Tech Forum

The tiebreaker circuit could also be implemented with relays, which wouldn't require any soldering or breadboarding. A relay would be required for each contestant, with as many poles per relay as there are contestants. The schematic I provided uses four 4-pole relays. For a higher number of contestants, relays could be ganged together (wire the coils in parallel) to obtain more poles. For each relay, pushing the corresponding button activates the coil; the normally-closed contacts of the other relays in series ensure that only one coil can be activated at once. A normally-open contact of the same relay placed in parallel with the button latches the coil so that it will stay on when the button is released. A normally-closed reset button (which could also be the on/off switch) will unlatch the relays.

Low cost multi-pole relays with screw-down wire terminal sockets can be found on eBay or at electronics surplus stores. Some even have indicator lights which would eliminate the need for separate lamps. Supply voltage would be selected to match the relay coil voltage. Since relays are mechanical devices, they have a finite switching time, which is usually on the order of milliseconds. It is theoretically possible that two button presses extremely close together could cause more than one relay to latch. To test this I put together a system like I described but with only two relays and buttons instead of four. The good news is that I was unable to make both relays latch on, even after many attempts at pushing both buttons simultaneously.

This is one of the simplest quiz circuits I've come across. I've built several versions. Some as simple as the first one, and a very complex one, but still using the same concept of parallel SCRs. www.techlib.com/electronics/games.html

There is a very simple old school tie breaker indicator circuit which I have used several times, It's inexpensive and works well.

Use a small neon lamp such as an NE2, and NE51 or any of the other types available. Use as many lamps as necessary for the number of stations desired. A latching type pushbutton or toggle switch in series with each lamp located at each players position. The trick here is that all of the series lamp/switch combinations are paralleled from one power source that being the 120 VAC line with a 68k-100k 1/2 watt resistor in series with one side of the line.

The principle is simple, with all of the switches off, there is no drop across the resistor. The first lamp that is energized, fires and pulls the voltage on the bus down to the point where no other lamp can fire. Neon lamps need a high voltage (generally around 65 volts) to fire but once fired will stay illuminated on much lower voltage, lower than any other lamp can fire.

The attached circuits (Figures 1 and 2) should do the trick. Each student station is equipped with a normally-open pushbutton switch connected to the instructor’s console via a cable of suitable length, terminating at a connector.

The instructor’s console consists of a bank of LEDs, one per student; a Master Reset pushbutton switch; and a suitable number of two-pin connectors into which the student pushbutton cables are plugged.

Each student readout consists of a “D” flipflop, an AND gate, an inverter, a diode, and an LED, plus associated wires, resistors, and capacitors as shown. The LED can be red or green, of any physical shape, having a maximum continuous current rating of 20 mA. The circuit is set for about 10 mA through the LED, which is very conservative and will provide quite adequate illumination.

The CMOS 4013B contains two “D” flipflops as shown, in a 14-pin package; power pins are 14(+) and 7(-). The CMOS 4081B contains four two-input AND gates as shown, in a 14-pin package; power pins are 14(+) and 7(-). The CMOS 4049B contains six inverters as shown, in a 16-pin package; power pins are 1(+) and 8(-). Be careful of this — applying “+” power to pin 1 is unconventional, but this is the way that the package is designed.

The circuit works in the following manner:  

• Assume that the Master Reset button has been pushed on the teacher’s console, asserting the +RESET bus. This forces all “D” flops to be reset, and all LEDs to be extinguished. The voltage level on the (diode-OR’d) -INHIBIT bus applies a logic-high signal to the D inputs of all flipflops, as well as to one of the inputs of each of the AND gates.    
• The first student to press a button enables the AND gate for her/his receiver, which sets his/her “D” flipflop to the “on” state. Immediately its Q-bar output goes low, pulling down (asserting) the -INHIBIT bus, which presents a logic-low signal to the D inputs of all flipflops and disables all of the AND gates, thereby inhibiting further clocking pulses to all “D” flipflops.   

This ensures two things:

1. that subsequent button depressions by other students cannot set the “D” flipflop in their receiver circuits to the “on” state; and
2. that a second depression of the winning student’s pushbutton will not set the “D” flipflop in her/his receiver to the “off” state, thereby negating her/his vote and inadvertently opening the process to a new round of voting.

The two resistors shown relative to the output and one input of each AND gate provides hysteresis and reduces noise sensitivity. As implemented, the input signal delivered to the 12K ohm resistor must rise above eight volts or fall below 4.5 volts before a high or low output level (respectively) will be realized at the gate output (assuming that the remaining input to the AND gate is held high).

The contacts in each pushbutton are debounced with a paralleled R-C network.

The 12 VDC power can be provided by a small wallwart regulated power supply. Anything that will provide 100 mA or more will suffice.

Construction Suggestions:

1. Note that the integrated circuit (IC) package schematics show the two 4013B elements, the six 4049B elements, and the four 4081B elements, and that each IC is bypassed by a 0.01 µF disk ceramic capacitor, to be connected with the shortest possible leads across the power input terminals of the IC.
2. Pin assignments in the circuit schematic are illustrative; consult the IC package schematics for actual pin-outs for each circuit element in any particular IC.
3. The tie-breaker circuit shows two 4049B elements connected in parallel. Both elements must reside in the same IC package.
4. Diodes are 1N914 or 1N4148 signal diodes. All resistors are 1/4 watt carbon composition. With one exception, the capacitors are all disk ceramic, of any type (Z5U or better), 25 volts rating. The polarized capacitor is tantalum, and is to be connected directly across the 12V power input to the circuit board.  
5. While the circuit schematic shows only three “student receiver” circuits, there does not seem to be any reasonable limit to the number that might be included, so long as +12V, circuit ground, and the +RESET and -INHIBIT busses are extended to all.
6. The input pins of all unused circuit elements must be connected to ground.
7. The student switches should be connected to the main console using twisted-pair cable to ensure minimum noise sensitivity. Mount the R-C debounce circuit directly across the pushbutton pins.
8. Dual-in-line package (DIP)-type ICs and IC sockets are recommended.

Datasheets for the three ICs may be found at:
www.nxp.com/documents/data_sheet/HEF4013B.pdf
www.nxp.com/documents/data_sheet/HEF4049B.pdf
www.nxp.com/documents/data_sheet/HEF4081B.pdf
A 1N914 diode datasheet is located at: www.vishay.com/docs/85622/1n914.pdf
Parts are all readily accessible from Digi-Key, Mouser, Jones Associates, Newark, etc.

I have modified a circuit that I used 27 years ago, which consists of a two input NAND (cd4011n) driving a set-reset flip-flop (cd4013n). The push button switch is connected to one 4011 input and goes high when pressed. The other input of the 4011 is held high by a resistor to VCC.

When the button is pressed, Q of the 4013 goes high, lighting the LED. At the same time, the NOT-Q output goes low and locks out all 4011 inputs including the one that pressed the button. The circuit remains in this state until the reset button is pressed.