led to trigger relay

Thread Starter

chini

Joined Feb 19, 2009
8
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
 

bertus

Joined Apr 5, 2008
22,270
Hello,

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

Greetings,
Bertus
 

Thread Starter

chini

Joined Feb 19, 2009
8
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.
 

bertus

Joined Apr 5, 2008
22,270
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
 

Thread Starter

chini

Joined Feb 19, 2009
8
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
 

bertus

Joined Apr 5, 2008
22,270
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).



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
 

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stellarpower

Joined Feb 19, 2009
26
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!;)
 

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Thread Starter

chini

Joined Feb 19, 2009
8
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
 

bertus

Joined Apr 5, 2008
22,270
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
 
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stellarpower

Joined Feb 19, 2009
26
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.
 

Søren

Joined Sep 2, 2006
472
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
 

Søren

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


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.
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.
 
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