Adding an LED to the console to light the stem-nut “Bike Watch”

Background

I’m cruising down the highway to go out with a friend. She’s about a two hour ride away, traffic getting out of the city was heavier than I expected, and I think I’m late. Not sure if I should pull up and call her, or just keep hammering… what’s the time? I’ve got to pull up my jacket sleeve and pull my glove down a bit with my right hand, while not loosing speed or wobbling around because I’m surrounded by hurtling four-wheeled death machines who are all waiting for me to make one mistake so they can pounce. Man I wish I had a clock on my bike, but either the Triumph ones are way too expensive for a clock, or the console’s already got a tacho in it.

So when I get home I get on the ‘net and order myself a “Bike Watch” from Twisted Throttle (www.TwistedThrottle.com). I got myself the version to go on the stem nut, and the one for the America/Speedmaster is the 30mm one. So, here I am again on the highway and I’m late as usual. What’s the time? Damn, I can’t see the thing at night!

A discrete, effective & simple mod is to mount one of the funky new “super bright” LED’s in the front of the tank console aimed at the watch. It’s also one of the cheapest mods you can do to your bike, costing less than $5.

Disclaimer: This worked for me and should work for you too, but if your bike blows up, its warranty is voided, your spouse leaves you, your dog dies etc it’s not my fault.

What the hell is this LED thing anyway?

If you’re familiar with LED’s you can skip down to “What am I going to need?”.

You probably know an LED as one of those funky little coloured lights that never blow and are mounted on everything from computer cases to microwave ovens. In technical terms it’s not actually a light, but a “light emitting diode”. They come in many sizes, shapes, colours and “brightnesses”. Like all self-respecting light bulbs it has two connections, but unlike a bulb these things have a right way and a wrong way to connect them- put them in the wrong way and they’ll either simply not work, or they’ll whimper and die. Another difference is that you can’t just connect them to a power source. LED’s have a maximum safe current, but if left alone will draw as much current as you can supply them which makes them go “pop” in short order. We need another small component called a “resistor” in line with it to limit the amount of current the LED can draw. LED’s also have a lens to direct the light in one direction, and these come in various angles. If you can find one you want a fairly narrow beam, as you really don’t want to illuminate your entire handlebars.

You can use any size, colour and shape you like, but for simplicity I used a 5mm round white one. It’s being used as illumination so I chose a “super bright” one. The brightness of LED’s is measured in “mcd” (millicandella) and I chose a 7000mcd. When choosing your LED you need to take note of the “forward current” (sometimes called current draw or current drop, or simply abbreviated “If”) and the “forward voltage” (Vf) or voltage drop. Forward current will be measured in milliamps (mA or If mA) and usually is around 15 – 50mA. Forward voltage is almost always 3.2V. They’ll be mentioned in the catalogue from the electronics shop or possibly supplied with a tech sheet that occasionally (rarely) comes with the LED.

Note: these LED’s are extremely bright – DO NOT LOOK DIRECTLY INTO THEM WHEN OPERATING! Yes, they actually can damage your eye, which makes it hard to look at the clock on your bike! Another thing to be aware of is that LED’s can only take a certain degree of heat, so if you’re not confident of your soldering skills get someone who is to help.

Resistors come in various values with each having an impredance value measured in Ohms (Ώ), kiloOhms (kΏ) or MegaOhms (MΏ), a power value measured in milliWatts (mW) or Watts (W) and a material type (carbon, metal film, ceramic etc) so we have to find the right one. To this end there’s a simple formula, or if you’re feeling lazy there’s heaps of “current limiting resistor calculators” on the ‘net – but more about that later.

What am I going to need?

Obviously we’re going to need an LED.  As mentioned I used a 5mm, round, white, super-bright 7000mcd LED which has a forward current of 30mA and a forward voltage of 3.2V. We’ll also need a current limiting resistor, and we’ll work out which one below. Other things needed are a 5mm allen key to remove the console mounting screws, approximately 15cm (6”) of a fairly fine gauge red insulated wire and the same length of black, a soldering iron & solder, a small length of heatshrink tubing, and a power drill & bits. Depending on your choice of connection method you might need a pair of “scotchlock” wire connectors, or the appropriate plug to connect to the bike’s existing wiring if you can find someone who sells one. (If you can, let me know!) You’ll also need a tube of silicon, a hot glue gun, or something similar to fix the LED into position. You won’t be able to use an LED mount due to the angle.

Working out the value of the resistor.

OK this can get a little technical but stick with me. The impedance value of the resistor depends on the values of the LED. The formula for finding the impedance of the resistor is:

Vin – Vf / If = R

Where Vin is the supply voltage (13.8V in our case), Vf is forward voltage of the LED, If is forward current of the LED (measured in Amps, 30mA = 0.03A) and R is the impedance of the resistor measured in Ohms.

You can plug these values into a resistance calculator (there’s a free one at http://home.cogeco.ca/~rpaisley4/LEDcalc.html) and it’ll give you a desired resistor value of 353.333Ώ and a power rating of 0.318W. We choose the first value HIGHER available resistor value which is 360Ώ. You can safely assume that a 0.5W metal film resistor will be fine for pretty much all LED’s. I’ve listed some common values below, assuming a 13.8V supply voltage. (Yes I know your bike only has a 12V battery, but the charging system actually runs at 13.8V typically.)

15mA – 750Ώ

20mA – 560Ώ

30mA – 360Ώ

50mA – 220Ώ

Putting it together – the electronic bits

The first thing to do is get the electronic bits together. If you examine the LED you’ll find that one side of the plastic lens is flattened, and that the same side has a longer leg. This connection is called the “anode” and goes to the positive side of the battery - the shorter leg (cathode) goes to the negative. The resistor has two leads and can be connected either way. You may attached it to either side of the LED, but to make things easier we’ll use the anode (positive) leg. Cut the cathode (negative) leg of the LED to about 10mm (½”) long and solder 15cm (6”) of black insulated wire to it. Use heatshrink to insulate the solder joint. Carefully cut the anode leg of the LED and one leg of the resistor both to about 10mm (½”) long each. Carefully solder the short leg of the resistor to the anode of the LED, then cut the resistor’s other leg to the same length and solder 15cm of red insulated wire to that. Insulate both joints and resistor with heatshrink.

At this point you can test your work by connecting the red wire to the positive terminal on a 12V battery (the one in your bike would be a good one) and the black to the negative. You might want to connect it briefly the first time by brushing one of the wires against the terminal in case you’ve connected things backwards. If all is well you should have a happy little glowing flashlight, if not trace the wires back to ensure that the flat side of the LED (which should have the resistor) is connected to the positive.

The next thing we want to do is connect it to the bike. Remove the 3 allen head screws from the tank console and examine the wiring inside it. Assuming you have no tacho or clock you should find a large connector which plugs the console into the bike, two connectors with two pins in each (one male, one female), and one connector with three pins. It’s this last one you want, as this is the wiring for the standard clock. You should find that the three wires in our connector are brown (negative supply), purple (constant on 12V supply for clock) and red with a black trace (switched 12V supply for clock’s light). Poke the black wire from the LED into the connector for the brown wire, then the red wire from the led into red/black wire’s connector, turn on the bike’s ignition and the LED should power-up. If you’ve managed to source the correct plug to go into this connector it’s a simple matter of soldering the wires in the appropriate place and plugging in, if not you’ll have to decide if this is a permanent fitting, or if you want to be able to return the bike’s wiring to standard. If permanent the neatest fitting is to cut the connector off the wiring, solder the LED’s wires into the correct positions and insulate with heatshrink (insulating the unused purple wire’s end too for safety). Otherwise you should use scotchlocks, or splice solder the wires into place as you see fit. I chose scotchlocks.

Putting it together – the mechanical bits

The first trick to mounting the LED is to determine the angle at which the hole needs to be drilled through the console to aim directly at the watch. I achieved this by removing the instrument blanking plate from the console (or the tacho if fitted) and sticking a little “blutac” to its top edge, connecting up the LED and sticking it to the blutac, then adjusting it to the correct angle. Note the angle by eye then drill at the same angle. You’ll be drilling through the chrome, so if you don’t have a drill press available it’s a good idea to mark the hole with a small file or similar first, to stop the drill bit slipping. Make an initial pilot hole that’s much smaller than needed so you can adjust the angle if required. When you think you have it right put the console back in place and put the LED behind the pilot hole to get an idea of where it’s aimed, and adjust by angling the drill bit in the hole if you need. Once it’s right, drill the hole out to fit the LED with a 5.5mm drill bit. You may need to file away the top section of the hole inside the console you seat the LED into the hole deep enough. When you’re satisfied it’s a simple matter of fixing the LED into place in its hole with hot glue, silicon, or whatever adhesive you have at hand. Put it all back together and enjoy. From now on you only have yourself to blame next time you’re running late.

Some final thoughts

I used a 7000mcd LED in my installation. As you can see, next to the yellowish standard lighting it looks quite blue. It’s also a lot brighter than it needs to be, and I discovered later that it reflects a little on a windscreen. It’s probably a good idea to use a significantly dimmer LED, I’d suggest a 2000mcd would be more than enough. It might also be an idea to experiment with a yellow or orange LED to get the light colours to match. Of course you may use whichever colour and brightness you like.

If you can find one bright enough you could use a 3mm LED instead of the larger 5mm one, for a less obtrusive fitting. You can also get pinhole LED’s (normal sized body but with a 1mm extension to the lens) for a truly discrete fitting, but these may not be bright enough or have enough spread to the beam.

You don’t really need red AND black wires as electricity doesn’t really care what colour the insulation is – but it makes it SO much simpler.

Connecting the LED to the tacho wiring gives the interesting effect of flashing the LED in time with the engine’s RPM, but isn’t what we’re looking for here and could be a little distracting. It’s also probably not real good for the bike’s electronics.

Most consoles aren’t squarely mounted on their tanks, so if the LED needs to be angled sideways slightly to get it aimed right, just go with it.