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How to detect when two signals have been above a certain level for a certain time

  1. Jan 14, 2013 #1

    I have two signals, I need a way of testing whether they go above certain threshold voltages for a period of 0.5 seconds. This must be non programmable. I am a bit stumped o how to do it.

    I thought of having these signals go into an AND gate and then have that AND gate feed into a timing circuit somehow, but I don't really know how I would design such a timing circuit.

    The 555 timer will not work because I need to start the timer, then have it wait, then send a logical high if and when the two signals have been above their threshold voltages.
  2. jcsd
  3. Jan 15, 2013 #2
    I have not given much thought yet, but the entry could be a pair of op-amps, to detect the above or under threshold, a 555 as a monostable: the above thresh triggers the impulse, the below thresh resets it. A second 555 is started at the same time and after 0.5s, both results are AND'd. If the first one was stopped, the result would be "0".

    I will look into this later today if someone else hasn't given a schematic.
  4. Jan 15, 2013 #3


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    welcome to PF :)

    it mite really help if you told us all what sort of signals they are ??
    give us all the information on what you are trying to do
    its pretty impossible for people to give you a good answer without the background

    are they audio ? RF ? maybe digital signals

  5. Jan 15, 2013 #4


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    maybe DTMF.
  6. Jan 25, 2013 #5
    Would have thought the 555 ideal. Feed an input trigger into the timer to start it off. If output is high and input drops reset timer. If timer times out the falling edge of the output clocks the high input signal into an edge-triggered flip-flop ('74). The flip-flop output will go high only if the timer times out. You can turn this f/f output level into a pulse using another 555 that resets the f/f. For a very fast pulse you might even be able to have the f/f reset itself. Might be a starting point.
  7. Jan 25, 2013 #6
    Might try a 4 bit counter with a 32 Hz oscillator. By running your anded signal into the clear pin of a 74HC193, you could hold its count at 0 until your condition was met. Then, the counter would wait 16 counts before dropping the carry output. This would be .5 to .53 seconds later.
    Another method would be to use an NE555 as a simple level detector. Tie the trigger and threshold lines together with a timing capacitor and a pull up resistor to Vcc. NAND your two signals and use the result to drive an NPN transistor to discharge the capacitor. When your condition is met, the NPN transistor will turn off and allow the capcitor to charge. Upon reaching 2/3's of the supply voltage, the output of the 555 will go high.
  8. Jan 25, 2013 #7
    Look into Schmitt trigger pulse generator circuits. Theres plenty of examples to generate your pulse. Have one gate to detect the leading edge. One gate to detect falling edge. Latch those two momentary signal and compare later.

    I' m on my phone atm but can assist in the rising edge/ falling edge method. There are other ways as well. Depends on your needs.

    Edit. I re read the above forgot to say delay the lead edge gate output to an RC Delay of 500 ms and use that to time the slowed down lead edge with the falling edge with another AND gate
    Last edited: Jan 25, 2013
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