My sister and wife are in education, and the students play a "Jeopardy" like game in the classroom. I wanted to make a circuit for them to use. After seeing several threads regarding game show circuits and attempting (unsuccessfully) to simulate many of them. I decided to attempt my own.

My circuit is for four players, and once anyone had pressed his/her button, their LED will illuminate and the circuit will lockout from any other inputs.

Step 1: Parts

The parts I used are listed below. The parts list includes part numbers from Digikey and Robotshop:

Qty - [Digikey #] Description
9 - [CF18JT4K70CT-ND] RES 4.7K OHM 1/8W 5% CF AXIAL
1 - [67-1153-ND] LED 3MM YELLOW DIFF PANEL MOUNT
2 - [A31112-ND] CONN HEADER VERT 2POS .100 TIN
2 - [S390QCT-ND] RES 390 OHM 1/4W 5% CF MINI
1 - [708-1403-ND] HOLDER BATTERY 1 CELL 9V
4 - [8714K-ND] STDOFF HEX M/F 4-40 .250"L ALUM
1 - [377-1217-ND] BOX ABS 4.61X4.61X2.36" BLK
4 - [CP-3502MJ-ND] CONN AUDIO JACK 3.5MM MONO
5 - [450-1110-ND] NO PUSHBUTTONS
1 - [on hand] SPST SWITCH
4 - [751-1116-ND] LED 5MM GREEN 15MCD GAP ON GAP
4 - [A100205-ND] CONN IC SOCKET VERT 14POS TIN
1 - [568-1702-5-ND] IC GATE AND QUAD 2INPUT 14DIP (4081)
1 - [568-1703-5-ND] IC GATE AND DUAL 4INPUT 14DIP (4082)
2 - [296-2033-5-ND] IC DUAL D-TYPE FLIP-FLOP 14-DIP (4013)
4 - [on hand ] 0.1uF CERAMIC CAPACITORS
1 - [on hand] 47uF ELECTROLYTIC CAPACITOR
1 - [Robotshop.com, RB-Hit-107] 50 Feet Heavy Duty Servo Wire 22 Gauge
1 - [Robotshop.com, RB-Spa-153] Break Away Headers - Straight

Step 2: Schematic of Main Logic

Here is the schematic. Note there are four identical sub-circuits, one for each player/team. Each logic chip has a 0.1uF capacitor between Vcc and ground and there us a 47uF capacitor on the power input. (“Vcc” is the positive power supply of the circuit. This circuit is powered by a 9VDC battery, hence Vcc is 9VDC.)

On the output side, there are four NPN transistors to drive the LEDs.

Step 3: Schematic of Supporting Circuitry

The supporting circuitry/wiring is shown on the second page of the schematic.

Now, let’s discuss the supporting circuitry. A 9 volt battery holder is used to supply power to the circuit. Ground is wired directly to the circuit board. Power is wired to a SPST switch (or equivalent) and then to the circuit board. On the circuit board is a connection for a power indicator LED. The LEDs current limiting resistor is on the board

The indicator lights are on a separate board, connected by five wires to the main circuit board. The LEDs are wired common anode. That is, their connection to the positive power supply is shared through a common current limiting resistor. Note this is NOT recommended practice, but as in normal operation of this circuit only one LED is lit at a time, it is acceptable.

The pushbuttons are connected to the board via plugs and jacks and a connecting five wire cable.

Step 4: Pushbuttons

The four pushbuttons are simple normally open pushbuttons mounted in a pill box, readily available from Amazon. They are often used by crafters, such as to hold beads or other small parts

For the wires back to the project box, I purchased 50’ of servo cable wire and pulled off one wire. This gave me a flexible cable like connection.

To connect to the circuit, I used mono audio plugs and jacks.

Step 5: Printed Circuit Board

A printed circuit board was designed to hold the components. It is a two sided board, where most of the bottom is left as a ground plane. This reduces the noise in the circuit during operation.

The board layout for creating the PCB is below. There is also attached a picture of an improved layout I developed. I manually laid out the printed circuit using ExpressPCB. The first one shown is the one I actually used in the build. The second one is a picture of an improved layout, where by using slightly different pins, I was able to create an orthogonal layout, simplifying wiring. Note that in version 2, I used a 4k7 resistor SIP instead of discrete components.

I have attached the ExpressPCB files in a zip file; note that version 2 has never been built and tested, but it has been validated.

Lockout.zip
Download

Step 6: Mounting

Everything is mounted in a project box. The circuit is built on a printed circuit board that I had made from ExpressPCB.

The PCB is mounted using hex standoffs to the bottom of the project box. First, I used the PCB to mark holes on the project box for the standoffs. Then, they were loosely attached (so as to give some wiggle room when screwiing the board on top). Once the board was attached from the top, the bottom screws were tightened.

When mounting the LEDs and pushbutton jacks, masking tape was used to protect the project box. A steel ruler was used to mark a straight line and then the holes for the jacks were laid out. For the LEDs, the PCB was used to mark the spacing of the LED holes. A small hole was drilled first, before the larger bit was used.

There are two switches on the side in addition to four pushbuttons on wire tethers. One is an on/off switch. The other is a pushbutton to reset the circuit for the next question. Also on the side is an LED used to indicate that the box is powered on.

Everything is wired to the PCB using 0.1" headers. I had some matching headers salvaged from old desktop PCs which were for two pins. These were used to connect the Power indicator, Power switch and Reset button.

For the jacks and LEDs, I used 6" servo cables cut in half. Then, each end was super glued together, making a 6 conductor cable. One of the pins is not used, to ensure the header is plugged in the correct direction.

Step 7: Operation

The heart of the circuit is an AND gate, connecting the pushbutton to the clock input of a D type flip flop. The flip flop is wired as a monostable latch, meaning that once it goes true, only a reset can change it back. The Set input of the flip flop is tied to ground, as it is not used. The Data input is tied to Vcc (+9vdc).

When the flip flop is reset, Q is set to 0; Q’ is set to ~9vdc (high output).

I've attached some pictures showing how the circuit operates when there are just two inputs. Blue lines represent a low value; red lines indicate a high level. The first picture shows the circuit state at the beginning, reset, ready to go.

When there is a rising edge on the clock input (the pushbutton is pressed), The Q output goes high. Hence Q’ goes low. The way in which the inputs are locked out is that the Q’ outputs are fed to an AND gate whose output feeds back to the second input of the AND gate between the pushbutton and the flip flop.

In other words, the pushbutton is pressed, sending a high signal (rising edge) to the AND gate, where the first press is passed on the flip flop. This cause the flip flop to change states, the Q output driving the LED and the Q’ output goes low, causing the next (quad input) AND gate to go low, causing all of the input AND gates to output low and which ignores any further button presses.

Note that the pushbutton inputs are not debounced. That is because they are input into a monostable latch, and hence as soon as one edge is produced, the circuit operates as expected. Bounce is irrelevant.

Step 8: Summary

This is a simple digital circuit for a game show. It can be used for a party trivia game or in the classroom for some entertaining learning. This is but one of many solutions to this application which can be found online.

In the future, I’d like to make it wireless, using a four channel remote control.