Our Pedal-Powered Office, Version 2

I wrote a post a few years ago describing the pedal-powered office we use to run our small business. I've made some signficant changes to the bike desk at the heart of that office since then, so I thought it was time to update that post and show what I'm using now.

Desk Frame

The desk is constructed mostly of annodized rectangular aluminum tubing. The desk moves forward and backward to make it easy to get on and off, and can be raised or lowered depending on the height of the user.

the desk pivoted forward

The seat can also be raised or lowered to fit the user via a quick-release lever.

adjusting the seat on the pedal-powered computer desk

I sewed the seat cushion using (what else?) an old foot-powered sewing machine I have. The back seat cushion has a mesh back to help keep me a bit cooler and is shaped to provide lower back support.

seat and back cushion

The frame folds to save space when it's not being used and to make it easier to transport. Wheels on the back end make it easy to roll from place to place when folded.

computer desk folded for transport

Generator

The generator is a Bofeili 250 W mid-drive brushless electric bike motor that I modified slightly to turn it into a three-phase AC generator.

pedal generator on computer desk

I really like this generator; it makes for a much cleaner, more compact, and maintenance-free system than the chain-driven generator I used before. The AC from the generator is converted to DC using 6 high-current Schottky diodes mounted on a PC board (more about this later).

Battery

A 12 V, 10 amp-hour lithium iron oxide battery mounted on the rear leg of the generator serves two purposes. First, it serves as a necessary buffer between the cyclical output of the generator and the fluctuating power requirements of whatever equipment I'm powering. Second, I use it as a short-term energy source to recharge phones or power our office router when I'm not pedalling.

LiFeO4 battery used on computer desk

I used to use a 12 V, 18 amp-hour battery, but this was heavy and made the desk more difficult to move. The LiFeO battery, although more expensive, weighs substantially less and makes the desk much easier to lift. It's lower capacity is not signficant, as I rarely drain the battery more than a few amp-hours anyway.

Power Sockets

The desk has four 12 V DC power sockets built into the front leg of the desk. These are used to supply power to my computer, monitor, and whatever else equipment you want or need to power.

power sockets and control box on front of computer desk

If the equipment to be powered is normally plugged into a wall outlet (e.g., a laptop power supply), you plug a 12 VDC-to-110 VAC automotive inverter into one of the power sockets and plug the equipment into the outlet on the inverter.

If it is powered via a USB cable, you plug a 12 VDC-to-5 VDC USB adapter into a power socket and plug the phone(s) or tablet(s) into the adapter.

If it is normally powered by an automotive power socket (e.g., my fan and my battery charger), you plug them directly into the power socket. No adapter is required.

Control System

A PC board mounted in a small box above the control box controls the system. This board converts the AC output of the generator to DC and turns everything on and off as necessary.

computer desk v2 control board

When not in use, the generator, battery, and power sockets are all disconnected from one another to prevent the battery from draining.

When you begin pedalling, a circuit on the PC board senses the output of the generator and turns on a relay, connecting the battery, generator, and four power sockets together. This switches on whatever is plugged into the power sockets and starts recharging the battery.

A current sense resistor mounted inline with the generator and each power socket measures the output current of the generator and the current drawn by each power socket. The difference between them is calculated to be the current going either in or out of the battery. A separate circuit measures the voltage across the battery, generator, and power sockets.

The power being produced by the generator and consumed by each of the power sockets is calculated using the power formula:

P (in watts) = V x A

To prevent damage to the battery and prolong its life, the control system will open the relay and disconnect the battery, generator, and power sockets from each other if any of the following occurs:

  1. the current either in or out of the battery exceeds its recommend limit,
  2. the battery voltage gets too low or too high, or
  3. the battery drains for longer than a (user-adjustable) period of time.

This third condition is how I normally turn my desk off--I simply turn my computer off and walk away from it. If I am recharging my phone and it's not fully recharged by the time I've finished work on my computer, I simply set the shutoff period for an hour or two before shutting off my computer.

Computer

I replaced the old Dell 15" laptop I used on the original desk with an 11.6" Chromebook that used about 1/2 as much power to operate. But lately I've been using a very energy-efficient Orange Pi Plus 2 computer and a 19" LED TV monitor instead of a laptop computer. I prefer the larger display, mouse, and keyboard, and the fact that they are all independent, replaceable components.

pedal-powered computer desktop

User Interface

Software installed on the computer I use reads the output of the PC board via a USB cable and displays the power data in a series of web pages:

screenshot of status screen of pedal-powered computer dashboard

screenshot of power and voltage graphs of pedal-powered computer dashboard

screenshot of daily summary table of pedal-powered computer dashboard

screenshot of settings view of pedal-powered computer dashboard

Planned Improvements

Although the desk has greatly improved over V1, there are still some changes I would like to make.

  1. Add the ability to turn the power on and off to the individual sockets from a control panel in a web browser. This would allow you to switch off lower-priority, power-consuming equipment when you're having trouble generating enough power to keep the battery charged.
  2. Build the Orange Pi+ 2 (or similar low cost, single-board computer) directly into the desk, and have the computer serve the dashboard over the local area network. This would allow any user to use their laptop with the desk and view their output in a web browser without installing any extra software.
  3. Use larger cross-section tubing for the main frame tube to reduce frame flex created by heavier riders or people who do not pedal as smoothly as I do.

I'll detail the process of making these changes in later posts.

written by
Jim Gregory (contact)
published
2016-10-05


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