The Complete Schematic Diagram
The Power LED Indicator
Figure 2 shows the complete schematic of the quiz
box electronics. As you study the diagram, you recognize
the SCR-LED-hand switch circuitry. Only three of the 10
SCRs are shown for clarity. As mentioned before, all 10
SCR cathodes are tied together and connected to R37,
a 100 ohm resistor. To reset the ON SCR to OFF,
disconnect the 100 ohm resistor. Transistor Q8, 3.3K
(R36), 1K (R38), and the SW11 NO push button switch
perform this task. Q8 is biased ON by R38 and R36,
thus shorting one lead of the 100 ohm R37 to ground.
Depressing switch SW11 removes the base bias on Q8
causing Q8 to turn OFF. This, in effect, disconnects R37
from ground causing the SCR circuit to reset.
At the time the RESET switch is depressed, the
BUZZER sounds, announcing the start of the next
question. Follow line “C” from RESET switch SW11 to the
top of the schematic of the past diode D12 and up to the
BUZZER. If toggle switch SW12 is closed, the BUZZER
will sound. Also, when a contestant depresses his or her
hand switch, the buzzer sounds for one second to let the
Quiz Master know a hand switch has been depressed.
This is accomplished by the BUZZER circuit Q3, Q4,
The monostable (one shot) one second timer gets its
command to turn ON the BUZZER from the base of
Q5, circuit (B), through the 100K ohm (R34) resistor
connected to the SCR cathode bias voltage (1.7V) signal
at the moment a hand switch is depressed. To turn ON
the BUZZER, the Q3 collector to emitter path must be
ON. Diode D12 (circuit “C”) is blocking the Q8 base bias
from going to zero via Q3’s ON condition. Without diode
D12, the SCRs would reset immediately after a hand
switch is depressed.
To conserve power, transistors Q1 and Q2 pulse the
LED D11 power ON indicator about every second. The
1% resistors R24 and R25 voltage divider bias the base of
Q1. However, when battery voltage falls to about 3.2
volts, the voltage divider voltage to the base of Q1 will be
below 0.6 volts. This will shut off Q1 and its associated
LED D11 indicating the batteries need to be replaced.
(The batteries will last a long time if you remember to turn
the Quiz Box OFF after each quizzing session.) The idling
battery current is about 7 mA, and when a hand switch is
depressed the current jumps to about 22 mA.
The 555 10 Second Timer
The contestants are given 10 seconds (you can
change R27 to suit your rules) to answer the question
after they depress their hand switches. Line “D” samples
the 1.7 volt signal from the SCR’s cathode circuit through
a 100K ohm (R33) resistor to the base of Q7. This turns
Q7 ON causing a negative going pulse to trigger pin 2 of
the 555 chip. The output — pin 3 — goes high, turning
Q6 ON which shorts the base of Q1, thus stopping the
pulsing power circuit and LED D11. After 10 seconds,
pin 3 goes low, Q6 goes OFF, and the power light starts
blinking telling the Quiz Master the 10 seconds have
expired. Pin 4 of the 555 is the reset function. R28 and
C7 reset the timer at power ON so the power LED D11
will start functioning immediately. Otherwise, the 555
can false trigger and go into its 10 second time-out when
the power switch is turned ON. You would then go into
cardiac arrest thinking that the power was dead. Diode
D13 blocks line “C” Q8 base bias voltage from interfering
with pin 4 start-up action.
■ FIGURE 3.