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Simple help with transistors!!


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OK I will be basic and I need you to be also:
I have a fuse that has a 7.5 on it.
Electricity presumably travels through it when certain lights within the vehicle light.
I want a relay to click on when these lights come on.
I can't draw too much power or put too high of a resistance on that circuit (with the fuse) or the lights won't work porperly.
Does that make sence?
Thanks

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Ahhhh why is this soooo hard to understand!!!  I really don't know a purpose either but it's just fun.
OK so I have things such as neons under my dash.  Whenever I turn on my parking/headlights the interior illumination comes on.  Right?  The numbers on your speedo light up and everything- all cars are that way.
I want the neons to turn on too.

So then for example when i shut the car off everything will turn off for me instead of putting in a seperate switch.

This is simplified senario but it'll answer my question.

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I understand that the contacts on the switch that turns on the lights and the contacts on the lights are difficult to reach. Therefore you want to sense that lights are on by detecting current in the fuse. Right?

The fuse that you want to measure current in will have a very small voltage across it when current flows in it. The voltage will be much too small to turn on a transistor or a relay, but enough to activate the input of a FET-input opamp that can sense a voltage at its positive supply. A TL081 is suitable. The opamp can drive a low current relay or drive a transistor that can turn on a big relay for your neon lights.

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::) That's exactly what I did...on my head-feet at the headrest with a trouble light!
Here's a small problem-I went to measure the voltage and current accross the fuse and I first made sure I was right by removing it and I turn on the lights and they worked!  :o  Then I thought they controlled whatever it is that makes the needles move on the gages (just a motor now?)...Well the car wouldnt even start..just turn over...maybe i read the diagram wrong or I think it could be a security thing.  You know how new cars are.  With the chips in the keys and all.

So you guys say there would only be a very small voltage but at a high current?
I get so confused between volts, amps, and watts.  I still don't understand watts at all--just music power!!!!!
Can't wait till my "properties of electricity" class next year so it all makes sence.
Untill I find a certain wire or fuse or relay I can work with this is held up.
I could however remove the fog light relay and put in my own with new wires.  Then I already have the switch.  No foggers tho.  I could even have 2 relays and a switch to go between them!
O well thanks guys!
Better go work on my electric ignition system for the 4th of july.

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More questions!!
I figured out a better way of doing it all.  I'm using the fog light system with basically 2 relays in parallel.
I tested the coil side of the relay socket and found it to be 12.8V and it maxed out my meter on amps it blinked on 14 then was 10A and beeped.
The wires going to the switch are only 22 maybe 20 awg.
This doesnt sound right.
The button it self is a pushbutton but it does not click on/off - its momentary as far as i can tell.
Is this some sort of logic circuit or something that when the putton it pushed it tells a circuit to send to the relay and the little fog light indicator in the gage panel?
The reason i asked this was to see if I could just splice a switch in the button wires but I guess not??  Will have to go from relay (can't get to wires)

How much do you estimate a 12V SPST relay would draw on the coil side?

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I got another transistor question (not like the other stuff had much to do with them)

Here's the situation:
--Istalled a sterio system in my boat.  Speakers are in the back facing forward-makes it nice for when the boat is parked so me can have beach music.
--Just got new deck for boat with included remote.  Deck faces rear of boat-towards speakers.  Remember speakers face the beach.

I want the (IR) remote to be able to work from the beach when we're not in the boat.  I just need to bend the beam around the corner.
I want to use an ir reciever and emmitter LED.

Do I hook the positive of the reciever to a transistor and hook a 3v supply to it and have that go to a emitter?  Negs are all grounded together?
Probably not that easy huh?

Basically i want the reciever to sit up on the dash (think of a car) and the emitter to be pointing at the deck.
thanks

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The datasheet for an IR receiver IC will have its max voltage and current ratings and have a typical receiving circuit. The TSOP series have the modulation frequency in their part number so you must match what your deck uses.

the datasheet for an IR LED has its max current rating.

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