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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.
SW3
+6V -o_|_o----+----------+----------->>--------+----------+----->>
| | | |
LAMP1 | | LAMP2
| | SW1 | SW2 |
+--A> |o <A-- o| |
| ==| --B> | |== <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> <A-- are
connected (jump)
--B> <B-- same deal
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! |
