In article , "HK" wrote:
I'm trying to help my daughter with a school project and while I know my way
around house wiring, I'm a relative newbie when it comes to low voltage.

Let's say I have a 6 volt DC power supply (4 "C" batteries). If I want to
drive a small motor and some lights, I just find 6 volt motors and lights
and wire them in parallel

Yes.

or series.

No.

Why is it that I can't light up a 12v light with a 6 volt power supply?

The filament resistance is too high, and/or your 6 volt source doesn't supply
enough current. Are you trying to light up an automobile light bulb with four
AA batteries?

I always test my batteries using a multi-meter and as the battery ages, the
voltage drops. With low voltage, the device (flashlight, etc.) still works
but the light is weak. So, wouldn't a 12v light just be weak if I use a 6v
power supply?

It depends on the current as well as the voltage.

Is there some kind of voltage threshold at which a device won't work?

Depends on the device. For something as simple as a light bulb, I wouldn't
think so. Try an experiment: see if you can light up an ordinary 60W
incandescent light bulb (120V) from a car battery (12V).

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.