Precedence Detector Schematic Circuit Diagram
Quiz Game Detector Operation:
The primary application of this detector is for quiz games, where it identifies the first person to press a push button through a buzzer sound and the illumination of an LED. Following this, the quizmaster can reset the circuit. In the schematic, push buttons S and S4 connect to the D inputs of the four bistables within IC2. Additionally, they link to the clock input of IC2 through OR gates ICia, ICib, and IC1. The Q outputs of the bistables then control D1-D4.
Initial State and Resetting Bistables:
Upon resetting the bistables with S5, briefly setting the CLR input low, all Q outputs become high, causing the LEDs to be off. When one of the push buttons, such as Si, is pressed, a high level emerges at the associated D input. This high level is directed to the clock input of the bistables, storing and subsequently applying the existing levels at the D inputs to the outputs D1.
Voltage Regulation and Circuit Disablement:
Given that both the switches and LEDs share a connection to the supply line through a common resistor, R4, the voltage at points beyond R4 diminishes to approximately 2 V due to the illumination of D1. The voltage across the switches, dictated by the potential divider R9-R10, amounts to only about 1 V. Consequently, pressing any switch other than S1 results in a voltage drop at the associated D input(s) and clock input, rendering the lc unresponsive. This effectively disables the circuit after the activation of one of the switches.
Buzzer Activation and Time-Controlled Deactivation:
The DC buzzer is driven by the Darlington T1. The base of T1 is linked to the LEDs’ anodes through R3 and C4. As one of the LEDs lights up, C1 conveys this low level to T1, initiating the buzzer. After approximately 0.5 s, C4 undergoes recharging via R2 and R3, leading to the deactivation of T1.
Current Variation with LED Activation:
Under normal circumstances, when the LEDs are inactive, the circuit draws a current of approximately 5 mA. However, when one of the LEDs illuminates and triggers the buzzer, the current surges to around 50 mA.
