Path: utzoo!attcan!uunet!cs.utexas.edu!know!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!aplcen!haven!uvaarpa!mcnc!thorin!oscar!tell
From: tell@oscar.cs.unc.edu (Stephen Tell)
Newsgroups: sci.electronics
Subject: Re: Headlight buzzers (was Re: Car Battery Rechargers)
Message-ID: <16627@thorin.cs.unc.edu>
Date: 9 Oct 90 17:15:50 GMT
References: <13766@mcdphx.phx.mcd.mot.com> <4233@rsiatl.UUCP>
Sender: news@thorin.cs.unc.edu
Reply-To: tell@oscar.cs.unc.edu (Stephen Tell)
Organization: University Of North Carolina, Chapel Hill
Lines: 55

In article <4233@rsiatl.UUCP> jgd@rsiatl.UUCP (John G. DeArmond) writes:
>hbg6@citek.mcdphx.mot.com writes:
>
>>>Since my concern was from leaving my lights on and not cold weather, I
>>>am now considering building a device I saw in a popular electronics
>>>magazine that buzzes if you leave your lights on.  (I don't have the
>>>magazine with me but can give details to anyone who wants more

Radio Shack sells a ready-made module for this purpose, and a few 
sonalert-equivilent buzzers if you want to make your own.

>Making a is trivial.  All one needs is a Sonalert or similiar device
>and a 1 amp diode (1n4002 works well).

[text replacd with picture]

			Diode			Buzzer
Headlights -------------->|-----------------------()---------- ACCessory
     						 +  -

>The circuit works as follows.  When both the ignition and the lights are
>on, there is 12 volts on both sides of the sonalert and thus no current
>flow through it.  When the lights are off and the ignition is on, there
>is voltage on the (-) side of the Sonalert and there is an effective 
>ground through the headlight filaments.  The diode is reverse biased,
>however and the sonalert remains quiet.  With the lights on and the 
>ignition off, the anode of the diode and therefore the (+) side of the
>Sonalert is at 12 volts while the (-) side of the sonalert is at an 
>effective ground through the de-energized load on the ACC terminal.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Since the diode is forward biased, the Sonalert sounds, alerting you to
>turn the lights off.  

Ah, but what if there is no de-energized load on the ACC terminal?
I installed a circuit like this (Actually the radio shack module, I was
lazy) in my 1983 Toyota Tercel.  The instructions said find an "Ignition"
wire (not accessory).  I couldn't locate one of those easily, and so used
the easily-available accessory power wire for the radio.  The result is that
the circuit works fine only if the radio is left on when you turn off the
car and not the headlights!  If the radio isn't on, it might beep a few times,
I think as capacitor charges someplace, probably in the digital clock
I installed at the the same time.  

So, unless you're in the habit of leaving other stuff on all the time too,
best find a wire that is only live when the engine is running.

>John De Armond, WD4OQC  | We can no more blame our loss of freedom on congress
>Radiation Systems, Inc. | than we can prostitution on pimps.  Both simply
>Atlanta, Ga             | provide broker services for their customers.
>{emory,uunet}!rsiatl!jgd|  - Dr. W Williams |                **I am the NRA**  


--------------------------------------------------------------------
Steve Tell      e-mail: tell@wsmail.cs.unc.edu usmail:  #5L Estes Park apts
CS Grad Student, UNC Chapel Hill.                       Carrboro NC 27510