Price - Stock No 210
This circuit will detect light falling
on the Light Dependent Resistor (LDR or CdS). When light falls
on the LDR the arrangement of resistors
at the base of the transistor T1 gives a change of
voltage. This biases or turns on the transistor
which turns on T2, allowing sufficient
current to flow to light the globe. The arrangement of
the resistors at the base of T1 is called a Potential Divider. It is
adjusted to suit ambient light conditions by the variable resistor VR1. A
typical use would be to set the project up in the refrigerator. The light will
light up in another room if you have enough wire, or you could put
a buzzer in instead of the light and catch whoever is
raiding the refrigerator.
Check Your Kit
- Check the P.C.Board for damage to the tracks in transit. Continuity of
tracks is best checked with an OHMS setting on a multimeter, or with an
Electronic Circuit Tester.
- Turn the board copper tracks down and begin to mount the components in the
locations shown on the Placement Sheet. The values of the Resistors are
determined by the colour bands. The resistors will fit
this Board more comfortably if they are mounted
vertically to the Board. The Trimpot will fit the holes and
position the legs correctly. Remember that the resistance is
varied between Centre and one outside leg. The other outside leg is a
passenger, but is soldered to improve firmness of the trimpot to the Board.
- Both Transistors are BC548 (or BC547) so they are
interchangeable. The drawing shows the way the flat on the body goes.
- The LED is polarised. The flat on the globe goes to the
K (neg) side. It won't work the other way round.
- If a buzzer is being fitted, the polarity must be
observed - Red wire is Positive (A), Black is Negative (K). A
power diode, D2 - IN4002- is fitted to avoid feedback
to the circuit from the LED and mainly, the buzzer.
- The battery snap is connected, Red to Positive. A slide switch
is included for a power on/off switch. One wire of the battery snap,
usually the red one, is cut and the switch soldered in series in
- The Light Dependent Resistor is soldered across
the two tracks shown.
- Soldering can be done progressively as components are
mounted if you like. Refer Soldering Technique.
To test the Electronic Candle, connect a 9V battery and
switch it on. Shield the Light Dependent Resistor from the
light source and adjust the trimpot until the LED goes off (and the buzzer
stops). Expose the LDR to light and the LED (and buzzer) will
operate. Differing ambient light conditions
will require adjustment to the trimpot. If the light is very strong
- say sunlight - you may have to replace the trimpot with one
of much higher value, say 100K or 500k or perhaps 1M.
If the project doesn't operate the process of trouble-shooting will
- checking each component for correct value,
polarity where applicable (LED, Diode IN4002,
- Inspect all soldering and resolder
any suspect joints, and watch for bridges of solder
across tracks which will create short circuits.
- As a party interest piece you can adjust the trimpot so that
when you shine a small torch at the LDR
the LED will light. If the LED is located away from the torch with
two insulated leads, say in the next
room, then the LED will light up
"on command", provided that the "command" is loud
enough for the guy in the next room with the torch to hear
and shine his torch on the LDR. Or perhaps you could conceal the LDR
under a cloth. Shine the torch at the spot where the LDR is
and the LED will glow in another place across the room.
- You will find it interesting to measure the different
resistance across the legs of the LDR under varying light flux. A
Multimeter set to the OHMS modes will show the enormous changes to
resistance across the legs. The Light Dependent
Resistor is a Cadmium-di-sulphide Cell as used in photography for light meters
and self-adjusting (automatic) exposure cameras.
- The Potential Divider consists of the arrangement of LDR1 on one
side, VR1 on the other side and the tapping point
between them. The Voltage at the
tapping point is varied by the proportion of the
values of these two resistors, and the total of the Voltage - Tap to Positive
rail + Tap to Negative rail = the
total supply Voltage (9V). Both the resistors are variable in this
case so a balance can be established to suit a range of light conditions. When
the Voltage at the Tap (the Base of Transistor 1) rises, the transistor will
switch on. This allows current to flow between C and E legs. The E
leg is connected to the Base of Transistor 2, so the voltage
turns T2 on. The output of T2 with this setup is very large so a fairly
large device could be driven. Transistors in this setup are called a
Darlington Pair. The GAIN of each of these transistors is about
100, but with Darlington Configuration the total Gain would be 100
X 100. This means that 1 milliamp at the Base of T1 would result in
10000 milliamps available at C or E of T2. R1,
R2, and R3 are protection metering resistors, and R4 meters the current to the
LED to approx. 20 milliamps, the specified draw of LEDs.
- Notice that the polarity of the Diode D2 is the reverse to what you would
expect, viz. the K band is to the Positive (A) rail. This
diode prevents damage to the circuit components that can occur from feedback
from buzzers, relays, or other output devices.
- Connect a Voltmeter from the B leg of T1 to Negative
rail. Switch the Candle on and
slowly increase the light intensity on
the LDR (by turning it slowly towards the light). Note the Voltage
reading when the LED just lights. This
will give the Switching Voltage of
the transistors. Check your reading against a Data Table for
this transistor (BC 547).
- Check also the different readings +ve rail/ tap and -ve rail / tap.
These will be different, but should add together to give Cell Voltage
(around 9 Volts).
Contact CdS electronics