Price - Stock No 220

Check Your Kit

Schematic Diagram




  1. To print the Board Refer Printing
  2. Inspect  the  tracks  for fine breaks and test the continuity of each track with an electronic circuit tester or ohm-meter.
  3. Identify the resistors by their colour bands and an  Ohm-meter.  Bend the legs so they fit the hole spacings, push the legs through from the plain side of the PCB and bend the legs to hold them in place. They may be soldered now if you like.
  4. C1  and  C2  will  push  into  their  locations,  but they are electrolytic capacitors; polarity has to  be  observed.   Look  on  the cylindrical  body  for an arrow pointing down one leg.  This leg is the -ve (K) leg.  Make sure the capacitors go the right way round.
  5. The eight pin socket can be soldered in place ready to  receive the 555 IC.
  6. The   PCBoard  layout  allows  for  either  a  Trimpot  or  a  Potentiometer.  Check and find which one is in your kit .  For the trimpot, bend one of the outside legs up under the body and push the remaining two legs (centre and one outside legs)  through  the  holes and solder off.  If using a potentiometer either solder its three legs to the donuts on the Board, or use wires to lead it off to its position in your case.
  7. The two LEDs can be mounted on the Board.  Check their polarity - the flat on the flange below the globe is on the K side.  The red LED is  a  "pilot"  that lights when the unit is switched on and timing.The green LED is in parallel with the buzzer and will light  when  the  set time  expires.  The  buzzer has red and black leads. Make sure that you solder the red lead to the A donut, and the black lead to the K donut.
  8. The switch is a Push-on/Push-off type.  Bare the ends of wires,  tin them with solder, push them into the holes  and  screw  the  screws down firmly.  The "ON" position can be found by connecting a continuity tester or a multi-meter and operating the switch.  The green LED with its 390R metering resistor can be connected to indicate that the  timer  is counting.  Work out the connections you will need to make so the LED will glow when the switch is on. An  interesting variation is to use a mercury switch and design a case that pivots like an hour glass.  To switch it ON you  will  need only to pivot it round, and back to the upright position to switch OFF.
  9. The battery snap also has red (+) and black (-) wires.  Solder them as the drawing indicates.
  10. Finally take the 555 IC.  Look at the top surface to find a dot  locating mark.  This dot must be at the end  that  the  drawing  shows.  Place the IC in an IC Inserting tool and push it into its socket. If no inserting tool is available, turn the IC on its side with its legs on a flat  surface  and  roll  the IC so the legs are bent slightly inwards.  Turn it over and repeat with the other set of legs.  This will line the legs up to fit into the socket.  Make sure the "DOT" is right and  push it into place.
  11. To  test,  turn  the  potentiometer anti-clockwise, connect a 9Volt battery and switch  on.   The  buzzer  will  sound  after  a  few seconds.   If  not  then check all components for values, locations and polarity, and check the dot on the IC is correctly positioned. Very few things can go wrong so the problem will be found in one of the above.
  12. To calibrate the timer, put a dot of correcting fluid  on  the knob  and  proceed  to  put matching marks on your case to indicate the times you require - hard boiled - soft boiled etc., using a clock  with  sweep second hand or a digital timer.


Contact CdS electronics

Home Page