Build Your "Who's First" Game Indicator Circuit!

Description

The circuit below activates a light corresponding to the first of several buttons pressed in a "Who's First" game. Three stages are shown, but the circuit can be expanded to incorporate any number of buttons and lamps.

Three SCRs (silicon controlled rectifiers) are connected with a common cathode resistor (50 ohm) so that when any SCR conducts, the voltage on the cathodes will rise approximately 7 volts above the voltage at the junction of the 51K and 1K ohm resistors, preventing the triggering of a second SCR. When all lamps are off, and a button is pressed, the corresponding SCR is triggered due to the voltage at the divider junction exceeding the cathode voltage. Once triggered, the SCR will remain conducting until current is interrupted by the reset switch, or the power is turned off and back on.

A 50-ohm, 5-watt resistor was chosen to produce a 10-volt drop at 200 mA when a single 25-watt lamp comes on. Larger wattage lamps necessitate a lower value resistor, and conversely. For instance, to operate 60-watt lamps while maintaining a 10-volt drop, the peak current would be 60/160 = 375 mA, and the resistance would be E/I = 10/0.375 or around 27 ohms at 3.75 watts. The SCRs are "Sensitive Gate" types, triggering on approximately 200 uA, with a gate current of around 1.5 mA when the first button is pressed. The 1N914 diodes in series with the buttons’ gates are used to prevent reverse voltage on the gate when a button is pressed after an SCR is conducting. The two 51-ohm resistors will be physically larger (compared to a 1/4-watt size) and should be rated for 5 watts of power or more. Caution is advised, and it is imperative not to touch any components while the circuit is connected to the AC line.

Adding a Buzzer:

The relay shown in parallel with the 50-ohm cathode resistor can be used to briefly power a buzzer with an external circuit through the contacts. A 1000 uF capacitor causes the relay to energize for approximately one second; longer durations can be achieved with a larger capacitor.

Parts List:

Quantity - Description

Circuit diagram