Introduction: Led Instrument Light Conversion

I have an old 80’s cycle (Suzuki Madura) with some very dim light bulbs illuminating several gauges. I thought it would be fun to update the gauges by changing the backlight source to LED’s. This would allow me to change the color on some of the guages, perhaps increase the brightness, and lowering power consumption. For the speedometer and tachometer I decided to go with Blue LED’s. For the other indicator I stayed with the same colors (Red=Gas, Amber= Oil, Green=Neutral, etc.)


  • · 3 Bright LED’s (size = 5mm) (my case Bright blue 11000-mCd is brightness @ 20 mA)
  • · Single sided Circuit-board material, and something to cut it with (hack saw, shear, etc.)
  • · 120 Ohm, 1/8 watt resistor
  • · Copper-Sulfide etchant (or alternative)
  • · Tank to submerge PCB containing etchant
  • · Printer & glossy photo paper
  • · Software to print (I use express expressPCB and expressSCH for schematic capture)
  • · 2 inches of wire
  • · Soldering Iron, and solder
  • · Wire cutter and needle nose pliers
  • Rotary tool with small drill bit, and "drill press" accessory (optional but handy).

Step 1: Getting the Compatible Bulb Sizes.

I first measured the bulb size, and determined that I could get 3 LED’s to fit in that envelope (see drawing of both. My bulb is a little longer, but fit for my application. You could have made it closer to size by laying out the PCB tighter). NOTE: One thing to note up front is that the LED’s tend to output light straight up, whereas the conventional light bulb is omnidirectional. That needs to be taken into account when you decide where to use this technique.

Information on the Blue LED I used: , T-1 3/4 (5MM), 11CD, 480NM; Bulb Size:T-1 3/4

(5mm); LED Color:Blue; Luminous Intensity:11cd; Viewing Angle:15ø; Forward Current If:20mA; Forward Voltage:3.2V; LED Mounting:Through Hole; Lens Shape:Round; Color:Blue. Purchased at Newark/Element 14 Part #04R6672 (Mfg #C503B-BAN-CY0C0461) for $0.20 ea.

Step 2: The Electronics: Determining Power, Current, Brightness, Etc.

Next needed to get the brightest LED’s in that size possible, and will want to run them off the 12 Vdc of the cycle. LED’s (unlike normal lights) have polarity (i.e. a Positive and a Negative side). So instead of adding a diode bridge to this, I just determined which lead was +12, and which was ground on the harness.

Next need to determine what size resistor to put into this. In theory a LED likes a constant current, and so a resistor is not the way to go. But in this instance I decided to just go with a resistor of the correct size. To determine the size of the resistor I took the nominal voltage of 12 V, subtract from the typical forward voltage of the LED (Vf – for my LED’s it was Vf=3.2 V) * number of Diodes. That came to be 12 -3*3.2 = 2.4 Volts. That is the voltage that will be across the resistor.

Next I need to determine what current to run through the LED’s, and per the spec page for my LED’s it is <= 20 mA. So to calculate the resistance I need I take the 2.4 V divide by current, ( ohms law) to get 120 ohms.

Finally what size (power wise) will I need to safely dissipate the 20mA. Again Ohm’s law says V^2/R = Watts or I^2*R to get watts. So 0.02 *0.02 * 120 = 0.044 Watts So a 1/8 watt resistor (0.120) should do the job.

Step 3: Laying Out a Circuit Board to Hold Elements:

Now I need to measure the bulb dimensions, and come up with a circuit board to hold the LED’s, the resistor, and pads that connect to the wiring harness. To keep things simple I decided to just do a single sided circuit board, and (similar to what is done with the bulb) run a wire from the pads under the bottom and back up to create contact points for the harness.

The PCB layout file is attached. There are a lot of different methods of creating DIY printed circuit boards. I like the method of printing out the layout at 1:1 scale on my Brother LaserJet printer and transferring this to a copper clad board. I will just cover the basics of that method here…

1) Using your favorite CAD program or circuit board layout program you create the design.

2) Then you place a highly glossy paper into your LaserJet printer, and print the design (in Black/white).

3) Cut that printout to size appropriate for you circuit board.

4) Clean the surface of the printed circuit board of oils etc, so the transfer will hold onto the copper tightly.

5) Place the printout printed side down on top of the copper material.

6) Heat this paper with a hand iron, being very careful not to move the paper in relation to the pcb material. Apply heat till the ink has transferred from the paper to the copper side of the board.

7) Gently soak this combination in warm water to make it easy to remove the paper without damaging the transferred ink. Once the paper becomes “soggy” gently pull it off the pcb material, and gently rub the paper till it is fully removed from the copper.

8) NOTE: You only want to remove the paper in those area’s where it had ink. If some of the paper remains attached to the ink areas that is okay. You are not ready to etch the copper to leave the copper traces needed to create the circuit.

9) Again there are several ways to remove the copper. I like to use either the “Etchant” solution that is available at your electronics store, or create my own copper sulfide solution (again there are instructibes that describe that process,so I won’t go into the details).

10) Place the PCB with trances into the etchant and slowly agitate. Over time the etchant will change color as it dissolves the copper from the PCB. Keep your eye on the pcb as you swirl it in the etchant. As soon as the copper is removed from the exposed areas (not covered by the transfer ink) stop, remove the pcb from the solutions, and gently wash to neutralize the etching reaction.

11) Now you need to remove the ink that was transferred, to expose the remainder copper traces that you will use to attach your components to. I like to use a fine steel wool to do that. Rib gently until you are left with nice shiny copper traces.

Step 4: Populating the New PCB With Components:

1) You next will need to drill out the holes for the LED legs, the resistor, and the leads that will make up the connection to the harness. I used a variable speed dremel tool, placed in a drill press accessory. Place the board under the drill and carefully drill into the center of the pads that will hold the components legs.

2) Now place components into the board, and solder them to the pads with a soldering iron. I did the LED’s first because in addition to needing the bend the legs they will extend all the way through the board and need to get trimmed flush on the back side so they don’t interfere with the LED legs on that opposite side. Solder the single LED first. Be careful to place the LED’s into the pads correctly. Remember the Flat side of the Led goes to the “negative”(or in this case ground”, and the round side goes to the positive. If you solder one of the LED’s in backwards, none of the LED’s will light up.

3) Bend the LED legs 90 degrees (at 0.16 inches from the bottom of the LED head), solder it in place. - WATCH THAT POLARITY! Then cut the protruding legs on the opposite side of the board FLUSH with the board.

4) Similarly solder the other two LED’s in place, after bending their legs 0.23” from the bottom of the LED's head (again make sure they are position with correct polarity). Flush cut the leads on the other side of the board.

5) Place the resistor, solder it , and flush cut its leads.

6) Next; place two wires into the holes just above the large pads at the bottom, on the side of the PCB without traces. Solder the end to the pad. Then wrap the wires around the bottom of the PCB and up the other side placing the wire down the center of the pad. Solder this side to the middle of the pads. These will make it easier to have good contacts in the wiring receptacle of the wire harness.

Step 5: Test and Install

TEST: I used a simple 9-V battery to test the circuit, making sure to put the Plus lead, and negative lead on the correct pad. You should see all 3 LED’s light up. Insert this into the gauge, to see the effect. If it is too bright you can put a larger value resistor inline with the LED. If you need to spread the light out more, you might try spreading out the LED tips, or even doing a light sanding of the LED's on tip and sides to diffuse the light.


· Stop using Ferric Chloride etchant! (A better etching solution.)

· Making PCB with a LaserJet printer. Or Much more detailed explanation here.

· Yet another Methods of making PCB’s with an inJet printer

There are many different methods, discussed online you can easily find with a web search. Find one that works for you.