Date:         Wed, 16 Dec 2009 23:22:48 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Audible Idiot "Light"...EE Inventor Wanted
Comments: To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
In-Reply-To:  <4B29954C.5020002@gmail.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 09:19 PM 12/16/2009, Rocket J Squirrel wrote:
>The Coolant Temp light on the 1.9 appears to have both ends going into a
>magic box according to Bentleys. It may be that one side of the lamp is
>tied high, or the other low, but without getting into the dash, it's
>unclear: they may both be active wires.

The temp light on all versions of Vanagon is a self-blinking 5 volt
LED in series with a 5.1v Zener diode, driven by a timing circuit
employing a 741 op amp.  This circuit powers the LED for about three
seconds any time the resistance to ground on the sender input falls
to or below about 35 ohms (I don't know the voltage that represents
but I'll find out).

In practical terms it would be easier to essentially duplicate that
circuit than to open up the gauge and get a drive signal from inside
it.  The circuit I'm envisioning would piggyback on the back of the
gauge terminals (+10v, ground and sender input) outside the flexible
circuit.  It would have inputs from the other lamps which I believe
are both switched-ground, wired-ORed with the
temp-light-circuit.  Might or might not need a buffer amp on the piezo drive.

Yours,
David