I am working on a project where I use the signal from 2 sensors in order to determine whether I want to trigger a timer circuit that then propagates a signal to latch an LED. The sensors give voltage output in the range 0.25 - 4.5 V, and I use a comparator to check if these voltages are greater than reference values. The output from the comparator goes to an AND gate and if both signals are high, the timer circuit is triggered. If the signal is high for a given amount of time, the LED turns on and latches (turning off LED requires a power reset).

I have all these steps working, but I need to figure out what to do if the sensors become disconnected/there is some downstream fault (open-circuit/short-circuit). Basically, I want the timer to trigger if either sensor is faulty or if both comparator outputs are high.

I have done a bit of reading and saw some mention of using pull-up/pull-down resistors with the comparator(s) but I am very much a beginner, so I didn't really understand what these were doing/how they were being used.

If anyone has an idea as to how to use these pull-up/pull-down resistors to accomplish the task, or if there are any alternative approaches, I am all ears!

  • \$\begingroup\$ Did you use 5v logic? \$\endgroup\$
    – K H
    Jan 29, 2021 at 8:38

1 Answer 1


Wire break detection is easy with a 4-20 mA sensor.

  • \$\begingroup\$ The issue is really like this: the sensor reading can be out of range (normal range from 0.25 - 4.75 V and 0.5 V to 4.5 V, faulty otherwise). If either sensor reading is out of range, I want to essentially bypass the AND gate and trigger the shutdown/timer circuit. I don't want to use 4 comparators to check this condition. Using pull-up/pull-down resistors to make both signals artificially high so that the AND gate opens high is what I am looking to do. \$\endgroup\$
    – EENoob
    Jan 29, 2021 at 6:12
  • \$\begingroup\$ Four Op amps are a must for your application. \$\endgroup\$
    – vu2nan
    Jan 29, 2021 at 10:20

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