LATCHING_PROBE

Published:2009/7/2 23:12:00 Author:May | From:SeekIC

TTL pulse catcher can be used as logic probe for IC troubleshooting. Uses Monsanto MV5491 dual red/green LED, with 220 ohms in upper lead to +5 V supply and 100 ohms in lower +5 V lead because red and green LEDs in parallel back-to-back have different voltage requirements. SN75454 driver circuit is cross-coupled to form latch or memory element. Positive-going pulse sets latch and makes LED change color to indicate arrival of pulse. Latch must be reset manually with RESET switch. Also useful for locating intermittents such as glitches; probe can be left connected to circuit under test, to see if latch hasbeen set by unwanted signaL-K. Powell, Novel Indicator Circuit, Ham Radio, April 1977, p 60-63.



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