Price - Stock No 266


Quizmaster is a device which uses electronics to decide who pushes their button first,  deciding who has the right to answer the question.


  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. Solder in place - see Soldering Technique.
  4. When trimming the excess resistor legs, use one to make the link as shown on the PCB Layout.  Solder the link.
  5. Insert the nine 1N4004 diodes, care needs to be taken to ensure the correct polarity.  Solder in place. 
  6. The 12 PCB pins can be inserted and soldered.
  7. Care needs to ensure the LED's are inserted with the correct polarity.  Solder.
  8. Ensure the SCR's are correctly orientated, solder in place.
  9. Solder the buzzer to its PCB pins, observe the polarity.
  10. Cut the figure 8 wire into three lengths.  The ends of the wire needs to be stripped to bare the copper wire.
  11. Tin the bare ends of the wire - refer Tinning wire.
  12. Identify the NC PB Switch.  It is normally closed (NC) and produces an open circuit when pressed.
  13. Use one of the three wires to connent this NC PB switch to its PCB pins.
  14. Attach the other PB switches to their PCB pins with the wire.
  15. Connect the battery snap.

Operation and Testing

  1. Connect a battery.
  2. With the switch in the ON position, press one button - the respective LED will illuminate.
  3. Try pressing the other buttons in turn - nothing should happen.
  4. Press the NC PB switch.  This resets the unit.
  5. Repeat the test pressing a different button first each time.

Technology Notes

This circuit uses three SCR's (Silicon Controlled Rectifier) to achieve the outcome.  An SCR is a diode which only conducts in one direction, just like a diode - note the A & K legs.  In addition it will only conduct in this direction when it has been triggered, this is done by applying a voltage to the G leg (Gate).  A SCR will continue to conduct even if the voltage to the gate is removed.  View the Datasheet for the SCR C106D1.
From the schematic diagram, the 470 R resistors tie the G leg of the SCR's to 0V, preventing  a false trigger. 

When a button is pressed, the G leg is taken high, the respective SCR conducts, this effectivly connects the A leg to 0V.  Current flows from +9V through the LED, resistor and  SCR - the LED is illuminated.

As stated before, the A leg of the SCR is taken to 0V, this has the effect of taking the G leg of the second and third SCR's low through the 1N4004 diode.  This means that even if the second or third buttons are pressed they cannot trigger their respective SCRs.

The second and third  buttons work in the same manor as the first - a totally unbiased 'fastest finger wins'.

Trouble shooting

Troubleshooting,  if necessary will involve careful checking of locations and polarity of components, in particular the diodes and the SCR. 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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