Help with adding a LED to my circuit

Discussion in 'The Projects Forum' started by nchoop, Jul 1, 2015.

  1. nchoop

    Thread Starter New Member

    Jul 1, 2015
    12
    0
    Hello,

    This is my first post to the forum. I have very limited knowledge of circuits but have no problem in building circuits even if I don't understand them fully.

    First some background. I'm in the process of installing a home automation as I renovate my house.

    One of the 'Modules' that I am using is able to switch up to 8 devices / relays. See attached PDF ODO002_DS

    I've been in discussion with the makers who have advised me to use a 1K resistor instead of the 4K7 one shown in the diagram as the relay coils I want to use are rated at 75mA. I have produced a schematic (my first ever one) which is basically the same as the sample one shown in their PDF with the addition of another resistor which through my research seems to be the done thing.

    What I'm trying to achieve is to add an LED into the circuit to indicate when the relay is switched on but I'm not sure where in the circuit to add it. I did think of replacing the diode with the LED but that wouldn't then stop the back current when the relay is de-energised as the LED would have to go the other way around.

    Any advice would be appreciated.

    Thank you.

    - Neil
     
  2. Randy 7140

    Member

    Jun 17, 2015
    32
    6
    This unit you are using is a specific type of Digital controller that is most likely proprietary to Idratek Products. You have to also consider what you are going to control this with. I'm guessing that Idranet connector connects to a specific type of controller. It is a digital/logic controller, so you may not be able to achieve what your trying to do. Do you have this part figured out yet?
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
    1,246
    You're ok so far. And you are correct that replacing the snubbing diode with an LED will not work.
    If the relay *coil* really is rated at 12 V and 75 mA, then putting an LED in series with the coil also won't work. Also, the base current from the controller is too low to get any brightness, so putting an LED in series with the base won't work.

    What will? Put the LED and its current limiting resistor in parallel with the relay coil and its snubber diode. Now the +12 connects to one end of the coil, the cathode of the diode, and the anode of the LED. The cathode of the LED goes to its resistor. For a normal LED and reasonable brightness, a 1K resistor will allow about 10 mA through the LED. Use a 1/4 watt or larger resistor. The other end of the resistor, the snubber diode anode, and the bottom of the relay coil all connect to the transistor collector.

    Note - if the controller really can source only 1 mA per output, then the transistor might not be "hard saturated". You can measure this with a digital voltmeter. When the relay is on, measure the voltage from the collector to the emitter (ground). It should be down around 0.1 V.

    ak
     
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  4. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    +1
    If the relay is energized, so is the LED. Depending on the LED and how bright you want it to be, you might get away with just 5mA. This adds less than 10% to the current load of the relay coil.
     
  5. nchoop

    Thread Starter New Member

    Jul 1, 2015
    12
    0
    The unit is part of a much larger standalone wired network that is controlled by a 24/7 PC which has been in operation since 2012. I currently have over 30 of various types of modules with probably another 10 - 15 to go as an ongoing project.

    ak -

    Thanks for that. When the module arrives, together with some others I'll get the circuit set up on the breadboard and test.

    wayneh -

    I'm not too worried about the LED being really bright so that'll certainly be an option.

    - Neil
     
  6. nchoop

    Thread Starter New Member

    Jul 1, 2015
    12
    0
    ak -

    I've just amended my schematic to make sure that I understood your instructions correctly.

    - Neil
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Almost. Relay coil pin 5 does not connect to the LED. It connects directly to the collector.

    ak
     
  8. nchoop

    Thread Starter New Member

    Jul 1, 2015
    12
    0
    duh !! Even I should have spotted that one, less haste more speed as they say.

    - Neil
     
  9. hp1729

    Well-Known Member

    Nov 23, 2015
    1,939
    218
    Two options.
    Add it across the relay or add it across the input.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Old thread, TS hasn't been around for months.
     
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