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Tuesday, 24 February 2009

Quiz-Show Indicator

Here's a simple, cheap circuit that will let you conduct your very own quiz shows. It has a lamp and a button for each player. When a button is pressed, it lights that player's lamp and locks out the other button until the circuit is reset.


+6V -o_|_o----+----------+----------->>--------+----------+----->>
| | | |
| | SW1 | SW2 |
+--A> |o | ==| --B> | |== ----- |o | | o| -----
SCR1 \ / | | | | \ / SCR2
\ / R3 ^ ^ R4 \ /
----- | CR1 CR2 | -----
| \ | | | | / |
| +--R2--+---+ +---+--R5--+ |
| | | |
| R1 R6 |
| | | |
GND ----------+---+----------------->>----------------+---+--->>

SW1,SW2 normally open momentary pushbuttons
SW3 normally closed momentary pushbutton
LAMP1, LAMP2 6V incandescent lamps
R1, R6 470 ohm
R2,R3,R4,R5 1 K
SCR1, SCR2 Small SCRs, not power type
CR1, CR2 1N914 diodes
+ connection
^ cathode of a diode
--A> --B>

Circuit Description

When the circuit is first powered up (or after a reset -- same thing), both SCR gates are held at ground potential by R1 and R6. Therefore, neither SCR will latch up, and both lamps will be off.

When one of SW1 or SW2 is pressed, the corresponding SCR's gate is pulled high, so the SCR latches on. Even if the switch is released, the SCR remains latched, keeping the lamp illuminated.

Diodes CR1 and CR2 ensure that only one lamp may be on at a time. Once an SCR turns on, it forces the other SCR's gate to remain at a low voltage, even if its switch is pressed.

It is probably possible to change the bulbs and the power supply to 12V with no other circuit changes, but I have only built a 6V system. The circuit does not draw current when the lamps are off, so it may be battery powered with no additional cutoff switch. I built the whole thing in a plastic shoebox.

Serving Suggestion: These are great fun in elementary school classes, and as the whole thing can be built for about $5, it's well worth letting the kids have fun while they destroy it!


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