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Would this circuit work?

Discussion in 'General Electronics Discussion' started by MyTechtime, Apr 30, 2015.

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  1. MyTechtime


    May 19, 2014
    This is a schematic for a smoke detector interface board. It would take the signal from the 2-wire smoke detector (draws small amount of power when in standby, current draw goes up when tripped), and convert it to an on-off signal. I'm just wondering if this would be able to detect that at all.[​IMG]
    The relay at the beginning is for the trouble circuit. The transistor would detect a resistor, installed across the detector and activate the relay.
  2. davenn

    davenn Moderator

    Sep 5, 2009
    hi there

    your layout and description doesn't really make a lot of sense
    I'm still trying to figure out what you goal is ??

  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    This will not work. The input of the opamp has a high impedance. There is no way the current through the smoke detector can flow. Here's a basic setup:

    R3 is a placeholder for the smoke detector. Positive supply is the top wire (sorry, I forgot to label it). Currrent through the smoke detector flows from 12V through R2 to ground. the voltage across R2 is proportional to the current, hence it will be low in idle mode and high when the smoke detector is triggered. You can set the voltage level by varying the value of R2.
    R5 and R6 are a voltage divider to create a reference voltage for the comparator. This reference voltage should be approx. midway between the inactive and active voltage level across R2.
    R1 and R4 form a positive feedback to give the comparator a hysteresis. Look up how this works here.

    The values for the various resistors depend on the smoke detector's characteristic and the (in)sensitivity you want to achieve. Note that a few capacitors in the right places (power supply, across R2, across R5) will stabilize the circuit against noise. The opamp you use needs to be chosen carefully such that its characteristics match the expected input and output voltages (typically for a single power suppl without negative voltage you'd use a rail-to-rail opamp).

    Attached Files:

  4. MyTechtime


    May 19, 2014
    Would that circuit be able to detect a trouble?
    I intend for the circuit to do a few things:
    1. Detect if an End of line resistor is present. It is wired across the two terminals of the smoke detector. This would activate the trouble relay.
    2. Detect the current draw increase of the smoke detector when it trips into alarm. This would activate the alarm relay.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    This resistor will increase the idle current. and thus trigger the circuit, provided the current is large enough.

    That's what this circuit is intended for.

    This circuit will trigger only one relay, not two different relays,
    If you need to supervise three conditions (idle,idle with parallel resistor, active), you will need aso called window comparator.
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