Circuit improves on first-event detection

Edited by Bill Travis and Anne Watson Swager

Lawrence Arendt, Oak Bluff, MN, Canada -- EDN, 8/16/2001

The circuits in Figure 1 and Figure 2 exhibit certain advantages over the circuit shown in the Design Idea in EDN, "Circuit detects first event," May 3, 2001, pg 89. The n-player first-event-detection circuit offers several improvements:

• It has fewer passive components. It needs only n diodes instead of (n²+n)/2 for three or more players. And, excluding the LEDs' current-limiting resistors, the circuit needs only n+1 resistors instead of 5n.
• The circuit uses less expensive ICs. The 74F74 or 4013 costs only 25% of the price of a LMC6762 (DigiKey catalog).
• The circuit offers inexpensive and simple scalability. You can easily add any number of additional player-event-detection channels to an event-detection configuration. All that's needed is that you connect the additional circuits to a common five-wire bus consisting of V_CC, ground, the Reset, the SwitchBus signal, and the CaptureInhibit signal. Thus, the wiring complexity is independent of n; in other words, it is O(1). Expanding the number of players for the original event-detection circuit requires additional diode-connected reset signals from each channel to all other channels, resulting in a wiring complexity that scales as (n²-2n), or O(n²).
• The improved circuit uses D flip-flops having reset and clear pins: either 74F74s for regulated 5V supplies or, with minor circuit changes, 4013s for unregulated 9V-battery supplies. For a 74F74 implementation, the D input of flip-flop FF0 connects to logic 1. The Q output of this flip-flop drives the SwitchBus signal. The Q outputs of FF! through FFn have a diode-OR connection to the CaptureInhibit signal, which clocks the clock input of FF0. All the  inputs for FF1 through FFn are connected through resistors to logic 1. Upon power-up or after you press the reset button, all the flip-flops' Q outputs are at logic 0 because of a pulse on the flip-flops'  inputs. The reset forces the SwitchBus signal to logic 0. When you press player-event switch m, the logic-0 SwitchBus signal connects to the  of the mth flip-flop, forcing Q_M to logic 1. Q_M now clocks FF0, forcing its Q output (SwitchBus) to logic 1. Because SwitchBus is now at logic 1, and applying logic 1 to the  input of a 74F74 has no effect, any further switch closures by player m or any other player now have no effect.

For a 4013 implementation, the flip-flop connections are the same as for the 74F74 circuit. Upon power-up, or after you press the reset button, all the flip-flops' Q outputs are at logic 0, because of a pulse on the flip-flops' Reset inputs. The Reset signal forces the SwitchBus signal to logic 1. When you press player-event switch m, the logic-1 SwitchBus signal connects to the Set input of the mth flip-flop, forcing Q_M to logic 1. Q_M now clocks FF0, forcing its output (SwitchBus) to logic 0. Because SwitchBus is now at logic 0, and applying a logic 0 to the Set input of a 4013 has no effect, any further switch closures by player m or any other player now have no effect.

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