First-event detector has automatic-reset function

The circuit in Figure 1 lets you indicate which game player presses a button first. Each button has a corresponding LED that indicates the pressing of the button. All other LEDs remain locked out until someone presses a reset button. When a player presses a pushbutton, the corresponding optoisolator turns on, which illuminates the appropriate indicator LED. The LED remains on after the player releases the pushbutton. The voltage at Point A pulls down to nearly 3.7V, which you determine by adding the forward voltage of the optoisolator’s internal LED, the phototransistor’s voltage, and the LED’s voltage: 1.3+0.6+1.8V=3.7V. The green LED then turns off.

Beginning at time T₁ (Figure 2), no other player can change the situation by pressing a pushbutton because switching on any other optoisolator requires a voltage exceeding 3.9V. Resistor R₁ depends on V_PS such that R₁=(V_PS–V_D1)/I_OPTOLED, where V_PS is the power-supply voltage, V_D1 is the voltage of diode D₁, and I_OPTOLED is the current of the optoisolator LED. Thus, for a 9V power supply, R₁ has a value of 1.5 kΩ. When a player presses the reset button, the player LEDs turn off, and the green LED illuminates. The voltage at Point A returns to 9.2V (time T₂ in Figure 2).

You can also add an auto-reset feature to the circuit. When Point A drops to 3.7V (time T₁ in Figure 3), the inputs at IC₁, pins 1 and 2, go low, and the output at Pin 3 goes high, charging C₁. After about 30 seconds (time T₂ in Figure 3), C₁ has enough voltage to force IC₁’s Pin 4 low. R₃ and C₁ determine the charging time. A pulse of current flows through C₂, which forces the voltage at Point A to nearly 2V. That action momentarily interrupts the current in any optoisolator LED. As a result, the circuit automatically resets, and the green LED lights. IC₁’s Pin 3 goes low, which discharges C₂ through R₂, resetting the circuit to its original state.

Click here for the illustrations:

Figure 1, Figure 2, Figure 3