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

Schematic Diagram

PCB Layout

  1. 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.
  2. 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.
  3. Both  Transistors are BC548 (or BC547) so they are interchangeable.  The drawing shows the way the flat on the body goes.
  4. The LED is polarised.  The flat on the globe goes to  the  K  (neg) side.  It won't work the other way round.
  5. 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.
  6. 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 line.
  7. The Light Dependent Resistor  is  soldered  across  the  two  tracks shown.
  8. 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 consist  of 


Contact CdS electronics

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