Precedence detector

The detector was designed primarily for use with quiz games. It indicates who has first pressed a push button by the sounding of a buzzer and the lighting of an LED. The quizmaster can then reset the circuit.

In the diagram, the four push buttons, S,-s4, are connected to the D inputs of four bistables contained in IC2. They are also connected to the clock input of IC2 via OR gates ICia, ICib, and IC1, The Q outputs of the bistables drive D1-D4.

After the bistables have been reset with S5 (which briefly makes the CLR input low), all Q outputs are high, so that the LEDs are out. If one of the push-buttons say Si, is pressed.’-a high level ensues at the associated D input. This high level is applied to the clock input of bistables, whereupon the existing levels at the D inputs of the bistables are stored and applied to the outputs D1 will then light.

Since the switches and the LEDs are connected to the supply line via a common resistor, R4, the voltage at points after R4 Will drop to about 2 V owing to the lighting of D1. Because of potential divider R9-Rio, the voltage across the switches will be only about 1 V. If then one of the switches, other than Si, is pressed, the voltage at the associated D input(s), as well as at the clock input, will be too low for the lc to react. In this way, the circuit is disabled after one of the switches has been pressed. Darlington Ti drives the d.c. buzzer. The base of T1 is connected to the anodes of the LEDs via R3 and C4. At the instant one of the LEDs lights. C1 passes this low level to T1, which then energizes the buzzer. After about 0.5 s, C4 is recharged via R2 and R3, SO that T1 is switched off.

The circuit draws a current of about 5 mA when the LEDs are out. When one of the LEDs lights and the buzzer sounds, the current rises to some 50 mA.

Precedence detector Schematic diagram

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