
Read: GUEST
Subj: Night Light Switch
Path: HB9ON<IW8PGT<IR2UBX<IW2OHX<UA6ADV<CX2SA<N0KFQ<GB7CIP
Sent: 180710/0844Z @:GB7CIP.#32.GBR.EURO #:28452 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                (New Jan 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

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
LFuse820RĴĴ>Ŀ <=
          Surge     |   __    __      48V        LDR<=
       Lamp       C2===  /_\    /_\      Ŀ       <=
                Vadj|    BRIDGE  +    Relay   \  
         / 2         |            ===    NPNĴ
         Relay          Ĵ>Ĵ C3100u   _____e/   
         \contacts                50v   __        18K
                    )      Pwr\_/LED      
N    

HOW IT WORKS
The small value of C1 a mains X rated Cap is used as high impedance current
source (Xc=33k) 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 7k
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 10k. 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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