Price - Stock No 270

This kit operates a relay for a specific time.  The circuit can be triggereg by a high going pule or a low going pulse.  As soon as it is triggered the LED illuminates and the relay energises, they remain in this state for a set time then turn off.  The time is set by the trimpot and in the configuration shown the range is from approximatly 1.1s to 12.1s. 

Other time ranges can be configured and please
Contact CdS electronics if you would like to customise this kit.




  1. Check  the  P.C.Board  for  damage to tracks.  Any breaks can be soldered over.  Check with a multimeter or electronic circuit tester.
  2. Identify the resistors, see Resistors.
  3. Insert the resistors and solder in place - see Soldering Technique.
  4. Insert the two 1N4004 diodes, care needs to be taken to ensure the correct polarity.  Solder in place.  see Diodes.
  5. Insert the IC socket taking care to orientate the locating dot.  Solder in place.
  6. Observe the orientation of the BC 548 transistor and solder in place.
  7. The 6 PCB pins can be inserted and soldered.
  8. Insert the trimpot and solder.
  9. Care needs to ensure the LED is inserted with the correct polarity.  Solder.
  10. Place the capacitors in their locations.  The 100uF electrolytic capacitor is polarised and must be inserted in the correct orientation.  Solder.
  11. Insert the relay and solder in place.
  12. Cut the figure 8 wire into three lengths.  The ends of the wire needs to be stripped to bare the copper wire.
  13. Tin the bare ends of the wire - refer Tinning wire.
  14. The switch is attached to either 'Trigger on high and +12V' or"Trigger on low and 0V'.  Solder the switch to it's PCB pins with wire.
  15. Connect another set of wires for +12V and 0V.
  16. The third wire is used to connect to the relay terminals - Common and NC or Common and NO. 
  17. Adjust the legs of the 555 to match the socket width.
  18. Insert the IC with the locating dot as shown in the placement diagram.

Operation and Testing

  1. Connect a 12V power source.  
  2. Press the button to trigger the kit - the LED should illuminate and the relay energise.
  3. After some time both the LED and relay will go off.
  4. Use the trimpot to adjust the delay for the time required.

Technology Notes

This kit uses the 555 timer in a one shot (monostable) mode.  View the Datasheet for the 555

When the button is not depressed, the 555's trigger is held high (12V) by the 10K resistor.  When the button is pressed, the trigger goes low (0V) setting the timed circuit into action.   The 555's output (pin 3) goes high, illuminating the LED and energising the relay.  The 555's discharge (pin7) and threshold (pin6) now allow the 100uF capacitor to begin charging via the 10K resistor and 100K trimpot.  When the voltage at pins 6&7 reach 8V (two thirds of 12V) the 555 sends it's output low, the LED and relay go off.  The 100uF capacitor is discharged via pins 6&7 and the system is ready for the next trigger.

Note the diode across the relay, this is a good example of physics in reverse.  When we energise the coil of the relay the current is turned into a magnetic field and changes the state of the relay.  When we turn the relay off,the collapse of the magnetic field is turned back into electrical current by the coil in the relay.  This current is in the reverse direction as the magnetic field is collapsing.  This reverse current can damage the 555 and is bled off by the diode. 

Trouble shooting

Troubleshooting,  if necessary will involve careful checking of locations and polarity of components, in particular the diodes, the 555 and 100uF capacitor.  Re-solder all joints and check to make  sure you have  not  bridged across  between  any  two adjacent component legs.  The end  of  a  broken  hacksaw  blade  sharpened  on  an  emery wheel is a good tool for cleaning between soldered joints.

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