This is one of several ways to make a flashing 3rd brake light assembly. I designed this to be easy to locate all the needed components and reasonably in expensive to build.

If you have any electronic experience, then the schematic is all you will need to make your light assembly. For the rest of the world, I will walk you through the details so you will understand how this device works and how it goes together.

It would  be much simpler to just get a red marker light and a 552 flasher and use this as your 3rd brake light. But that wouldn't be fun. And you wouldn't have to spend all your time adapting these little LEDs into some sort of light assembly and figuring out why it doesn't work correctly. But bucketheads like doing things a little differently! So here goes.

First, look at the schematic and get familiar with it as I walk you through starting from the top to bottom.  The first illustration you see is a view of a typical LED. You will notice that the 2 legs are different lengths. The longer one is the ANODE or, positive leg. and the short one is the CATHODE or negative leg. Because LEDs are "Light Emitting Diodes" the polarity is important. Diodes have the ability to block or pass voltage like a one-way street. So pay particular attention to how you connect the diodes.

Next, you will notice 3 rows of 9 LEDs. Each row of  LEDs is an array in series, and the rows are in parallel to one another. Because of the parallel rows, each row operates independently of each other, making in essence 3 light bulbs. The only reason for this is to produce lots of light! You can just as easily use 1 array or 50.

The next circuit you see is the flashing circuit. It consists of 1each 2N2222 transistor, 1 each flashing LED, and 1 each 10 Ohm resistor. The transistor is there to handle the load of the LED arrays. A transistor consists of 3 legs. A BASE, COLLECTOR, and EMITTER. The EMITTER leg goes to the main power, the COLLECTOR goes to the LED array(s), and the BASE connects to the flashing LED. The package the transistor comes in will illustrate which leg is which. This varies from mfg. to mfg. So pay attention to this.  The flasher LED switches the transistor on and off. Then the 10 Ohm resistor is limiting the current to the flashing LED. The flasher LED cycles at the rate of approximately 1/2 second on and 1/2 second off. This varies a little when the supply voltage changes.

Now to answer your burning question as to why you would want to go to all this trouble to make an LED 3rd brake light. The really big plus is that LEDs are small and extremely efficient. So you can put these LEDs into anything. They are already red so no lens is required. They only use 50 milliamps so they don't pull any load on your battery and do not produce heat. They also are instant on! They actually fire 1000 times faster than a conventional light bulb. And as far as brightness goes, a high intensity LED is 500 times brighter than a bulb (lumens per watt). LEDs typically never burn out, unless their voltage exceeds the limit. The down side to all of this, is that LEDs are fairly directional in emitting light and they do not tolerate over-voltage.

All of these parts are readily available through Radio Shack but you would be better off getting them from an electronics surplus store. The LEDs are about .12 each in surplus but .99 at Radio Shack. The 2N2222 is about .19 in surplus or 1.20 at Radio Shack. So find a surplus store! The flashing LED is about  .99.

NOTE on the transistor: The arrowhead is the EMITTER. Actually, any NPN transistor will work just fine. The NPN designates the polarity of the transistor. So any NPN transistor that can sink 100ma or better will work. Stay away from Power Transistors though... they require more current than the flashing LED can produce. It (power transistor) will overload the flasher. 

You can leave out the flashing circuit and directly connect the LED arrays to your brake light switch. This will give you a high intensity 3rd brake light without the flashing. 

Just so you will know...Current travels in th opposite direction of voltage. Current is what does all the work, voltage is the "push".

This is OHM's Law. Where P=Watts, I=Amps, and E=Volts

This simple law tells you everything you could possibly want to know about Watts,Amps,and volts of a circuit..    

This is OHM's Law where E=Volts, I=Amps, and R=Resistance

This law will tell you the resistance, volts, and amps of a circuit.

So, if you had a light bulb that you wanted to know how many watts of power it was then; it would be...P=I*E (the first foumula.) You need to know the VOLTS and AMPS is draws. Then multiply the VOLTS times the AMPS and the result is the WATTS! So if your bulp pulls say, 4 amps and the voltage is 12, then the WATTS=48. On the other hand, you may know the watts and want to know how many amps this bulbs pulls. So, let's say you have 100Watt bulb. Now let's see how many amps it pulls: that would be: I=P/E (the first formula) OK, here goes... 100(watts)/12(volts)=8.333...AMPS

That was easy... now let's see what the resistance of that 100 watt bulb is... That would be the (second formula) where R=E/I or, 12(volts)/I(8.333 amps)=1.44 Ohms of resistance!

OK, no more BORING formulas!!! But you will be surprized how handy they are when doing your wiring on the ol' T Bucket...

Note: I was at Auto Zone and had seen an LED 3rd brake light assembly. It was about 18" long and appeared to have 20 to 30 LED's. So I bought one to try it out. It works excellent, and cost only 18.00. This could be a better way to go especially if you are not into electronics.