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Sensors, Actuators, and Robotics » 2-Wire Automotive Sensors
October 13, 2011 by tonysq |
I'm taking off from the temp sensor project for an automotive project. The problem I am having is that many of the temperature sensors in a vehicle are a 2-wire type that has a sensor signal and sensor ground. Sensors such as air temp, coolant temp, oil temp and fuel level all have these type of sensors. They read as a variable voltage with a scan tool. I'm thinking that I have to use a pull-up resistor like this: Am I on the right track? Thanks, Chris Squier |
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October 14, 2011 by Ralphxyz |
Yeah I believe you are correct, or at least for a Dallas One Wire bus:
The quote is from WikiPedia I got some DOW tempsensors that NOTER was using for a project, I have not done anything with them yet. Ralph |
October 14, 2011 by tonysq |
I'm a real novice so I'll have to study what you posted but I'm confused because I'm not talking about data communication. I'm talking about reading an analog variable resistence using the same wiring setup as the vehicle manufacturer. I visualize these sensors as a "volume" potentiometer where you're just using 2 of the posts... What confuses me is that they use only a sensor ground (lowref) and a sensor "signal" that carries >0 and <+5 volts for the ECU to read. There is no seperate 5v feed in these 2-wire sensors. Thanks, Chris |
October 14, 2011 by Ralphxyz |
Yeah, read up on the DOW (Dallas One Wire).
Again from the wiki article:
I believe the sensors, like for temperature, are actually digital IC devices not analog potentiometer! So there actually is a data communications. At least for the DOW bus that is how they work. Now posible for the automotive system you are looking at, it is an analog system, in which case you would obviously not need pullup resistors. Ralph |
October 14, 2011 by 6ofhalfdozen |
Any chance you have the data sheets?? That might clear up if they are 0-5V analogs or DOW type like Ralph suggestions... (I have worked with some odd-ball pressure trandsucers that are 4-20mA output with only two wires as well, so you never quite know unless the datasheet says....) |
October 14, 2011 by tonysq |
Nope, these are definately analog. I am using a simple 5K pot from radio shack to measure the upper and lower limits with the scan tool, it's alot cleaner than trying to mess with a used fuel pump module. I set the pot to read "full" on the gauge, read the voltage over OBDII with the scan tool then measure the resistance on the pot. I can do the same thing with the coolant temp. |
October 14, 2011 by Ralphxyz |
Are you sure the OBDII is directly reading the device? I thought OBDII reads the car's computer which reads the various sensors? Or is there an analog mode on the scan tool? Also some sophisticated electronics systems will multipurpose a wire and depending on what is being seen on a wire behaves in different manners so when your scantool "sees" a pot (constant voltage) on the wire it behaves one way and when it sees a data stream on the same wire behaves differently. Anyway if it is a voltage you are needing to monitor than a pullup resistor should not be necessary, to answer your question. Ralph |
October 14, 2011 by tonysq |
I'm sorry, I didn't mean to confuse the issue more. The ECU is reading the sensors. The scan tool uses OBDII to get the info from the ECU. So, PC0 to a resistor to ground will give me a voltage value? Experiment time tonight. Thanks |
October 14, 2011 by tonysq |
I think this is what I'm trying to go for... http://ikalogic.com/proj_dig_thermo.php I kept wanting to say thermistor but didn't think I had the right term. Chris |
October 14, 2011 by Ralphxyz |
Ok now:
No! The ADC (PC0) is expecting 0 to AREF (with Nerdkit standard setup @5 volt). Now as to the "thermistor". A thermistor is a variable by temperature resistor. To use a thermistor you would stick one leg into a + voltage source and the other leg to PC0. Ralph |
October 14, 2011 by tonysq |
Thanks Ralph. The problem is that in the automotive world they stick one leg to a common ground and the other (measured signal) leg actually has +11.72v DC supplied by the ECU. I just took a reading with the meter and that's what it says. I reversed the +/- leads and the common wire to all the sensors is definitely the ground side. That why I think I'm looking for a voltage divider setup. I had the mis-understanding that it was 5v so I'm glad I never hooked it up to the breadboard.... sheesh. Measure twice, cut once. Chris |
October 14, 2011 by tonysq |
Ok, Here are the results of my work this evening. I set the potentiometer attached to the ECU, read the R with a meter and the V reported by the ECU to the scan tool. I then recorded readings through a full resistance range to a table. I set the V to Vout and the R to Rb in the formula:
When I solved for Ra:
The Ra values for the voltage divider resistor were all over the place from 500 ohms to 950 ohms... After banging my head against the math wall for a couple hours I ended up deciding I was just going to set up my voltage divider circuit the way I wanted to so that I could get a useful reading. I went back to the original formula, set the upper and lower Rb values from the actual sensor and used the WAG method to set Ra at 500ohms. I started out with 5v Vin and generated a list of expected Vout's. Just for kicks I set the Vin to 12v and my Vouts almost EXACTLY matched what I saw on the scanner. Close enough that rounding by the ECU could account for the mismatch. So here's my dilemma. If I stick with Vin = 5v then I get Vout from 0.2v to 1.5v. If I set Vin = 12v then I get Vout from 0.5v to 3.66v which gives me twice the resolution. BUT, if the Rb goes to an open then the full 12v will be sent to the microcontroller, right? Is that bad news for the chip? What's the best way to handle that? Thanks a bunch for the help |
October 18, 2011 by BobaMosfet |
tonysq- If the sensor acts like a variable resistor, then being in series, you should be able to tap off of either sensor lead, go through a 1.8K Ohm resistor (Dissipating 27.2mW), which is to drop the voltage level to not more than 5V at about 3.8mA, max, right into your ADC input pin. Engines generate a lot of noise; you might also have to put a ferrite bead on it, and a small bypass capacitor (.1uF) to help smooth out the signal. BM |
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