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R/C Relay Driver

Discussion in 'General Electronics Discussion' started by DEEGE, Apr 23, 2017.

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  1. DEEGE

    DEEGE

    5
    1
    Apr 23, 2017
    Hi,
    I am new to the forum and hope someone can help with this circuit I have found.
    I have built and tested it with and it works, up to a point.
    The circuit is set up to trigger, with the 100k trimmer when switched from a TX spare channel.
    The circuit output works and lights the test LED.
    The problem is it will not trigger the relay...
    The transistor stage work perfectly when tested separately so I am assuming the comparator is not supplying enough output to trigger the BC 547 transistor.
    Can anyone suggest any different transistors or other components that may make it work better?
    Regards,
    DEEGE
     

    Attached Files:

  2. Chris Hill

    Chris Hill

    12
    4
    Apr 17, 2017
    If you have concerns about the op-amp's ability to drive the transistor, why not replace the bipolar transistor with a small MOSFET? A 2N7000 should work, and has the advantage that it doesn't need continuous IB to remain switched on. You would need to take the gate connection direct to pin1 of the LM358, though, for the MOSFET to have a sufficiently high driving voltage.
     
  3. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    What current does the relay draw from 5 Volts? That's quite a low value base drive resistor you have. The max collector current for the BC547 is 100 mA so pushing more current into the base than is needed serves no purpose. And you might actually damage the transistor. Please supply the specs or data sheet for the relay you are using then we can advise further.

    Thanks
    Adam
     
  4. DEEGE

    DEEGE

    5
    1
    Apr 23, 2017
     
  5. Audioguru

    Audioguru

    2,944
    655
    Sep 24, 2016
    1) The opamp has no power supply.
    2) Since the LED lights then the base-emitter of the transistor is blown up or the pins on the transistor are connected wrong.

    A LED needs at least 1.5V but the maximum base-emitter voltage of a BC547 is about 0.8V so if the transistor is good then the LED will never light up.
     
    Last edited by a moderator: Apr 24, 2017
  6. davenn

    davenn Moderator

    13,658
    1,888
    Sep 5, 2009
    and you also don't show any overall power supply for the circuit

    how about you complete the circuit and then repost it

    Dave
     
  7. DEEGE

    DEEGE

    5
    1
    Apr 23, 2017
    Hi,

    Sorry for any confusion.

    The new picture shows the set up as it connects to the R/C receiver

    R-C Relay Driver.JPG


    [Mod edited to show image in post]
     
    Last edited by a moderator: Apr 24, 2017
    davenn likes this.
  8. Bluejets

    Bluejets

    4,227
    897
    Oct 5, 2014
    Pick a different comparitor.
    When loaded, the 358 will drop it's output too low.
     
  9. DEEGE

    DEEGE

    5
    1
    Apr 23, 2017
    Hi

    Thanks for the reply.

    Have you any suggestions for a replacement op amp comparator chip to use in this

    circuit

    The input voltage from the BEC is held at 4.8v

    The op amp is a preferable circuit as it keeps the part count and thus the weight down

    Regards

    DEEGE
     
  10. Bluejets

    Bluejets

    4,227
    897
    Oct 5, 2014
    Pick a lv comparitor that has rail to rail switching.
     
  11. DEEGE

    DEEGE

    5
    1
    Apr 23, 2017
    Hi.

    I have replaced the BC 547 transistor with a 2n 7000 mosfet as suggested.

    With the output pin 1 of the comparator connected direct to the mosfet gate the circuit now works fine

    Thanks for all your replies

    Regards,

    DEEGE
     
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