ICM7556 Monostable Touch Switch doesnt work

Discussion in 'The Projects Forum' started by cpalmer60, Feb 2, 2012.

  1. cpalmer60

    Thread Starter New Member

    Jan 31, 2010
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    I have a touch-activated circuit (5V power source) that works fine with an NE556 chip, driving the Output up for about 8 seconds (timing circuit consists of a 4.7microfarad cap and 1.5M resistor), driving an NPN transistor that powers an LED (see attached circuit). As a monostable circuit, after 8 seconds the Output drops to 0V until I ground the Trigger again. When I replace the NE556 chip with the low power CMOS ICM7556 (this circuit will run on a battery and I need to draw as little current as possible), the Output stays high for the 8 seconds, and then after a barely visible drop in voltage (Voltmeter hardly moves) Output goes high again, acting like it's an astable instead of monostable circuit.

    I understand that CMOS versions are much more sensitive, and I am wondering if the circuit is being activated by a stray signal, etc. I am currently only using one of the 556 timers, and read somewhere that maybe the open pins on the second timing circuit could be interfering with the first circuit.

    I imagine it is my lack of electronics experience (especially with CMOS devices) that is the main culprit, but any help in figuring out how to get the ICM7556 to act like a "normal" 556 would be most appreciated!
     
    Last edited: Feb 2, 2012
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I bet it is being re triggered with stray capacitance.

    when using cmos u need some decoupling. Try adding decoupling caps at the inputs first

    Wait.. u should not have the reset pin open circuited, tie it to the positive voltage and u need to have the trigger high. Leaving it open catches noise u know. Try a 10M R from trigger to 5V
     
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  3. cpalmer60

    Thread Starter New Member

    Jan 31, 2010
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    Thanks for such a fast response! So even if I tie the trigger to 5V via the 10M R I can still momentarily tie the trigger to ground to trigger another timing sequence?

    And regarding decoupling caps, would ones in the .01 uF range be about right?
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    100nf close to IC supply Pin.
     
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  5. cpalmer60

    Thread Starter New Member

    Jan 31, 2010
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    Sorry it has taken so long to respond, but I finally changed the circuit based on the feedback in this forum, and your advice on stray capacitance and adding the decoupling capacitors, along with adding the resistor (ended up using a 20M) between trigger and 5V and did the job!

    Thanks again!
     
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