I've been approached by a local group who are interested in putting on some
'green' Christmas lights in the High Street this year using pedal power.

I was sent a document outlining the idea but, unfortunately, it seems to be
based on a lot of press reports of other schemes which I suspect have cost a
lot more than their nothing pence target ...

Now my background is electronics, so I've never had any dealings with
generators other than the Miller dynamo on my mum's old bike (pre-war, 28"
wheels, cast iron frame, no gears) which was handed down to me over half a
century ago. Oh yes! There was also the little motor from my Trix construction
set when I was about 10 which could be made to light a 2.5V torch bulb - just!

I've stripped out all the 'why we should do this' and so on bits out and a lot
at the end - which does mention the H&S aspect, by the way. What follows are
the sections I've commented on - including my comments, of course.

What does the team think? Has anybody ever got involved with anything like this
before? And is my car alternator idea worth pursuing?

The originator already confesses to having zero electrical knowledge, so I
don't think that point needs further elaboration!

================================================== ===========

] PRECEDENTS OF PEDAL POWERED LIGHTS

I do so love it when journalists get involved with technical matters €“ they
can be a constant source of mirth unless, of course, anybody makes the fatal
mistake of taking them seriously!

] - Belsize Park, London
] http://www.hamhigh.co.uk/news/belsiz...o_the_metal_1_
727643

Doesn't say they managed it - just an appeal for bits. "We are currently
looking for an old exercise bike, a small 12volt generator and a battery ..."

] - Chorlton, Greater Manchester
] http://menmedia.co.uk/southmancheste...news/s/1183002
_celebrating_a_pedalpowered_christmas

Better, but "The turbine costs £500 ..." etc.

] - Copenhagen - UN Climate Summit:
] http://news.bbc.co.uk/1/hi/world/europe/8387187.stm

Hardly sounds like one 80V 300W generator (see below) ... "a number of bikes
are (each) connected to a different section of the tree ..."

] - Video of Copenhagen (larger-scale than ours will be - but same principles)
] http://www.youtube.com/watch?v=59JYRyzSfz4

Unfortunately, you don't see what they are using for generators!

] - Make Your Own Xmas - Rob and Roberta Smith at Tate Britain
] http://www.tate.org.uk/britain/exhib...s/default.shtm

Reading between the lines, this was all done commercially (see last paragraph)
at an undisclosed cost. The only thing free about this is the cycling!

] - Electric Pedals bicycle generator: http://electricpedals.com/shop/bicycle-
generator/

300W ...? One horsepower = 746W so 300W is nearly ½ hp! 300W is actually 0.4hp
but this is assuming 100% efficiency! If 80% efficiency could be achieved, the
physical input would be ½ hp.

I note they have a €˜shop but no pricing or any way to place the order ...
Weird?

Delving further, we find: http://electricpedals.com/shop/bicycle-generator/

Ah! This bit mentions "a 10m high tree with 2400 LED lights powered by the
public using 10 bikes and generators ..." So that's 240 LEDs per bike - but no
mention of the power demand of each LED ...

There is some further info on the Tate tree, though: "an 11m high tree with 30
halogen bulbs powered by 8 bikes ..." but, again, no details of the bulbs'
consumption but if we assume 12V 10W bulbs, that's approx. 4 per bike, so 40W
per bike. Finding suitable bulbs in weatherproof enclosures won't be cheap,
though ...

Perhaps this will help? http://electricpedals.com/how-it-works/

At last, some more realistic figures! "An average human can maintain 40 to 50
watts for an hour or more ..." - a tad short on the previous 300W then! Oh yes,
the generators: "Recycled 250 watt electric-scooter motors work a treat ...".
Have you got a source for these ...?

] TECHNICAL SPECIFICATION FOR PEDAL POWERED LIGHTS

] - Motor - expect 0 to 80volts (the faster you pedal, the higher the voltage);
] can generate over 300watts.

Rubbish! No way can you expect an average person/bike to expend ½ hp
continuously (See above) As for 80V, Id expect a 12V motor (under load) to
generate something more like 12V! You might also like to look into the H&S
aspects of generating anything above, I think, about 40V €“ bearing in mind that
this is outside and almost certainly in wet/damp conditions!

] - If we want to store power, need a diode, DC/AC inverter and a battery

What is the inverter for? Assuming the generator voltage is matched to the
battery, a diode simply stops the battery attempting to drive the
motor/generator in the absence of physical energy (from the bike). If an AC
generator is used, it also rectifies the AC to produce DC to charge the battery
but note that this is a very simplified explanation though.

] - LED lights

I note that no specification is given for these! However, as the intention is
to use whatever is freely or cheaply available, this comes down to Hobsons
choice, anyway ...

Environmental and physical constraints probably rule out any idea of building
anything with sufficient mechanical strength from component parts €“ which would
also be extremely time consuming.

LEDs are intrinsically low voltage devices (2 €“ 3V typical) but care has to be
taken to limit the current through them or they can (and will!) instantly self-
destruct!

Adapting available lights will depend on how they are designed to work in the
first place. Low voltage lights (intended to be driven from a €˜wall-wart)
should be the easiest to match to the generator.

Mains driven strings might be a problem as it could be very difficult to sub-
divide them into lower voltage strings without causing considerable physical
damage. Each sub-string will then need to have its own current limiter added as
there is unlikely to be more than one in the original.

It may be easier to use a DC/AC inverter here to feed the string/s with 230V
AC. Note that most inverters are designed to work on 12V DC only. 24V and
12/24V types are available but less common. This has a bearing on the choice of
generator.

Provided thick low voltage cables are used to minimise energy loss, inverters
could be mounted high up out of reach (for safety purposes).

* Alternator alternative

If a suitable automotive expert is available, how about using a car alternator,
rather than a DC generator. A scrap car can supply a complete
alternator/regulator/battery set-up. (Virtually any old car battery should do €“
most get replaced because they cant start a car but are still good enough for
the current purpose €“ if youll pardon the pun ...)

As an alternator, as its name suggests, generates AC, rather than DC, which
would ease the problem of producing a range of voltages, if required, using
suitable transformers.

] NECESSARY RESOURCES - (I've deleted this section except for one paragraph I
] added

* Protective earth. Even a small DC generator can generate some nasty peak
voltages if inadvertently run unloaded. Overall, were looking at a fair bit of
power here, so a PE may be mandatory, even if everything is kept to 12V. Can be
a bit awkward on a large paved area. Would the council let you lift one of the
paving slabs ...?

] COSTS

] The aim is that they should be free, with materials obtained through
] Freecycle or donations.
] Paul Allen says there may be a small cost. LED light strings cost c. £10 for
] 50.

--

Terry