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Build this low-cost quiz game adjudicator If you’ve ever wanted to risk all the prizes and go for the cash jackpot, then this is the project for you. Called the Quizmaster, it lights a LED & briefly sounds a buzzer to indicate which of four players pressed the button first. By DARREN YATES Imagine it. You’re sitting down with two other “brains” in the local Gulargumbone Sale of the Month Championships. You’ve just got to make it through the 60-seconds “fast money” and all the prizes are yours –a year’s supply of toilet paper in your choice of pastel colours, a $100 gift voucher at Spud Murphy’s second­hand farm 70  Silicon Chip machinery depot, all the icecream you can eat in a week, plus various other (mainly useless) household supplies. However, your eyes are firmly fixed on the cash jackpot which grows by $2.78 each night. The compere, in thongs and stubbies, bellows out, “Hey, Raelene! What’s tonight’s grand total cash bon­ anza?” Raelene, re­galed in the latest fashion wear from the local opportunity shop, informs the audience, most of whom are now asleep with excitement, “Tonight’s bonanza is $38.75 minus the cost of the beer. We’ll put the one minute up on the clock and your time starts ... now!” Starting out $4.23 behind your opponent, you charge through the fast money as if you knew the answers before the questions were even asked. It comes down to the last question. You have to get in first and answer correctly or you lose the lot. “What’s the name of Bullhead Jones’ prize pig?” Knowing full well that the answer is Beethoven, you thrust your arm to­wards the heavens, expecting to be awarded all the prizes. D5 1N4004 470 16VW 9V RESET S5 4 6 100k 12 14 PLAYER 1 S1 3 PLAYER 2 S2 7 PLAYER 3 S3 11 PLAYER 4 S4 15 100k 100k Q5 BC557 82k 16 100k 100 16VW 47k 10k 3-12V BUZZER E B 5 Q6 BC337 C B C S1 E 1k S2 S3 S4 R1 IC1 4043 R2 Q2 R3 Q3 Q4 R4 100k Q1 2 D1 1N914 D4 1N914 D3 1N914 D2 1N914 22k Q1 BC557 E B 9 22k 10 22k 1 22k C Q2 BC557 B C LED1 B E C VIEWED FROM BELOW A K Q3 BC557 B E C A A 8 E  K LED2 A  K Q4 BC557 B LED3 A  K E LED4 C  K 680  THE QUIZMASTER Fig.1: the circuit is based on IC1, a 4043 quad RS latch. When one of the PLAYER buttons (S1-S4) is pressed, the corresponding Q output of IC1 switches low & turns on its associated PNP driver transistor (Q1-Q4) to light one of the LEDs. Q5, Q6 & the associated 100µF capacitor are used to drive the buzzer. Your opponent, whose arm was broken by a freak and mysteri­ ous accident during the last round, raises his plastered arm a full second after your own mighty effort. However, the compere who unbelievably loses his glass­es just after reading the ques­tion, fails to see your arm rocket upwards and awards the ques­tion and all of the prizes to your opponent. And the moral of this sorry tale? – if the compere had been given a Quizmaster, this would never have happened! Circuit diagram Let’s take a look at the circuit diagram of the Quizmaster – see Fig.1. As you can see, it uses a single 4043 IC (IC1), a buzzer, and a few transistors and LEDs. Inside the 4043 are four tri-state RS flipflops. The reset pins (R1-R4) are connected to their corresponding PLAYER buttons, while the set inputs are tied together and connected to the RESET button (S5). The circuit detects which of the four PLAYER buttons is pressed first and disables the other three buttons until the RESET button is pressed. When S5 is pressed to start the game, the four set inputs are pulled high and so the Q1-Q4 outputs at pins 2, 9, 10 & 1 also go high. These outputs drive PNP transistor stages Q1-Q4 via 22kΩ current limiting resistors. Thus, when the RESET button is pressed, transistors Q1-Q4 will all be off and none of the LEDs will be lit. IC1’s Q1-Q4 outputs also drive a 4-input AND gate made up of diodes D1-D4. When all four Q outputs are high, the output of the AND gate is also high and thus Q5, Q6 and the buzzer are all off. This high is also applied to the commoned side of the four PLAYER buttons. Normally, the four reset inputs on IC1 are held low by 100kΩ pull-down resistors. However, if one of the player buttons is now pressed, the high output from the AND gate is fed into the corresponding reset input and this causes the associated Q output to go low. This low then turns on the associated PNP driver stage to light the correct LED. At the same time, the output of the diode AND gate goes low and this prevents any of the other switches The four PLAYER switches are housed in discarded 35mm film canisters. Mount each switch on the lid of its canister & feed the connecting lead out through a hole drilled in the bottom. July 1993  71 10k PARTS LIST 82k PLAYER 4 D5 PLAYER 3 9V BATTERY 470uF IC1 4043 Q6 1 D1 22k 22k 22k 22k D2 D3 D4 PLAYER 1 RESET Q1 K 100k LED1 DC BUZZER 1k Q2 K LED2 100uF Q5 47k 680  100k 100k 100k 100k PLAYER 2 Q3 K Q4 K LED3 LED4 Fig.2: the parts layout on the PC board. Be sure to use the correct transistor type at each location & take care with the orientation of polarised components. The pin connections for the transistors & LEDs are shown on Fig.1. from resetting its associated flipflop. This means that the remaining player switch­es are effectively disabled. Transistors Q5 and Q6 form a simple monostable circuit which drives the DC buzzer. It works like this. When the output of the diode AND gate switches low (ie, when one of the PLAYER buttons is pressed), PNP transistor Q5 turns on and provides base current for Q6. This turns Q6 on and so the buzzer sounds. The 100µF capacitor between Q5’s emitter and the positive supply rail now charges via the 1kΩ collector resistor. As it charges, the current through Q5 tapers off and the voltage devel­ oped across the 1kΩ resistor drops. Eventually, after about 0.2s, it drops below 0.6V and Q6 and the buzzer turn off. This means that the buzzer only gives a brief burst of sound, to indicate that one of the players has responded. The buzzer then remains off but the relevant indicating LED remains on to show which player pressed his/her button first. Pressing the RESET button now resets IC1 and turns the LED off again to rearm the circuit. Power for the Quizmaster is supplied by a 9V battery via reverse-polarity protection diode D1. The circuit draws only a few microamps of current while in reset mode, so there’s no need for a power switch. Construction All the parts for the Quizmaster, except for the five push­button switch­ es, are mounted on a PC board coded 08106931 and measuring 144 x 87mm. Fig.2 shows the parts layout on the board. Begin the board assembly by installing the 10 wire links. These should all be as straight as possible, to avoid possible shorts to other components. If necessary, you can straight­en the link wire by clamping one end in a vice and then stretching it slightly by pulling on the other end with a pair of pliers. The resistors, capacitors, diodes and semiconductors can now all be 1 PC board, code 08106931, 144 x 87mm 1 3-12V DC buzzer 1 9V PC-mount battery holder 5 PC-mount 3.5mm sockets 5 3.5mm plugs 4 rubber feet 5 plastic 35mm film canisters 5 normally-open momentary pushbutton switches 1 10-metre length of light-duty speaker cable 1 9V battery 4 self-adhesive rubber feet 2 3 x 10mm-long machine screws & nuts 3 8BA machine screws & nuts Semiconductors 1 4043 quad RS latch (IC1) 5 BC557 PNP transistors (Q1-Q5) 1 BC337 NPN transistor (Q6) 4 1N914 signal diodes (D1-D4) 1 1N4004 silicon diode (D5) Capacitors 1 470µF 16VW electrolytic 1 100µF 16VW electrolytic Resistors (1%, 0.25W) 5 100kΩ 1 10kΩ 1 82kΩ 1 1kΩ 1 47kΩ 1 680Ω 4 22kΩ mount­ ed on the board. Be sure to install the correct transistor at each location and check that the IC, transistors and capacitors are correctly oriented. The accompanying table shows the resistor colour codes but it’s also a good idea to check them on a multi­meter as some of the colours can be difficult to decipher. RESISTOR COLOUR CODE ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 5 1 1 4 1 1 1 72  Silicon Chip Value 100kΩ 82kΩ 47kΩ 22kΩ 10kΩ 1kΩ 680Ω 4-Band Code (1%) brown black yellow brown grey red orange brown yellow violet orange brown red red orange brown brown black orange brown brown black red brown blue grey brown brown 5-Band Code (1%) brown black black orange brown grey red black red brown yellow violet black red brown red red black red brown brown black black red brown brown black black brown brown blue grey black black brown corresponding player numbers. This labelling can be done using transfer letter­ing or a suitable marker pen. Testing The leads from the momentary contact pushbutton switches are terminated with 3.5mm plugs & these go to matching 3.5mm sockets on the PC board. Once these components have been mounted, install the bat­tery holder, the buzzer and the five 3.5mm sockets. The battery holder is secured to the PC board using three 8BA screws and nuts, while the buzzer is secured using two 3mm x 10mm screws and nuts, with two additional nuts used as spacers. You will have to drill two mounting holes in the PC board to suit your particular buzzer. The five pushbutton switches are housed in discarded 35mm film canisters – see photo. All you have to do is drill a hole in the lid of each canister to accept the switch, plus an exit hole in the base of the canister for the switch lead. The switch leads can each be run using two metres of light-duty speaker cable. These leads are terminated with 3.5mm mono plugs to match the sockets on the PC board. Finally, the PC board can be fitted with four rubber feet and the input sockets and LEDs labelled with their To test the unit, plug in the external switches and install a 9V battery. The circuit should now fire up in one of two ways – either with all the LEDs lit or with all the LEDs off. This may sound a bit imprecise but the initial state of the circuit will depend on the state of the flipflops inside IC1. If all the LEDs are on, check that they all go out when the RESET switch is pressed. Now check that the buzzer briefly sounds and that the appropriate LED comes on when one of the PLAYER switches is pressed. The remaining PLAYER switches should now have no affect on the circuit and the LED should remain on until the RESET switch is pressed again. If it doesn’t work, first check that all components are correctly positioned and that there are no missed solder joints or solder splashes on the copper side of the board. This done, check that pins 5 & 16 of IC1 are at +9V when the battery is installed. If one of the LEDs fails to light, check its associated driver transistor and check that the LED has been correctly ori­ented. Similarly, if the buzzer fails to sound, check the circuit around transistors Q5 and Q6. In particular, note that Q5 is a PNP type while Q6 is an NPN type so don’t get them SC mixed up. Fig.3: this is the full size pattern for the PC board. Check the etched board for track defects before mounting any of the parts. July 1993  73