Date:         Fri, 20 Sep 2002 15:38:51 -0500
Reply-To:     Chris Mills <scmills@TNTECH.EDU>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Chris Mills <scmills@TNTECH.EDU>
Subject:      El-cheapo-voltage_metero-project
Content-type: text/plain; format=flowed; charset=us-ascii

I am wanting to duplicate (in function at least) something I saw in a
motorcycle mag for use on my motorcycle and in my VW. Would be useful for
boats, tractors, jet skis, RV's, etc. The catalog price is quite steep
compared to my quick mental estimate of the parts.

It is a simple looking little thing that has three colored LED lights that
tell you that the charging system is GOOD, NOT SO GOOD, and SOL - - - -
green, yellow, red.

Can any of your engineering types tell me how to create this circuit? I
understand the LED's and resistors to get the 12 volts down to a usable
form, but what controls the threshold at which each light comes on and off?

I'm assuming transistors. I realize there is 10 gazillion different
varieties with different voltage ratings and I'll be happy to do that hunt,
I just want to get a better idea of the circuit.

With an experimental version built with relays I figure we would have
relays with a minimum switching current and the relay would not close until
that threshold was met, thus keeping the yellow or green LED's off if your
charging system was only producing enough juice (10V) to trigger the red
LED. The red would be straight wired to the system, the yellow relay would
be fed through a resistor to keep it off until 12 volts (or whatever level
we choose), and the green relay would be isolated by a larger resistor that
would put it's threshold higher than that. In theory we could even add a
forth LED to indicate overcharge and that would limit the last LED to 15V
or higher.

And 14V all three LED's would be on unless you included wiring that would
remove power from the other relays when the green LED was lit. As the green
relay closed it would open the power to the lower relays (yellow and red).

This could all be done with transistors - right? What type would I be
looking for?

As I am an amateur I have to ask a question: transistors will do the very
same thing as a relay for smaller loads - right?

And why not a $10 voltage meter? Well because I like to take the simple and
make something complicated out of it and b/c a few small LED's could be
mounted ANYWHERE and would be quite useful in Ultravans and Westfalias. You
could have LED's for the main battery and another set for the coach battery.

Ciao!

Chris Mills
Cookeville, Tennessee
ICQ# 5944649