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Four Station Quiz

Here we have another Quiz circuit using a number of CMOS ICs. Now the number of contestants can be easily increased even though the present circuit shows only four. It uses a two input Quad AND gate (74HCT08), a four input Dual AND gate (74HCT21), a dual D flip flop (CD4013), and a 555. Truth tables of the ICs are given here. If any one of the four contestants presses the switch first, LED corresponding to his station lights up and the alarm goes on. All other switches will be immediately disabled. The. circuit has to be reset by a switch at the master to start the next question.

The circuit is shown in Schematic 28. It consists of four ICs. 74HCT08 has four AND gates with two inputs each. 74HCT21 has two AND gates each with four inputs, of which only one AND gate is used in this circuit. The other can be used for expanding the number of contestants. CD4013 dual D type flip flop and two such ICs are used in the present circuit. There is one inevitable 7555 used as an astable multivibrator for alarm.

There are four push button switches, one for each contestant. Each switch is connected to one input of AND gate of 74HCT08. This input is made low by IK resistance and can be pulled up by the push switch. All the other input gates are made common and connected to the output of 74 HCT21. Outputs of 74HCT08 are individually connected to the four set inputs of flip-flops in both CD4013s. Reset pins in CD4013 are made common, pulled down by IK resistance and connected to reset switch. This switch is used to reset the circuit for the next question.

Now Q outputs of CD4013 are made common through diodes Dl, D2, D3, and D4 and connected to the reset terminal of 7555. 7555 is wired as an astable multivibrator but its reset pin is pulled low by 3.3k resistor. Four LEDs D5, D6, D7, and D8 are connected at the Q outputs of CD 4013 to indicate which switch was pressed first, Q outputs are individually connected to the four inputs of one AND gate in 74HCT21.

Let us take the case of Si contestant. As soon as he presses the switch, if he does so first, the output of his AND gate (Ul A) goes high. This makes the SET pin of CD 4013 (U3A) high. Now output Q will be high and Q will be low. LED1 connected to Q will glow.

Now when Q goes high, it also brings this reset pin high through IN4148 diode and the alarm goes on. Now Q which is low is connected to an input gate of 74HCT21 (U2A) (It has four inputs). Out put of this IC is connected to all the second input gates of 7408. Hence they will ail be pulled low. Now even if any other switches are pressed, they will not respond. There is a provision for an LED at the individual station taken from Q output to show which station has responded first. So there two sets of LEDs, one set at the master and the other at individual contestants.

All the Q outputs of both CD 4013 are connected to the reset pin of IC7555 through individual diodes (IN4148). IN4148 diodes are fast recovery version, which look like small zener diodes. Hence when any one of the Q outputs becomes high, alarm will be activated at 7555. All the reset pins of CD4013 are connected to a switch (S5) which acts as a master switch. To normalize the operations, reset should be pressed by the master. Then the quiz can start all over again. The fastest will now get the priority and all others will be disabled.

74HCT08

Pinout and truth table are given in Figure 35 and Figure 36 respectively This IC has four AND gates with two inputs each in a single packing. As you can see from the truth table, output will be low if any or both of the inputs are low. Output goes high if only both inputs are high. It is an inverse of NAND gate already discussed.

74HCT21

Pinout and truth table are given in Figure 37 and Figure 38 respectively This IC has two AND gates with four inputs each in a single packing. As you can see from the truth table, output will be low if any or all of the inputs are low. Output goes high if only all the inputs are high.

CD4013

Now you have four ICs to deal with. Vero board will suffice, but think well for the best position of ICs so that a rninirnum number of interconnections are made. Switches must be rugged to stand the excitement of quiz competitions and should stand repetitive punches. You need to take three wires from the main console to individual contestant stations. One will be the power line and another is in the input line of AND gate, another is for the indication LED at the station. To save number of wires going to the stations this LED is given from the Q dashed output of the Flip-Flop. Power is regulated 5V, unless you can procure 74C versions of the ICs. Ground all unused inputs particularly of 74 HCT21.

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