18-08-2012, 03:44 PM
Kit 79A. LDR LIGHT/DARK ACTIVATED RELAY SWITCH
1Kit 79A..pdf (Size: 26.84 KB / Downloads: 56)
We wanted to do a light/dark switch as a kit. But we found
there were several types of basic circuit. This is one type
of circuit. Two other types may be found in Kits 79B and
79C. You may read the documentation on our website at
http://kitsrussoft.html
By reading about all three Light/Dark Kits you will be able
to decide which circuit is the best for your particular
application. If your application is in very bright or very
dark conditions then you can easily experiment with
different resistor values to get better control over the
switching range you need.
There is nothing original in these circuits. We have taken
them from published material (see end of this paper for
References.) But we have redesigned them each onto
similar PCB's to allow easy comparison. And in addition a
light/dark option has been incorporated into each.
We say 'light/dark' because each circuit has a PCBmounted
switch on-board. In one switch position a lightto-
dark transition will activate the relay. In the other
position a dark-to-light transition is required.
ASSEMBLY INSTRUCTIONS
Before you do any construction we suggest that you
connect a resistance meter to the LDR's - the component
with the transparent face and a spiral pattern inside it - and
note how the resistance depends inversely on the amount
of light falling on it. Note the wide range of resistance.
Play with the LDR in very bright and very dark conditions.
A feel for what is happening in the LDR will help you
understand the circuit you are about to build. In the dark,
the resistance is very high, typically around 1M ohm. In
bright light it is low, typically 1K ohm. The peak spectral
response of the LDR (VT936G from EG&G) is at 550nm.
The continuous power dissipation is 80mW and the
maximum voltage which can be applied to it is 100V.
CIRCUIT DESCRIPTION
The circuit depends on a light sensitive device called a
LDR, light dependent resistor, as already described above.
The resistance of the LDR depends on the amount of light
falling on it. The snake-like track on the face of the LDR is
a cadmium sulphide (CdS) film. On each side is a metal
film which is connected to the terminal leads. If you played
with an LDR & resistance meter as suggested above then
you will know what it does. The CdS LDR used in these
kits are relative slow response devices. This one has a
time constant of about 100msec. So it is quite fast enough
to switch on/off when people pass or run through it. But if
you wanted to use a LDR for fast light-activated
photography then other LDR materials, or a different
circuit would have to be used.