Hi,i just want to do an indicator to a car gauge(temperature).
Let say if the temperature rise a bit than normal condition,then this indicator will trigger a LED or buzzer.
But i dono how the circuit should be and i have no idea how that gauge works.....

need help...

 

The guage is likly to be voltage based in which case the solution is relativly simple, just a single comparator.
However there is little point in expanding on that without further info ..
Can you provide a wiring diagram / physical charictoristics of the guage itsself?

If not you will have to 'test' how it works which may involve stripping things apart which you probably wont want to do.

Al

 

Just leave the car's original gauge out of things and find the sensor wire(s) leading from it, which should have a voltage that varies according to the temperature it's monitoring.

Send that voltage to a comparator circuit and adjust as necessary.

 

Modern vehical electrics / electronics 'can' be odd to say the least ....
It wouldnt be unusual to find that a guage is current based or even physically electro-thermal all sorts of strategies are employed to avoid light mechanical instruments that are suseptable to mechanical shock.

In addition they are likley to be connected directly to the ECU these days and potentially being driven by a PWM so they are actually measurung RMS power so you may find that the job isnt as simple as you first thought once you start.

I learned through painful expeeriance to expect just about anything and I have been out of that game for some time now but I dont expect it got simpler whilst I wasnt looking.

Of course it could simply be a volt meter connected to an NTC beed who knows, I was just suggesting that forwarned is forarmed....
Al

 

Yeah...a comparator circuit is what i was thinking.

My car use carburator....so no worries of ECU.

 

Most of the newer ones do go directly to the ECU but due to costs involved I don't think they've changed from plain thermistors.

 

If it just a 'volt / current' meter, damped or otherwise, you may well be fine with marshallf3 origional suggestion. and any relativly high impedance method of sensing a voltahe level / setpoint.

I only mentioned the other 'potential' complications because I have fallen foul of them several times myself.

Al
marshallf3

marshallf3

 

Lets say i has created this comparator circuit,where do i need to hook up the input?
The gauge has an electromagnetic mechanism and a circuit....i think the circuit convert thermostat signal into voltage,but not sure

 

The circuit probably helps to convert the fairly non-linear signal of a thermistor to a more linear voltage for the gauge.

Pretty much your choice where you tap off a signal for your comparator circuit, wherever's easier sounds best to me.