LED Replacement in Bicycle Headlight

This Instructable shows how to replace an LED in a modern bicycle headlight. The adjective 'modern' here refers to a headlamp with integrated electricity storage in a capacitor (5.5 V, 1.0 F).

The combination of a dynamo-powered (alternating current, 6V) LED lamp (direct current) with integrated storage ensures that both the headlight and the taillight remain on, even with the bicycle halted (that is, without dynamo power). The lamp shown here remains on for about four minutes, which increases visibility and thus safety, for example when halted for traffic lights. With dynamo power the lamp shines more brightly than in the capacitor-powered mode, which can be considered as a parking-light functionality.

Localizing the problem in the broken lamp was quickly done by Victor Vejvoda who ran a Repair Café in an ISO container during the Amsterdam Fabcity Festival, a 'makerspace for urban innovators' in Summer 2016. Victor indicated that the Printed Circuit Board (PCB) inside the lamp usually is sufficiently protected electronically, but that it's often the LED itself that burns out. This was indeed the case, which is illustrated clearly in the pictures in Step 2 of this Instructable.

The process of this repair is easy and straightforward:

1. Firstly, open the lamp housing and localize the LED;
2. Then replace the LED with a spare one, and;
3. End up by closing the lamp again.

Steps 1 to 3 in this Instructable elaborate on these three actions. Step 4 highlights some lessons learned: for the greater part the repair has been successful but regrettably a minor problem remains: at high speeds (probably at higher dynamo voltage) the LED switches to capacitor-powered mode...

This Instructable was first published on 3 October 2016 under a Creative Commons Attribution license by Openproducts.org at Instructables.com. See Step 5 in this Instructable for some words on how the contents of this Instructable may be used for other purposes.

Through the Openproducts Twitter account an animated GIF was published on this Instructable (retweets are welcome, see early October 2016). Among the 30+ Instructables published by Openproducts there are some that also are bicycle-related, see the gallery here. Perhaps Openproducts' online shop is also worth a visit:https://openproducts.etsy.com.

After having opened up the lamp the electronics on the printed circuit board (PCB) looked quite intact. After removing the reflector from the lamp's front cover the LED becomes visible, and the dark color shows that the LED got busted. Looking carefully allows you to see the dark spot also when looking through the glass (without having opened up the lamp). The pictures show it all (note that the LED is facing backward, i.e. it is placed in the center of and shining at the parabolic reflector behind the cover glass of the lamp).

The next step shows how to replace the broken LED with a spare LED.

To replace the broken LED cut through the connectors and remove the insulation (here transparent synthetic material). Strip the wires, apply some tin to the thread and to the legs of the LED. Put heat shrink tubes (here colored green) on the wires and solder the connectors of the LED to the wires. Remember that an LED has polarity: the longest leg is the anode and needs positive current (so connect it to the wire that often is red or measure the voltage to be sure), the other leg is the cathode (connect it to the wire that often is black). Position the heat shrink in place and heat it up the to fix it around the wires and to make it insulate the connectors in order to avoid a short-circuit.

The next step shows how assemble the lamp again.

Put everything together again.

The next step highlights some lessons learned.

Some lessons learned during this LED bicycle headlight repair:

• First of all: how could the LED burn? This is really exceptional, usually these LED lamps have a very long lifetime. Possibly the underlying reason is that the lamp has unintentionally been connected wrongly, even without having the taillight attached (this was after an instant repair on the road). Therefore: always look up in the manual how to connect the lamp to the dynamo and where to connect the rear-light, don't just try and see (strangely enough the lamp also lights up when you connect it much differently than suggested in the manual).
• What went well in this LED replacement: simply replacing a burned LED by an arbitrary spare one (with the same color) at first sight works all right. I bought the LED in the electronics shop around the corner and I didn't have much choice. Main criterion was the size of the LED that should be identical to the original LED (5 mm, 0.2 in).
• The repair has been mostly successful but a minor problem remains: at high speeds the LED switches to capacitor-powered mode, which is quite unpractical. Reducing the speed below a certain threshold makes the lamp lightens up again (i.e. it changes back to dynamo-powered mode). This indicates that possibly the electronics inside the lamp protect against high dynamo input voltage, which may be triggered earlier because of the different properties of the replaced LED.
• I opted for the 'warm white' LED which was said to be less powerful than its bright white-blueish alternative (but it looks much nicer). Instructables Member nqtronix has published a Practical Guide to LEDs which can assist in picking the right LED.
• In case this newly placed LED burns again (at the date of publishing this Instructable it is still functioning – albeit with the above-mentioned limitation at higher speeds – after many cycling kilometers) I intend to choose a higher power yellow LED and if needed I lower the forward voltage with a small resistor. Surely it is advisable to do some voltage measuring beforehand, this information can help in the process of selecting a new LED.
• The electricity storage functionality through the capacitor still works fine after the repair. This means that it wasn't damaged when the LED burnt out.
• A word on costs: the spare LED was really cheap so I judge this repair to be very cost-efficient. The problem occurring at higher speeds is unpractical but relatively easy to solve (that is, reduce cycling speed).

The next (and final) step elaborates on the Creative Commons license under which this Instructable was published.

This Instructable is being made available through a Creative Commons Attribution (CC-BY) license. Republishing this Instructable is allowed, provided it is being attributed properly (cite the name Openproducts as an author and link to www.openproducts.org, www.instructables.com/member/openproducts, or to the original Instructable. For other arrangements send a Private Message through the Instructables member page (www.instructables.com/member/openproducts).