Article View: sci.electronics.basics
Article #135Re: Alarm delay in car?
From: Terry Pinnell
Date: Wed, 21 Aug 2002 16:59
Date: Wed, 21 Aug 2002 16:59
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3506 bytes
DaveC <anon@example.net> wrote:
>On Tue, 20 Aug 2002 23:43:45 -0700, Terry Pinnell wrote
>(in message <l9d6mucf2ve5sbkfusk31beknvnd16hkbc@4ax.com>):
>
>> http://www.terrypin.dial.pipex.com/Images/CarSensorDelay.gif
>
>Terry,
>Thanks for your suggested circuit. It is one that has many merits for
>*digital* inputs, ie, those that have "switched" sensors. When pressure gets
>low or temperature gets high, the switches close, the alarm sounds.
>
>For an analog "gauge" circuit, the sensor changes resistance to ground based
>on temperature (or pressure). Using your circuit with a temperature gauge
>sender will, probably, trigger the alarm. But at what temperature this
>happens is anyone's guess. It will depend on the sender's resistance at a
>given temperature, and the CMOS 4011 NAND gate input's threshold.
>
>This is why I'm asking for some kind of calibratable input so that I can
>adjust the trigger point when the alarm goes on.
>
>Any ideas?
I must have misunderstood. In your opening post you said there were
already lights or LEDs on your dash ('idiot lights'). I understood
these are activated when temperature or oil pressure go outside their
safe limits (you specified 'max' and 'zero' respectively). I thought
the add-on circuit was required to sound an additional alarm if either
sensor was still in its unsafe state 10 secs after engine start.
Assuming you could access both light circuits (which are inherently
digital in this context), that's what my suggested circuit would do.
If instead you want to set your own limits, and you can get access to
a varying output voltage from each sensor, then you need to insert a
simple comparator ahead of each.
Terry Pinnell
Hobbyist, West Sussex, UK
Technical Details
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