Circuit detects first event

Kelly Flaherty, National Semiconductor, San Mateo, CA -- EDN, 5/3/2001

The circuit in Figure 1 is a 'first-event' indicator, like a game show's 'who's first to answer' detector. It indicates which of the two momentary switches, S₁ or S₂, closes first by latching the corresponding channel, IC_A or IC_B, to a high state. As either of the outputs latches high and lights its respective LED, it locks out the other channel and prevents it from triggering. The other momentary switch, S₃, resets either of the latched outputs to its initial low (LED-off) state. At the initial condition, the positive input of each comparator is approximately at 0V, because both outputs are low. The negative inputs are at V_BAT/11, as set by voltage divider R₄ and R₅. In this initial condition, assume that S₁ is momentarily closed. The positive input of IC_A becomes 5/6(V_BAT), as set by voltage divider R_2A and R_3A. Because 5/6(V_BAT) is greater than V_BAT/11, the output of IC_A goes high, and the positive input of IC_A latches its threshold to approximately V_BAT/6.

Correspondingly, the negative input of IC_B latches to approximately V_BAT, thus preventing S₂ from triggering IC_B's output high. S₃ resets (turns off) either of the active outputs by pulling the inverting inputs one diode drop below V_BAT. Both channels are then in their initial condition and ready to go again. The LMC6762 dual comparator is a good fit for this application, because it draws only 7-µA quiescent current, and it has rail-to-rail inputs and outputs. The comparator's sourcing capability allows it to easily drive an LED. Figure 2 shows how you can cascade two first-event detectors to obtain more channels.