ELECTRONIC CANDLE

INSTRUCTION SHEET

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.

TROUBLESHOOTING

If the project doesn't operate the process of trouble-shooting will consist  of 

• checking  each  component  for  correct value, polarity where applicable  (LED,  Diode  IN4002,  Transistors,  Buzzer).
• 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.  MAGIC!

TECHNICAL NOTES

• 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).

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