Hi,
I am in the process of creating a LCD display with an Arduino Uno for my BMW Z3 which shows values like ambient air temperature, engine oil pressure and temperature, limited slip differential oil temperature.
Now I got everything aligned besides the oil temperature sensors. Ambient temperature is sensed by a DS1820, oil pressure by a transducer which outputs a voltage proportional to the pressure.
The problem with the oil temperatures is that I cannot sense them with DS1820 because the oils can easily exceed +125°C in demanding environments. So before fiddling around with PTC and voltage dividers I want to ask if there are similar sensors as the DS1820.
If not do you have any recommendations for a PTC which can be used and results a reasonable accuracy (+- 1 K is fine) without op-amp? I do not want to use an op-amp because it would increase the pcb size significantly and I hope it is possible to get that accuracy without one.
I had a look at the PT1000 and ST-20M. https://produktinfo.conrad.com/datenblaetter/175000-199999/186465-da-01-ml-TEMPERATUR_SENSOR_de_en.pdf
It turned out that the ST-20M is much more feasible because its resistance changes more over the relevant measuring range which results a bigger voltages change and that bigger voltage change can be measured more precise with the 10-bit ADC. Right?
Are there any experiences with the PT1000 without op-amp? As previously mentioned I really do not need super high accuracy. What I do need is a screw in sensor. The thread typ does not really matter because I probably need to adapt that anyway. It would be lovely if the sensor was also waterproof. PT1000 or ST-20M are around 20 USD. It would not bother me if you could recommend a cheaper soulution.
Z3bastian:
I do not want to use an op-amp because it would increase the pcb size significantly
Considering a typical OpAmp is about 4x5 mm in size (and you can get them in smaller packages if you want), that must be an awfully small PCB, if that's causing a significant increase in size!
For your sensor, high temperature and hostile environment, I'd be looking for a thermocouple. Of course it does need some kind of amplifier, but those are pretty small as well.Shouldn't take more than 10x20mm of board space.
There are multiple reasons why I do not really feel the need for an op-amp.
its the first time where I design an actual pcb so I do not want to clutter it more than necessary. Currently its the size of an 1602 LCD.
KISS, as I said I am not demanding a super high accuracy so would think it should be fine without op-amp
the op-amp needs additional resistors which also need space
I understand your point but I really want to keep it simple. The engine bay of a car is not a too hostile environment in my opinion. It should not even be over 200°C in the sensor area.
Are there NTCs with similar nearly linear behavior like the ST-20M? I would appreciate suggestions which are actually available as screw in sensor. I found a few sensors which were not available as screw in version.
Get an automotive oil temperature sensor ( these are USUALLY some type of thermistor - later BMW aren’t !) and use that - it will have a thread etc for mounting .
Measure its resistance at a few temperatures and then you can decide power, resistor divider values etc.
If you google these sensors , you can find characteristics too/instead. ( VDO for example).
BTW oil temp/pressure maybe available via the ECU ?
Z3bastian:
Are there NTCs with similar nearly linear behavior like the ST-20M? I would appreciate suggestions which are actually available as screw in sensor. I found a few sensors which were not available as screw in version.
Linear response shouldn't really be a concern. You can linearize it in code.
Oil temperature is definitely available via OBD. Pressure isnt. But I want a display which is always available no smartphone OBD solution. The Z3 does not have an alphanumeric display like E46/E39/... so there is no way to inject it there.
I tried a automotive NTC for coolant temperature which doesnt really work for temperatures >100°C as the resistor change between 40 and 100°C is not even 1kOhm. I couldnt try an oil temperature sensor yet but I will get one to try it out.
Z3bastian:
I tried a automotive NTC for coolant temperature which doesnt really work for temperatures >100°C as the resistor change between 40 and 100°C is not even 1kOhm.
That does not make sense. What matters is percentage of change.
If a 1k thermistor changes 1k that's good. If a 100k thermistor changes 1k it's not.
Thermistors need a pull up (or down) resistor, and that resistor value has to match the thermistor resistance at the temperature of interrest for the largest range of returned A/D values.
Don't just use a 10k resistor for a 10k thermistor (@25C).
If you measure engine oil temps, you might want the highest resolution between 50C and 130C.
So you look at the thermistor resistor/temp table for it's resistance at about 90C.
And pick a pull up resistor of the same value at that temp.
Leo..
