```In message <4l45t3\$qs3@news.fonorola.net> - marka@spots.ab.ca (Mark Achtemichuk
) writes:
:>
:>I would like to ask anyones help out there in designing a simple circut as
:>follows:
:>
:>The basic idea is that a dual colored LED (red and green) lights green when it
:>receives a 12V lead.  When that 12volt lead disappears, the LED must turn red.
:>If I remember correctly the LED's work by switching their polarity for the
:>different colors.  This is where I run into trouble.
:>
:>The circut will be used to test fuses in my car.  When a fuse blows, the LED
:>will turn from green to red.  The circut will have a constant 12 volt supply as
:>well as the 12 volt probe.  The LED's are rated at 3 volts so I assume it would
:>be easiest to put a current limiting resistor on them vs. building a power
:>supply.
:>
:>Can anyone help me with this?  I would appreciate anything anyone has to offer.
:> You can reach me at:  marka@spots.ab.ca
:>
:>Thanks again.  I work better with car engines than electronics <grin>.
:>

Mark,

OK, after reviewing the other posts, permit me to offer my suggestion.
I see what you are looking for is a probe which can be run along a series of
fuses to determine if they are good or not.  The simplest circuit that
will meet your description is:

1Kohm 1/4 watt
|-------/\/\/\------- To +12 Volts (vehicle battery)
-----<---|>|-----|
Probe tip and   |-------/\/\/\------- To 0 volts (vehicle ground)
bi-color LED          1Kohm 1/4 watt

The end of a fuse will be at one of three states - +12, (power side of the
fuse or load side of a good fuse); ground (load side of a bad fuse with load
on); or open circuit (load side of a bad fuse with load off).

When the probe is touched to +12 current will flow through the LED to the
resistor junction, causing the LED to glow green.  When the probe is touched
to ground current will flow from the battery through the LED to ground and
the LED will glow red.  When the probe is touching anything not electrically
connected the LED will not glow.

I would build it in a clear plastic probe handle so you can watch the LED
and the probe tip at the same time.  I haven't experimented with LED
brightness, it may be desireable to reduce the resistor values to increase
brightness.  Just watch the power dissipation of the resistors and the
LED.  (I think most LEDs can handle up to 100 ma.  That translates to 120
ohm 2 watt resistors).

Next, thanks to all who made the other suggestions.  Several of them
gave me the ideas for this design.

Lastly, Sam, would you care to check my work?

Bill

```

Date: 24 Apr 1996 17:07:01 GMT

Original Subject: Re: HELP designing a simple switch circut!