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Strange phenomenal?

Discussion in 'Electronic Design' started by [email protected], Apr 17, 2007.

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  1. Guest

    Hi Guys,

    Recently I constructed a circuit using a voltage comparator, relay,
    transistor, some diodes and resistors. The output of this circuit is a
    LED. By having a specific range of input, the voltage comparator will
    enable the LED to be lit. LED is lit thru' the energizing of the
    relay. Theorectically, all calculations made are correct and the
    circuit is properly soldered. However, the LED will not light up
    unless I TOUCH certain parts of the circuit (with my fingers or any
    metallic objects). What can probably cause such an occurance?

  2. Tam/WB2TT

    Tam/WB2TT Guest

    My guess is that either:
    1. Circuit is oscillating. Add a few .1 uF bypass capacitors.
    2. Bad connection.
    3. You left unused CMOS inputs floating. Connect ALL unused inputs to GND or

  3. Guest

    Tks. I resoldered everything and tied the floating pins.

    The circuit I built is meant to measure resistance. It works perfectly
    for various loads(resistors values) but when i attempt to measure the
    resistance of some position sensors(Rx) it does not work. I measured
    the resistance of the Rx with a multimeter and replace that load with
    a resistor of identical value. Again the circuit works for resistors
    but not the position sensor circuit. Thinking it might be due to the
    input voltage, I reduced it to 2.7V but still it doesn't work.
  4. If what you are saying is that it doesn't work when you use long leads, then
    it is probably some kind of oscillation. Try putting a .1uF cap across the
    leads like Tam said. Can you show us a schematic?
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