Transistor switch issue

Discussion in 'The Projects Forum' started by kbaker, Jul 18, 2013.

  1. kbaker

    Thread Starter New Member

    Jul 7, 2013
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    Hi all,
    this is my first post and I'm a newbie trying hard to learn!
    My project is a small fan activated for a certain length of time when a thermopile sensor detects a high temperature.

    I use an LM358 to amplify the thermopile output from about 0-5mV to 0-5V (which corresponds to 0-100C). If a high temperature is detected it is supposed to activate a transistor switch which triggers a monostable 555 timer circuit (it is retriggerable). The output of the 555 timer uses another transistor switch to activate the fan.
    The actual temperature is not important but its not supposed to activate at normal room temperature.

    My problem is that the monostable circuit is triggered immediately and does not turn off. I replaced the transistor with a push button switch and it works fine so I think my problem is related to this transistor. (See the attached circuit diagram).

    I've tried changing resistors R2 and R3 without effect. The NPN transistors are 2N3904 and the PNP one is a 2N2906.

    I feel like I'm missing something obvious. Any advice appreciated! Thanks.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
    1,097
    Try this circuit.
    And also why you add this PNP transistor?
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    I'd have to check the data sheet, but I don't think the PNP transistor is doing anything for you. The 555 is retriggerable and as long as the trigger input is active, the discharge pin will have a low-impedance path to ground.

    I don't think your problem is with the transistor itself. Try using your pushbutton in place of the LM358 output and see if it still works.

    If so, then the problem is most likely that you are trying to amplify a signal that is too small. The opamp has input offset voltages that are can be larger than your max thermopile voltage.

    I would search for thermopile amplifier circuits.
     
  4. LDC3

    Active Member

    Apr 27, 2013
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    Let's see.
    The LM358 has an output of about 1V at RT. The produces a base current of about 1.5μA.

    Ohh, I see the problem now. The change in the base current for the NPN occurs over several seconds. This is much longer than the RC time constant for the 10K and 100nF. Therefore it doesn't change the voltage at pin 2 on the 555. It worked with the switch since the change was faster than the RC time constant.
     
    Last edited: Jul 18, 2013
  5. ScottWang

    Moderator

    Aug 23, 2012
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    Using Jony130's circuit.
    R3 → 10k.
     
  6. kbaker

    Thread Starter New Member

    Jul 7, 2013
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    Wow thanks for the lightning fast replies!
    I understand that the PNP is required to make it retriggerable (http://www.doctronics.co.uk/555.htm#retriggering). Removing it doesn't solve the issue but strangely it make the 555 output variable and unstable. To see the output easier I replaced the fan with an LED and it appears flickery with the PNP removed.

    LDC3: Your idea about the rate of change in base current is interesting. Any suggestions on how I can change it?
     
  7. LDC3

    Active Member

    Apr 27, 2013
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    Replace the NPN with a unijunction transistor. If the components are properly selected, it should retrigger the 555 and keep the fan on longer.
     
  8. kbaker

    Thread Starter New Member

    Jul 7, 2013
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    Keeping the fan on is not the problem. It won't turn off.
     
  9. LDC3

    Active Member

    Apr 27, 2013
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    It turns off with the switch, right? Then using the original circuit with the UJT should work.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Only if the output of the thermopile amplifier is reasonable. If the opamp has an input offset voltage of 5mV, then the output of the amplifier will be what it would be at 100°C when it is actually 0°C.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I agree. An op amp with low input offset is required, with a Schmitt trigger replacing the NPN transistor. The RC differentiator is still required.

    Question: What happens if, after the monostable times out (fan turns off), the thermopile is still too hot? You're screwed.
    Where does the heat come from?
     
    Last edited: Jul 18, 2013
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