The Bus on the Wheels goes Round and Round…
People who work with electricity will often refer to something as a bus. This derives from the term bus bar, for a section of copper that carries high currents. The term bus is also used in computing, where it means the part of a computer that shuttles information around—we often refer to a 16-bit or 32-bit bus.
The root is omnibus, for something shared or providing for many, and for my purposes in this entry, bus means just that. (We could similarly say that several electrical outlets on a wall of your house form a bus.)
As part of my dynamo comparison program (see also here) I realized that I would need to build a bus on my bicycle.
The reason is that I want to be able to test four different kind of dynamos. Two of those dynamos mount near the rear rim, one mounts behind the bottom bracket, and one is in the front hub. Each dynamo has to be connected to a headlight and a taillight and, potentially, to a USB charger (to enable recharging a phone while riding). That’s a lot of connections. Further, one of the dynamos requires installation of a thermistor between the dynamo and the lighting system.
Since I didn’t want to spend a lot of time rewiring the bike every time I did a comparison, I decided to install a common bus on the bike. My plan was to build junction boxes at the front and back of the bike, and provide jacks (connectors) on these boxes through which the various devices could be connected. I ended up with this circuit:
To make it easier to read the diagram, every connection J1 through J5 is assumed to have a “ground” connection, even though it isn’t drawn in. In fact, every connection does. I used standard RCA plugs and jacks:
Each connection supplies power (through the central pin) and ground (through the shell). Consequently, my lighting system need not depend on successful grounding through dropouts, headset bearings, or any other primarily “mechanical” system.
I used small plastic boxes for the junctions (and in the rear, to contain and protect the thermistor—the one supplied with the dyno is a halogen bulb, and I get nervous about the possibility of damaging anything made of glass, so….). The front box is bolted to a spare light fitting on the front rack, providing easy connection to a hub dynamo (if and when the bike is so equipped) and a headlight. The rear box is mounted on the rear rack, providing access to any rear-mounted dynamo and/or taillight. The wire between the two boxes is taped to the underside of the top tube with helicopter tape.
One nice feature, and a good reason to have a bus like this even when you’re not testing dynamos, is that when a rear dyno is used, there is a spare connector in front that can be wired to the USB charger. Similarly, when a front dyno is used, there is an extra connector in back.
Finally, there is a secondary connection to the bus through the thermistor for those rear dynamos that require its use.
My next step is to get the dynamos aboard the bus!
junction boxes. First, you’ll notice that the rear box is very slightly melted (you try working in a tiny plastic box with a soldering iron!). Second, sorry about the photo quality. It was pretty late.
In the top photo, you see the front junction box with the Edelux cable wrapped around the front rack stay and plugged in, and one open socket. In the lower, you see the cable from the bottom bracket dynamo on the left, and the cable to the taillight on the right The special thermistor input (J4) for the Velogical unit is on the other side of the box. The box itself is mounted on the rack stabilizer stay.