Total Newbie Here, where to start

FYI, your attachment isn't working

UPDATE:
I was surprised. It seems exponential in that it grows really fast at the end. But because of that and the fact that I had a analog volt meter I was unable to get accurate readings towards the end of the curve. And after 11v at 1/2 way up to high, it was such a minor change from 11v at 1/2 to 12v at 3/4. I assume 12 is the max and it goes by an order of magnitude of .25v towards the last 25%

Low = Cold temp
High = Hot temp
1/3 = needle is at 1/3rd between Low and high
1/2 = medium
3/4 is our end point = hot and turn on
Full or high = broken system or bad food

Battery read 14v with the generator on
Low = 8v
1/3rd of the way to high it was 11v
1/2 of the way it was 11.5 estimated. Towards the end it was such a slow difference along such a big adjustment on the gauge.
1/2 to 3/4 of the way was around 12v and hard to differentiate.

So I would like the circuit to buzz and blink the LED at 11.75ish (This would be the Ux trigger point right?) to start and be safe and understand what to do. I could then see when it turns on or go buy a digital volt meter to get closer to "hot" aka 3/4. However now I have a starting point with 11.75 volts to turn on the switch and to be safe in calculations. Having the calculations and diagram will be great. I can refine them when I get a new digital volt meter, if my mom will let me get one.

I'm not an engineer or anything, but when I go to college I think it would be interesting so I want to start getting into it now with my ideas. Your help would be awesome.

Hi again (because of zone time difference),

Let us agree on some basics so that we can be surer on how to define our project here by real given data.

So far, we talked about a resistance and a voltage. I guess you like talking about voltages since we can measure them anytime by a voltmeter.
Also a resistance can be measured rather easily by an ohmmeter if the element (resistor) has no power applying on it (that is, there is 0 V across it prior to measurement). But what does the meter do in order to give the resistance reading? In case of a resistor, the simple formula could be applied:

V (DC voltage across the resistor) = I (DC current flowing in it) x R (its DC resistance value)
That is (if * means multiplication):
V = I * R
Therefore
R = V / I

So if the meter has an internal constant current (I) source (that is, its delivered current stays the same no matter if the voltage on its two terminals changes), measuring (V) then dividing it by (I) gives the value of (R). For instance, natural electric sources around us are close to the 'constant voltage source' instead, like batteries (that is, ideally, it applies a constant voltage on a load. like a resistor, no matter how much current the load takes). So to have a constant current source (within a limited range of voltage) we need to build an active circuit which could also be called a 'current regulator'.

Back to our formula, if I = 1 Amp (though this is a too high value for a meter, R would be equal to the measured V. So if V=1 V then R=1 Ohm, V=2 V, R= 2 Ohms... etc

But there is a fourth factor which is worth to always consider. It is the power (P); generated or dissipated. Also its calculation is simple:

P = V * I
or
P = V * V / R = I * I * R

Let us apply these two formulas on our project here.
We are talking about R in the vicinity of 40 Ohms and a voltage across it about 11 Volts.
I = 11 / 40 = 0.275 A
P = 11 * 11 / 40 = 3.025 W
Could those rather high values be real just for a sensor?
I am not familiar with car meters so I can’t give a practical comment on this.

On the other hand, should the car battery generator run before the meter and sensor can work properly?
You know, the nominal battery voltage is 12 V and it may be as low as 10.5 V (before a recharge is necessary).

For instance, how were you able to know about the sensor resistance?

Kerim

Here is one principle circuit.

If someone has time to calculate values
and simulate circuit.

View attachment Temp_alarm_circ_02.pdf


Inverted Alarm function >> cross-connect R4 left point and R5 left point

Kerim,

The original resistance of 51. (+/- 4) ohms was defined by the manual to be the point at which it would reach 90 deg C which is the breaking point.

I tested the voltage coming out of the thermistor at different intervals as the gauge rose in temp.


Kak how do you guys simulate circuits? A program? Which one?

I expect, that Temp_alarm_circ_02 exactly meets the requirements. There's however one point, that should be clarified. Is the gauge behaving like a series resistor, or somewaht different, e.g. like a current source? If would assume the former, but if you want exact operation of the limit switch you should check. The circuit is perfect for resistive (voltage divider) behaviour of measurement circuit, otherwise it could be improved. In traditional (last century's) cars, there has been often a common voltage regulator, reducing the gauge supply to e.g. 8 or 10 V to make the operation independent of battery voltage variations.

FVM,

You lost me. I hope this helps see what the circuit/ gauge is doing or how it is ran. After talking with my dad we decided to use the temp gauge as a system bc of all the info we can get on it, diagrams etc. See attached diagrams. First is full page then 1st half and then 2nd half, then a closer look at the temp system.

Ah, look at the last pic. Wire going in the side of guage (47c) is ground. Top wire (from #8 at the top of gauge console) is ignition positive, and bottom (wire #3) is variable negative sent from thermistor.

Full view:


1st half:


2nd half:


Insert warning system between 47c and 45
47c is the gauge circuit:

wow reading this whole thread makes my head hurt....more so than my graduate EE classes. ha.

It looks like you have a great idea, but I would recommend learning the language of schematics and electronic symbols, and basic circuit theory first.

But no problem,,, since that is what you are trying to accomplish here!

Best of Luck!

---------- Post added at 09:50 ---------- Previous post was at 09:44 ----------

Is it possible to sense a Voltage or Current somewhere in the system to trigger your alarm? Sensing a change in resistance isn't as straightforward.

templemark said:

Is it possible to sense a Voltage or Current somewhere in the system to trigger your alarm? Sensing a change in resistance isn't as straightforward.

Click to expand...

Hence my problem. I wish it was that easy vs a resistance triggered alarm. Couldn't we just trigger it on at 11.75 volts and off lower than that? I already made the basic alarm, but it wouldn't work in the 3 cases I gave, fuel level, car temp, and fridge temp.

how do you guys simulate circuits? A program? Which one?

Click to expand...

About circuit simulators

What is Electronic circuit simulation

Electronic circuit simulation - Wikipedia, the free encyclopedia

there is a List of free electronic circuit simulators

___________________________________________________________

Your first circuit simulator
-simple for experimentals and learning circuits, easy to use, not real tool, but start with this

watch first...........
‪Falstad circuit simulator‬‏ - YouTube

Free Educational Great Java Circuit Simulation
Circuit Simulator Applet

_____________________________________________________________

for bigger circuits and more professional
I suggest Tina - TI from Texas Instruments (free)

SPICE-Based Analog Simulation Program - TINA-TI - TI Tool Folder

>>> try other simulators too

KAK

That video was awesome, cool stuff

The circuit diagram shows, that the temperature gauge has also a ground connection in addition to 12 V power supply. This opens all options for internal voltage regulators and other cicruits, that won't work different from a simple voltage divider with 12 V power supply. To find out the exact operation mode, you should measure the voltage across the sensor with varying battery voltage and determine, if it's constant or varying relative to the supply.

how do you guys simulate circuits? A program? Which one?

Click to expand...

I made a small demo for you.

Temperature Alarm Circuit Demo

Circuit Simulator Applet

KAK