controlling 12 volts with 3.5 volts

Discussion in 'The Projects Forum' started by trickyrick, May 7, 2014.

  1. trickyrick

    Thread Starter New Member

    Sep 18, 2013
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    Hi guys i have a motion sensor runs on 12v dc and has an output of 3.5 volts I want to use that 3.5 volts output to operate a small 12 volt relay. I tried a SCR applying the 3.5 volts to the gate of the SCR but that didn't work.
    Any suggestions
    Thanks Rick
     
  2. #12

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    Nov 30, 2010
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    2n7000 'Fetlington'
    Besides turning on a SCR with DC supply requires some way of turning it off?
    Max.
     
    #12 likes this.
  4. #12

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    I often seem to forget how good those are at low voltages.:(
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
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    From another thread.
    Is 12vdc available to power relay?

    Sub. relay for the meter shown?
     
  6. trickyrick

    Thread Starter New Member

    Sep 18, 2013
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    Thanks for the replys guys
    this is the info on the relay
    Nominal Voltage (VDC): 12V
    Coil Resistance (Ω) (± 10%): 400Ω
    Power Consumption (W): 0.36 W
    Nominal Current (mA) (± 10%): 30 mA
    This is the info on the motion sensor (HC-SR501 PIR Motion Sensor)
    Voltage: 5V – 20V
    Power Consumption: 65mA
    TTL output: 3.3V, 0V

    or the web site http://www.electrodragon.com/product/pir/
     
  7. #12

    Expert

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    R1 in the drawing by inwo will be 900 ohms if you don't choose the mosfet or a 3.3 volt relay.

    The nearest common value is 910 ohms.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    Corrected.....................................................
    As per #12.
     
  9. #12

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    A bit more information. Relays need a diode to stop them kicking the brains out of the transistor. Use this drawing except the resistor is 910 ohms or use the mosfet with no resistor at all.
     
  10. trickyrick

    Thread Starter New Member

    Sep 18, 2013
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    Thanks guys
    I only had a 2N3819 so I ordered the N23904 on Ebay will try the circuit as soon as it comes in
    Rick
     
  11. trickyrick

    Thread Starter New Member

    Sep 18, 2013
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    ok got the item and wired it up last night works great thanks for the info
    I have a question though.
    I understand somewhat how the transistor works and the diode but I'm not to sure y the diode is needed
    Rick
     
  12. #12

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    Read post #9.
    Relays need a diode to stop them kicking the brains out of the transistor.
     
  13. trickyrick

    Thread Starter New Member

    Sep 18, 2013
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    Yes I read that but im not exactly sure what that means.
     
  14. MaxHeadRoom

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  15. #12

    Expert

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    OK. I'll break it down.
    When the transistor is conducting, the current through the relay coil makes a magnetic field. There is energy stored in that magnetic field. When you turn the transistor off, the magnetic field keeps current flowing in the same direction it was flowing a microsecond ago, and it has to go somewhere or it will develop a fierce voltage if it can't find a current path. That voltage will punch through the transistor if you don't give it an escape route. The diode is the escape route. The persistent current produces a voltage higher than the power supply voltage and the diode just lets it run around in a circle through the relay coil until it gets used up warming the resistance of the coil.
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    With the reverse/opposite polarity across the coil.;)
    (as the link shows)
    Max.
     
  17. trickyrick

    Thread Starter New Member

    Sep 18, 2013
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    Ok thanks guys that makes sense

    Thanks for explaining
     
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