Hi guys, can you help a newbie to electronics and programming out?
I engine swapped my car, and would really like to have a way to see my coolant- and oil temperature. There are already (factory) sensors in the engine I can you use for this purpose. I can't stress enough that I want to use these sensors because they fit the engine and are proven capable of whitstanding the harsh enviroment of the engine bay.
I think the biggest problem is not having any data for the sensors no beta value or table and the fact they're low resistance (both around 1.2k ohm at 22 degrees). The expected operating temp will be 85-105 degrees for coolant and 110-130 for the oil. Obvious resistance will be very low at these values.
I've found the steinhart equation which can help me to determine the resistance curve, but what would be the route to go to transform this to a usuable voltage? Within the examples/tutorials I've found a voltage devider is used with more or less the same resistance as the themistor, but this will lead to a high current. Do I need to use a wheatstone bridge? Or is there a better/easier solution?
The accuracy I aim for is +/- 1 degree in the operating range.
Thanks in advance for putting up with a probably "stupid" question
No not really, I just want to keep the engine save. When it shows 105 and in real life it's 110 deg. C I wouldn't be pleased. But like a said it's an aim not an absolute necessity. In designing a system there should be some goal right? Do you think it isn't feasible?
I know next to nothing about this problem, but couldn't you use some kind of switch and only sample for a few moments every second or so? So, close the switch, wait a few millis for voltage to stabilize, measure, open switch? Even if you have "high" current while the switch is closed, as long as it's only closed 1% of the time it shouldn't be too much of a problem.
Also, how high is high? If you're at 3 kOhm at 5v logic levels that is around two milliamps of current, isn't it? That's a lot if you're running on a battery but if you're running on a car's engine's output is it a big deal?
Yeah but you're talking about it being 105ºC and showing 106ºC. For something like coolant temperature 5ºC would be significant if it was within a certain range but not otherwise. You should bear this in mind when worrying about the calibration of your sensors. Think about what the critical temperatures are.
Feasible but maybe needlessly complicated considering the sensors you are using. These are not sensors designed for lab quality measurement rather ruggedness and longevity.
It seems that this is your main concern, but I don't understand why. At a 12V excitation, that is less than 12mA of current, and probably what's being used for your existing gauges. With a voltage divider of the same resistance, now you're at 6mA.
Why is the current a concern? Self-heating is not likely to be a problem, considering the environment it will be operating in.
There will be a warning light eventually. To put things more into perspective underneath the car I'm talking about. This was originally delivered with an aircooled engine with ~50 hp, I changed this to an water cooled engine with 115 hp. So a lot more thermal energy needs to be dissipated. I didn't want any cooler to be visible so quite a few challenges were faced guiding the air and have sufficient surface to cool down. When I'm confident all the engineering I've done works the actual temperarures will be come less important.
My idea was to use an arduino. If I use the 3.3v to supply voltage to the sensor which with a quick measurement at ~85 deg. C give 230 Ohms this will lead to a high current right? When Ohms law is used I=U/R this will be ~ 3.3/230= 14mA Dammn you're absolutely right cedarlakeinstruments! feeling realy dump right now. Not sure if I've mixed up units before then.....
Edit; potentially I found the problem. I used a used sensor to my original measurements in Excel. Before starting this post I've did some measurements on a new sensor, seeing the values I thought I messed up units k ohms instead of Ohms, so I added the k (Ohms) in the cell. Maybe I did mess up, on the moment I don't know. I will redo the measurements on the original sensor, but the reason to cool it down instead of bringing it to the operating window was to rule out measurement error, which isn't needed if we're talking about k Ohms instead of Ohms.
As long as you knew.