Honda S90 - LED Tail Light

Plans for a new S90 tail light using LED's.

Following a couple of breakdowns from a flat battery the tail light was discovered to be drawing too much current. It was a 21w/5w unit instead of the recommended 6W/2W bulb. This specified bulb was not easy to obtain and so an LED replacement was made to fit in the existing housing.

My electronics knowledge is very poor but thankfully a friend at work was able to help with the design.









Some strip board was used to connect the LED's in an array.

Two earth connections were designed to also be stand-offs to bring the PCB forward of the original bayonet fitting. They were made from 12mm long brass bar, drilled to suit the lens fixing screws.

The two stand-offs were soft soldered to the board 68mm apart, to line up with the existing lens mounting points.

Using the lens as a template the PBC was cut to a profile which would sit inside the lens shape.









The first circuit design used the LED's wired in series to ensure the correct voltage. For example the white number plate LED's were rated at 3.2 each. So 2 of these in series on the bike's 6v battery would be about right.









Although the light worked as intended on the test battery, when connected to the bike a problem occurred.

The bike battery was actually more like 6.8v and with the engine on and revving, a peak of 8.9v was measured.

The result was that all the LED's were toast.


So a second circuit was fitted with resistors for each group of LED's. This protected the LED's OK but because the LED's were grouped, they were dim and flickered on tick-over. The brake light was also not bright enough in the daytime.



The revised circuit (shown on the right) had resistors for each LED and also had some diodes. The diodes were wired so that the brake light would include all LED's in the daytime, but at night 4 LED's would be used for the tail light and the remainder for the brake light,

This circuit design used all white LED's for a nice bright lamp. There were twice as many brake light LED's to get the contrast from the tail light.

Click to enlarge









The Resistor values were calculated as follows.

First R1 was calculated.

Typ LED Voltage = 3.2

Max sustained LED current 30mA

Max bike voltage = 8.9v

R = V/I

Where V is the voltage drop required across the resistor.

And I is the maximum LED current.

R2 = (8.9v - 3.2v) / 0.03A = 190Ω

Check power requirements of resistor.

P = I x R2

Where I is the total current for each group of LED's

P = (0.03) x 190 = 0.17w

Closest Resistor of higher value = 200Ω. 0.6Watt.

For R2 the equations were the same but the current voltage was less because of the voltage drop across the diode.

Typical diode voltage drop = 0.7v. so Supply voltage to tail lights = 8.9-0.7 =

R2 = ((8.2v - 3.2v) / 0.03A = 166.6Ω

Check power requirements of resistor.

P = I x R1

Where I is the total current for each group of LED's

P = (0.03) x 166.6 = 0.14w

Closest Resistor of higher value = 180Ω. 0.6Watt.









This image shows the complete PCB. The number plate lights were soldered in place and then bent downwards to point towards the plate.

The group of 4 LED's in the centre were for the tail light and outside groups were for the brake light.









To connect the light board to the bike wiring 2 wires were soldered between the PCB and the contacts behind the bayonet fitting.









The earth contact was achieved by fitting springs between the light lens and the PCB as shown here.

The springs would ensure that earth bushes would remain in contact with light back at all times.

















Light on test.

These two images show the contrast between the tail and brake lights, the camera was used on the same setting for both photos.

Tail light

Brake Light