led to trigger relay

Discussion in 'General Electronics Chat' started by chini, Feb 19, 2009.

  1. chini

    Thread Starter New Member

    Feb 19, 2009
    8
    0
    hey

    im trying to get a led to trigger a 12v auto relay (not goin to happen that easy:))
    i couldnt find any 2v relay on the market so after a bit of research i found that you can use a transistor to trigger the relay. the funny thing is i wouldnt have a clue on how to do this and dont no which transistor and resistor to use so if anyone could help me it would greatly be appreciated.
    the relay is a bosch 12v 30a and to trigger it is a led i think around 1.8v
    thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    Can you make a drawing of your idea ?
    How do you want the led to trigger the relays ?

    Greetings,
    Bertus
     
  3. chini

    Thread Starter New Member

    Feb 19, 2009
    8
    0
    ill make a drawing if u want but its for a shiftlight to set off a relay
    when the tacho hits a certain rpm the led shiftlight will go on, i want that to trigger a relay which will have the negitive side of the coil attached to it and so when the light turns on the relay triggers and sends the coil to ground making it like a rev limiter.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    The led must be driven from some device (transistor or IC).
    Probably there is also a resistor in series with the led.
    Do you have acces to the led (and resistor) ?

    Greetings,
    Bertus
     
  5. chini

    Thread Starter New Member

    Feb 19, 2009
    8
    0
    yer i have access its just im limited to wat i can do because i dont no wat im doing if u want i can get a photo of the circuit board
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    Can you make a photo of the component side and the coppertrack side ?

    Greetings,
    Bertus
     
  7. chini

    Thread Starter New Member

    Feb 19, 2009
    8
    0
    yer but ill have to wait till tommorro cause its like 10 at night here in aus
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Ok, I will wait and see.
     
  9. chini

    Thread Starter New Member

    Feb 19, 2009
    8
    0
    [​IMG]
    here is wat i want to do i think
     
  10. chini

    Thread Starter New Member

    Feb 19, 2009
    8
    0
    here are the pics of the board

    [​IMG]
    [​IMG]
     
  11. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    When you follow the trace (red) from the led to the resistor, you could try to measure the voltage after the resistor (points 1 and 2).

    [​IMG]

    Do you know if the led is bound to the + or - side ?
    I think we still need some transistors to drive the relays.

    Greetings,
    Bertus
     
    Last edited: Feb 20, 2009
  12. stellarpower

    Member

    Feb 19, 2009
    28
    0
    Here is a little demo in transistors (powerpoint in a zip folder. If you can't open it, reply, and I'll change it). I'm not sure what the calculations for the values are, but you can find them out here or elsewhere on the web. If you already know this, and/or I misunderstood this, I'm sorry, I've got it wrong. Also, I apologise for the bad layout in the slide show, but I didn't have enough time to make it any better.

    P.S. 30 amps for the relay doesn't sound right at all. Can you double check that?

    Have Fun!;)
     
  13. chini

    Thread Starter New Member

    Feb 19, 2009
    8
    0
    bertus ill measure if for u as soon as possible
    stellarpower i was just wondering why would i have to split the power at the emitter to a led cant i just run it straight to the relay and i think it is 30amps because it has 12v 30a on the casing of it.
    thanks guys for all your help
     
  14. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    The 30 A is for the contacts, the currect of the coil will be lower 1 - 2 A perhaps.
    I will try to find some data.
    I overestimated the relays coil, the coil resistance is between 60 and 100 Ohms.
    The current will be between 120 and 200 mA.

    Greetings,
    Bertus
     
    Last edited: Feb 21, 2009
  15. stellarpower

    Member

    Feb 19, 2009
    28
    0
    If you still want the LED to light, it's best to put the LED after the transistor. If it went before, there would be less power going to the base, and the amplification would be relatively small. However, if you put the LED after, you have all the 1.8 volts ish going to the transistor, and giving a lot more power at the emitter. Then, split it in parallel to go to the relay and the LED, and use a resistor to stop the LED hogging all of the power.
     
  16. chini

    Thread Starter New Member

    Feb 19, 2009
    8
    0
    hey thnks guys for all your help
    now i was wondering wat size tranistor and resisitor would i need
    thanks
     
  17. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    You have to measure if the side of the LED that doesn't go to a resistor is going to ground or to B+ (or something else, which is unlikely).

    When you have done that, use the relevant circuit from this file
     
  18. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    The amplification has to be higher if less current is available, but it is not determined by the input current.


    If the LED is not in circuit, the output driving it will be either 5V, 12V or whatever. If taken over the LED, after the resistor, you know the design criteria, which is the 1.8V
    You only need 1 mA or less to drive the transistor - hardly noticeable on the LED, if at all.
     
Loading...