Although expensive, I’m a big fan of the Brompton luggage system. Much like the fold of the bike itself, it is simple, elegant and involves the only the minimum amount of fannying about. It is this functional design which inspired me to finally revisit one of my old projects; USB charging using a bicycle dynamo. Whilst my previous attempt was rather crude (both electrically and aesthetically) the Brompton luggage system presented the opportunity to do this idea right.
You will need:
- A Brompton (or other bike which uses the Brompton luggage system, such as the Circe Helios) with luggage block and either an A, C/T or S bag luggage frame (plus bag) made after Brompton switched from their older all steel frame design to their current tubes and black plastic one.
- Switched* dynamo lights (and a dynamo)
- At least six M3 x 5 mm (approx) cheese head screws (flat tops)
- At least eight M3 washers
- At least two M3 nuts
- At least eight small (red insulation) 3 mm ring crimps
- Some double (bell) wire, or two strands to twist together (1.5 m approx)
- A 2.5 and 3 mm drill bit (and a drill)
- A set of pliers
- A flat head screwdriver of suitable size for the M3 screws
- A few square cm of thin (approx 0.5 mm) sheet metal (a bit if old drinks can might do in a pinch)
- It would also be advantageous to have a set of M3 taps, although a screw can be used for tapping if you do not have taps.
If you don’t have any of these parts, consider ordering them from Farnell (for reasons which will become apparent later).
The first task is to drill two holes on the top of the luggage block using the 2.5 mm bit as shown in the picture and then tap the holes (this can be done, with sufficient patience using one of the M3 screws if you do not have taps). Fit ring crimps (remove the insulation and crush the crimp closed with the wire inside the crimp using pliers) to a length of wire long enough to reach from these holes to the connector on the dynamo and connect the crimped ends to the luggage block with a screw and washer each. Connect the other end to the dynamo connector in parallel with your existing (switchable) dynamo lights. If you want the wiring to look tidy, fit a couple of P clips on the luggage block to route the wire.
Next comes the luggage frame. Drill two holes on the plastic part of the frame just above the cut out for the luggage block, as shown in the picture below and tap these holes as before. Make a pair of connectors from the sheet metal as shown below and drill them with the 3 mm bit. Fit these connectors to the luggage frame using a screw and a washer each in the holes tapped in the frame. The top of the luggage block does not usually make contact with the top of the cut-out on the frame, so your contacts will need to protrude downwards enough to make good contact with each of the screws on the top of the luggage block.
Congratulations, you now have a set of terminals supplying ~6V AC on your Brompton luggage. What you do with it is only limited by your imagination.
I decided to build a new USB charging circuit, based on this post. The post contains a list of all the parts you will need to build the circuit neatly on a bit of strip-board (veroboard) from the Farnell catalogue. I added a PCB USB socket from Maplin so that the 5V DC produced by could be used to charge a range of devices such as smartphones, MP3 players or GPS devices and a simple plastic enclosure to keep it all together. I drilled a pair of 3 mm holes for two screws to be used as electrical connectors and linked these connectors to those on the luggage frame using a ~400 mm section of paired wire with 3 mm ring crimps on each end. When using the referenced post, if you buy different but equivalent parts, be aware that capacitors 1 and 3 are polarised type and capacitor 2 is a non-polarised type.
The circuit on the strip-board (the layout may appear confusing due to an earlier mistake).
I added a switch to the enclosure so that the circuit can be switched off when the lights are in use. The strip-board and components can just about be stuffed into the remaining space in the enclosure, which has had a hole cut in it to allow access to the USB port. In the mark II I will probably think more carefully about the arrangement of parts and the inputs within the enclosure before I start drilling. The wire linking the assembled enclosure to the luggage frame is routed through a hole which had started to develop without any assistance from me in the bottom of the C bag’s left pocket.
The circuit charges my Android smartphone when the front wheel is spun by hand and I will update this post after I take it for an extended test ride in the near future. When running the charging circuit, simply turn off your dynamo lights. When using your dynamo lights, be sure to turn off the charging circuit. Happy charging.
* If you add additional switches, non-switched lights and a bottle dynamo could be used in place of a hub dynamo.