reed relay to a single led trigger

[rollingbear]

Hi there I'm new to this forum and have need of some information on how to fix my problem. I am building a custom car and want to add a dash mounted external led tach light but, have the unit hidden under the dash and a hidden light. so i bought an external adjustable tach light on ebay but i want to trigger a larger external led light so i bought a 12v 24led 3rd break light that i want to install in my dash vent so when i hit my rev limit it will light up. the circuit board inside the tach light has a single led and i want to know how trigger the 12v 3rd brake light off of the led or off the circuit with a reed relay.
what i want to do is have the reed switch trigger off the red led leg and go to ground on the other side of the relay and then have 12v go through the reed to the 12v b+ side of the 3rd brake light. i hope to use the 12v in and ground coming into the board for the relay and just need to know how much voltage is coming out of the red led and if it will triger the reed switch or not.
the reed switch i was looking at is 5vdc and 250ma will this work in this circuit? if not what and how to make it work thanks

the circuit board
http://images.thesamba.com/vw/gallery/pix/971386.jpg
http://images.thesamba.com/vw/gallery/pix/971385.jpg
the 3rd brake light
http://images.thesamba.com/vw/gallery/pix/971384.jpg

[pebe]

No problem getting it to work - I can do you a circuit.
But can you give the maker and the part number of the reed relay you want to use?

[rollingbear]

hi there thanks for the help... the reed relay i bought is from radio shack it is a 275-232 relay

[pebe]

The output pins of the PIC micro are getting near to their maximum current allowed if you drive the reed relay direct. So you need a transistor buffer.

Here is the modified circuit. Remove the LED and its feed resistor. Then add the components shown on the right.

Pin 5 goes low to turn on the LED, so it will now turn on the 2N3702 instead, which will then turn on the relay. The 1N4148 diode stops voltage spikes as the relay turns off.

Components are not fussy – you can use any similar diode and PNP transistor. I cannot find the coil resistance on the Radio Shack site, but have found a similar one elsewhere, so I assume the resistance is about 250ohms. Best of luck.

[rollingbear]

hi there , wow that's way more complicated that i thought it would be. i am pretty green, but i think i understand what i have to do. i found the 2N3702 transistor and the 1N4148 diode on ebay and i have the reed relay. i think the last thing i would need( is that a 470 ohm resistor in the right bottom corner?) also can you please tell me where the vss 0v post would be on the board? i am mostly a laymen and have never taken an electronics class although i would love to so i can understand this better. i am very good at auto electrical relay circuits and stereo installs but this is way different.

[rollingbear]

pebe is there any way you could number the posts on the picture of the bottom of the board so i know what is what when i go to do it. thanks that would be so helpful if you could. thanks

[pebe]

Yes, the resistor is 470 ohms.

My circuit showed the PIC with the pin numbers marked on it.
There is a simple rule for identifying pin numbers:- Look for the notch on the top of the IC at one end. Looking at the top, the pin to the left (anticlockwise) from the notch is always Pin1. The other pins are numbered successively from that one, counting in an anticlockwise direction. That rule holds true however many pins the IC has.

I have marked the pins on your photo of the underside.

[rollingbear]

thanks buddy you are the best. i am going to order the components and may have more questions in a few weeks when i try and do it. but thanks so much for you help and input. i will have to spend a lot more time on here reading and learning...