New to electronics... building a simple circuit.

OK so I got this FM transmitter for use in my car which is designed to hook into the main 12V line of the car. I found a 12V line under the seat that I wanted to use, and decided also to install a switch with led indicator to turn it on and off and show when it was on/off. Anywho, my current knowledge of circuits only let me to come up with a circuit that doesn't work.

Here's a pic of the circuit (I dont have a drawing prog and I madde this really fast but this is what it is.) (edit: the box thing is the transmitter)

**broken link removed**

I'm sure you're laughing because the resistor lowers the current too much for the transmitter to work but yea... I can't figure out how to do it.

(the resistor is 470 ohm and the led is a normal led im using a 3v forward voltage and it runs off 20 mA)

The possible solutions I thought of were:

-get a seperate source for the led and hook it up using a DPDT switch (but I would really prefer to do this with one circuit)

-use transistors to boost current? I don't relaly know how to calculate this or anything though.

Please help! thank you so much.

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You can't give a led 12volt's you have to use a voltage regulator to reduce the voltage.

Also you have to draw something on eagle cad or something cause that doesn't look right.

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Also do you know how to use a voltage regulator?

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Also the switch goes to the negative not the positive.

Sorry but you got that circuit all wrong.

I will post the actual schematic in a hour.

If I understand what you want it is to make the LED glow when the FM transmitter is on.

If so, then all you need to do is to connect a LED in series with a 1k resistor and connect this across the FM transmitter. I'll try to draw it

+12V-----switch-----A------FM tx----------Gnd

A----LED---Resistor---Gnd

In case you don't understand the diagram, connect the anode of the LED to point A and the right hand end of the resistor to Gnd.

By the way, the "-12V" in your diagram is ground, not -12V.

The negative plate of the battery is connected to gnd which is 0V not -12V.

Len

I was putting the picture together while the last correspondent was posting. **broken link removed**
The switch can be in either the pos or neg (0V) lead but it is usual to put it in the pos.

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Well it depend's what people learned with, I use negative and I think it is most common.

when i started typing there were only four posts in this thread.. :lol:
anyway this is what i had , and i'm sticking with it.. :lol:


actually that should have worked ! unless the transmitter requires more than .025 A ..it shoud say on the unit..??
or the unit does not work when less than 12 V is applied..
one equation to remember is V=IR..
v is voltage
I is current
R is resistance..
bet you never thought that you would have a use for algebra??
but with a simple manipulation
gives I= V/R
12V/450 ohmsR = .025A..

you have the diode positioned correctly also..

williB said:

actually that should have worked ! unless the transmitter requires more than .025 A

Click to expand...

That's an input of 300mW. Hardly realistic.

hey thanks for all the replies guys!

It's great that you guys are so willinging to help a person new to electronics and the boards too.

Anywho, I will try David Bridgen's circuit.. although I have already tried a very similar one. In mine, I had it wired the same but had the resistor in front of the LED rahter than after it. This kept the LED from lighting however as all the current went the way of the transformer and wouldn't even try going through the resitor.

WilliB i used V=IR to discover that I needed a 450 ohm resistor, I've been trying to tech myself about electronics and it's always nice to see hwo people use equations...

ljcox, what is the "A" in your quick diagram? I'm thinking it represents a break or whatever in the circuit where the led part is. sorry for the confusion... I have much to learn and though -12V meant that it was the voltage running back but now that I think about it that makes no sense.

robotic info.. I'm not sure I can use a voltage regulator because the transmitter needs 12V. unless I could break off from the 12V line and create another line with a voltage regulator in it? Sorry i'm probably incorrect.. getting overloaded here..

Lastly, there were no specifications given on the transmitter itself. However, I do know that when its current is reduced with the resistor it does not work and when it gets the 12V w/o resistance, it operates with whatever current the car battery generates.

Again, thanks so much, I will definately be surfing these boards and try to learn more.

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I way's talking about the led because to much voltage will burn out the LED but you could fix that by using a resistor.

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Roboticinfo said:

I way's talking about the led because to much voltage will burn out the LED but you could fix that by using a resistor.

Click to expand...

yea... I mentioned how I am using a 470 ohm resistor... I also mentioned how that is what is keep in gmy ciurcuit from not working...

also, I believe I'm correct in saying that a resistor cannot decrease voltage levels. As it only lowers current. The V=IR equation can prove this with the porportionality between variables or whatever.

Thanks for all the posts still!

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I know I build robot's and I am very knowledge in electronic circuit's

yea an 470 Ohm wod work.

the circuit works like a zener diode circuit the voltage over the led regulates to the LEDs forward breakdown voltage.

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Roboticinfo said:

Well it depend's what people learned with, I use negative and I think it is most common.

Click to expand...

That's rather bizzare!, switches are almost always in the positive lead, it would be VERY unusual to switch the negative for an application like this.

The basic reason is that you switch the LIVE wire, in a car this is crucial, as all the bodywork is at 0V (negative of the battery), if you switched the negative the entire unit would float to 12V and be a potential safety risk.

One exception to this is the interior light in a car, simply because it's easier to short it down to ground, rather than have to provide two wires.

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I guess your right on that, but I usually but not alway's use my switch on negative.

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Roboticinfo said:

I guess your right on that, but I usually but not alway's use my switch on negative.

Click to expand...

Perhaps you should change it then?, come in to line with the rest of the world :lol:

Things are usually done for good reasons! - and this is one of them.

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Hahaha ok, since you got a avatar I guess I could do that then.

drawagoat said:

..... although I have already tried a very similar one. I had it wired the same but had the resistor in front of the LED rahter than after it.

Click to expand...

I doesn't matter whether the resistor is before or after the l.e.d. Just get the l.e.d. the right way round.

This kept the LED from lighting however as all the current went the way of the transformer and wouldn't even try going through the resitor.

Click to expand...

The voltage is across both the resistor/l.e.d. combination and across the transmiter, and it will drive a current through both paths.

It would help you a lot if you read up about Ohm's Law and resistors in series and parallel.

hey thanks guys everything is working now. David's plan was actually my original schematic but my implementation was VERY poor rsulting in the circuit I showed you. anywho, thanks so much for helping me realize my mistake. Sorry if my newness was a pain to you! I could post some pictures of the finished product if anybody would care to see it..