Date:         Sun, 11 Jun 2000 00:09:39 -0400
Reply-To:     David Beierl <dbeierl@IBM.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@IBM.NET>
Subject:      Re: Fridge burner LED
Comments: To: Bill Davidson <wdavidson@thegrid.net>
In-Reply-To:  <005301bfd34f$2b0fdc00$14a0e0d8@wd1000086onem>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 22:45 6/10/2000, Bill Davidson wrote:

>What I would like to have is two LEDs on the dash instrument panel... one
>that would light if the fridge is on 12v

Ok, that one's easy -- just connect your LED from the hot side of the
heater to ground.  Use at least 470 ohms in series, increase it if the LED
is brighter than you want.  If it's not bright enough, get a brighter LED.


>Now how would I do that? Where would I get enough power to light the propane
>LED on the dash when we barely have enough power to light the stock fridge
>LED on the LED panel?

There's plenty of power available, the LED is running off an amplifier that
can supply something like 30 milliamps, more than enough to light an LED to
full brightness.  And the two chips on the board contain 8 amplifiers, but
only 7 are used, so there's a spare if you need more drive. The problem is
getting the amplifier to notice that the thermocouple is running, because
the maximum output (in the working gas  valve circuit) is less than ten
millivolts, and might only be two or three.  Incidentally, this output is
*negative* to ground.  In the stock LED panel the ground reference is taken
from the supply wire, and will go several millivolts positive when you add
loads like the sink pump -- that's why the LED lights up bright when you
run the pump.  To make things more predictable you'd want to take the
reference ground directly from the body of the gas valve, or the
thermocouple itself.

But here's a simple mod that will give you a second flame LED that works
just like the first one, foibles and all: connect pin three of the IC1
(left side chip, #1 amplifier which is unused) to the same pin on
IC2.  Connect a 2.7k resistor from pin 2 of IC1, in parallel with the same
resistor coming from pin 2 of IC2.  Connect two resistors to pin 1 of IC1
-- a 470 ohm to drive the LED, and a one-megohm back to pin 2 to set the
gain.  See the top half of the schematic at
http://pws.prserv.net/synergy/Vanagon/Pilotmod_schematic.gif.  You're
simply taking the spare amplifier and duplicating the existing setup to
drive a second LED, instead of the way I wired the bottom half of that
schematic.  The thermocouple is a *very* low-impedance circuit, so I doubt
that adding the second amp will make any detectable change in operation of
the first one.

There are two tweaks you may want to add:  To control the sensitivity of
the amplifier, i.e. how hot the thermocouple has to be to bring the LED to
full brightness, add a one-megohm or greater pot in series with the one
megohm resistor btw pins 1 and 2 of each amplifier.  To control how bright
"full" brightness is, add a 2k or 5k pot btw the 470 ohm resistor coming
from pin 1, and the LED itself.

Make sense?  Obviously I can do this sort of mod for you if you want.

d

David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"