🚀 go-pugleaf

DaveC wrote:

> 
> Ed,
> Thanks for the very good ideas.
> 
> This circuit will work with "idiot light" sensors (thermal and pressure
> switches) on my truck. But what changes should be made to accomodate the
> temperature gauge (analog sensor -- reduces resistance to ground as temp
> increases) in my car?
> 

Dave,

In the car, you'll have to determine the point on the temp
gauge that you want to sound the alarm.  That'll require
some digging around in the dashboard, so that you can
get good access to the gauge.  What I would do is this:
connect a variable DC supply to the gauge with the
gauge disconnected from its normal connections. Then
I'd increase the voltage from the supply, until the needle
on the gauge rose to the "trigger point" - the point
at which I'd want the alarm to sound. I'd measure the
voltage at the trigger point.  With that voltage known,
you can then build the circuit to sound the alarm.
For precision, you could use the comparator or bridge
circuits mentioned already.  However I doubt that precision
matters.  I'd just use a zener diode and a transistor,
just like in the delay circuit.  You can "pad" the
zener voltage if you need to with a series diode to
raise the voltage (by about .6 volts per diode) at which 
the alarm will sound.
For example, let's say your measured "trigger point"
is 10 volts.  You could use a 9 volt zener and pad it
up to about 10.2 (depending upon the needed B-E voltage
to saturate the transistor) with a diode.  Breadboard
the circuit first, to see the exact voltage that sounds 
the alarm - then apply that voltage to the gauge to 
ensure that the needle moves to the area where you
want the alarm to sound.
I believe that the gauge reading won't be affected
by the addition of of your circuit - but it could
be. If so, add a second transistor to create a
darlington pair as described in an earlier post.
Then you can use a high ohms resistance series
with the zener in the base circuit. That will avoid 
"loading down" the original circuit to the gauge, while 
allowing your alarm circuit to work.