Nuts and Volts - August 2009

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, and Q5.

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.