Re: [ST] Helmet camera

[Neil Lindsey <neillindsey@xxxxxxx>]

On Thu, 24 Apr 2003, Mike Bostock wrote

> BTW it requires a 12V regulated power supply (the bike can peak to
> 13.5 volts) any easy way of regulating this?

On Thu, 24 Apr 2003, Bill Swartz wrote:

> That shold be easy enough with a typical car cigarette lighter type power
> adapter that puts out various voltages, commonly used for powering cd
> players, radios, pocket tv's, etc.  You should be able to find one at
> Radidio Shack

Sorry for the delayed response (I only subscribe to the digest).

I am a 'gEEK' (and proud of it Blake).

If the camera was intended for automotive use and the manufacturer says it
requires a 12V regulated power supply, then I'd say it is OK to hook it up
directly to the motorcycle electrical system, because automotive (and
motorcycle) electrical systems ARE regulated to a degree (there is an upper
and a lower limit, but the range is relatively high).  Automotive electrical
systems are also notoriously NOISY, so additional electrical filtration may
be necessary to isolate sensitive electronic equipment.  Presumably a device
intended for automotive use is either designed to withstand such electrical
interference or the details as to what is required will be specified in more
detail.

If, on the other hand, the device was NOT designed for automotive use (I
wonder why they would have chosen 12V as the power requirement if this is
the case), then usually they mean a regulated power supply that does not
vary appreciably.  The Radio Shack device mentioned above DOES NOT provide
regulated power, not by a long shot.  These devices only get you 'in the
ball park' and are intended for devices that can tolerate quite a variation.
The voltage will drop as the load (current drain) increases.  Don't believe
me? -- check the output with a multimeter.  A properly regulated power
supply does not vary with the load current, so the open-circuit voltage
should read the same as when it is loaded down by the device it is powering.

To create a properly-regulated 12V supply, the first thing you need is an
unregulated power source that is ALWAYS higher than the target regulated
output.  Such is not the case with automotive power systems, since the
output can go down to (or, in some instances, below) the 12V nominal
reading.  There are ways to up this source voltage.  Some are more
complicated than others, but a 'voltage doubler' made with capacitors may
suffice for low current applications.  Also presuming a low current
application, you may be able to regulate this increased voltage with nothing
more than a carefully-chosen resistor and a zener diode (12V in this case).
If the current requirements are higher then you may need more circuitry that
involves a power transistor to pass the additional current required for
increased loads.




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