Design Con 2015

Circuit detects first event

-May 03, 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, ICA or ICB, 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 VBAT/11, as set by voltage divider R4 and R5. In this initial condition, assume that S1 is momentarily closed. The positive input of ICA becomes 5/6(VBAT), as set by voltage divider R2A and R3A. Because 5/6(VBAT) is greater than VBAT/11, the output of ICA goes high, and the positive input of ICA latches its threshold to approximately VBAT/6.

Correspondingly, the negative input of ICB latches to approximately VBAT, thus preventing S2 from triggering ICB's output high. S3 resets (turns off) either of the active outputs by pulling the inverting inputs one diode drop below VBAT. 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.

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