Sensors with resistance outputs

All sensor specific discussions! Temperature, pressure, steering angle, brake and throttle, etc. Post adaptations of OEM-style sensors, and also your clever DIY hacks and custom designs here too!

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rharris19
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Location: Houston, TX

Sensors with resistance outputs

Post by rharris19 »

Forgive my lack of an even elementary of how these things work. My miata's fuel and coolant temp sensor both give a resistance (Ohm) signal as an output, but the race capture wants 0-5V. What do I need to do to make this work if possible?

brentp
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Post by brentp »

Hi,

For a resistance style sensor you will need a pull-up resistor. If the sensor is two leads, one lead goes to ground, and the other lead is connected to the Analog input of Race Capture/Pro. Then, you would connect a resistor between the analog input and the 5V voltage reference on RaceCapture/Pro. This creates a 'voltage divider' for the sensor that allows a varying voltage to be seen by the analog input.

Clearly, a diagram would be helpful here!

Selecting the resistor value is depends on the resistance range of the sensor. 2.2 K for ECU style sensors, which have a 700-3000K range; 240 - 1K ohm for a gauge type sensor (25-240 ohms).

However, are you tapping into the existing sensors on your car, or are these separate sensors you're connecting? If they are existing sensors the car's wiring may already have the pull up resistors in place. Let us know either way.
Brent Picasso
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rushy
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Post by rushy »

If i wanted to use a Resistance Temp Sensor that has the following outputs

http://www.syxthsense.com/sensortable?subcategoryID=1

How would I then convert them into 0-5v outputs? For example which sized Pull up resistor would you recommend?

brentp
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Post by brentp »

If you're using that sensor you can use a 1K resistor as a pullup. Note, that sensor doesn't have a wide resistance range; about 600 to 1500 ohms according to the chart. With a 1K pullup that gives you about 2v to 3.9v input range on the port.

This is a handy voltage calculator. put 5v as the input voltage; then put 1K as R1 and the temperature sensor value (which varies) as the R2 value. It will calculate the output from the divider, which will be fed into the analog input of RaceCapture/Pro:

http://www.raltron.com/cust/tools/voltage_divider.asp
Brent Picasso
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rharris19
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Location: Houston, TX

Post by rharris19 »

Thanks Brent. I will have to do some research on understanding what you said, but I have a few people that could help me that would are more than capable of comprehending that.

I will be tapping into the OEM fuel level sender and possibly the OEM temp sensor. The Autometer Elite gauge I have for my water temp has a Data Logger output to go to a computer that i just found, so I should be good on that. I just confirmed it is a 0-5V output and I have all the values from autometer.

http://www.autometer.com/download_instr ... 8-00rA.pdf

The OEM Fuel Level sender on a miata is 7-95 Ohm if that helps.

brentp
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Post by brentp »

I actually have it on my short list to do a diagram for the installation guide, because it's such an important topic! I'll post here when I have it up.
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FieroLibre
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Post by FieroLibre »

Can someone post a picture of an actual connection they made for the pullup? I understand the logic, but not sure how to put the resistor in the line to a pigtail for the race capture box.

rharris19
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Location: Houston, TX

Post by rharris19 »

I used the online calculator you put up Brent and I am coming up with an even smaller resistor. My sender is 7-97 Ohm and it is looking like a 100 Ohm resistor would work well. Am I doing this right?

I could use a 240Ohm, but it would give me the range within 0-5V that a 100Ohm would. Does that really matter if it is only using a range of .14 to 1.4 volts with the 240 Ohm?

brentp
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Post by brentp »

Hi rharris,

Are you trying to piggy back off of the existing sensor for the dash, or is this a sensor that's stand-alone, dedicated to RaceCapture/Pro?

If you're piggybacking off of the gauge you do not need a pull-up resistor since the equivalent pull-up circuit is already in place, provided by the vehicle wiring. In this case you'd want to measure the voltage on the sensor output ranging from hot to cold and plot the curve between the two points. If the voltage goes over 5v, you will need a voltage divider to get it down to 0-5v.

The other option is to use a dedicated sensor. this is much cleaner electrically; it does not interfere with existing gauges and gives much more accurate readings since the voltage reference provided by RaceCapture/Pro is regulated to 5v.

This clearly deserves it's own wiki page!
Brent Picasso
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rharris19
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Joined: Mon Jan 07, 2013 10:40 pm
Location: Houston, TX

Post by rharris19 »

This is the OEM sender that I am tying into, but it is wired directly to my current gauge, which is an autometer digital fuel level gauge. I have gutted the wiring of the car and it is a single wire that goes directly from the OEM sender to the aftermarket gauge.

I will see if I can mess with a multimeter and figure this out.

If it is still a resistance reading, would having a range of only .14 to 1.4 volts using a 240 Ohm resistor be acceptable?

brentp
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Post by brentp »

So if the sensor is hooked up to the gauge you will measure a varying voltage as the fuel level changes - the gauge forms the other leg of a voltage divider circuit.

That's why you won't need to wire in a pullup resistor for the sensor; it would be redundant.

Using a volt meter, measure the voltage between the sensor terminal and ground when the tank is empty, 1/2 full and full. with that information you'll be able to plot a line between which you can use in the analog calibration process.

Please post the readings here and we will help you along!

Thanks,
Brent Picasso
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troubleonwheels
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Post by troubleonwheels »

So Brent, if I understand correctly...

-with a ONE WIRE resistance-type sensor-

1. That one wire is inserted into one of the Race Capture Pro analog inputs
2. A resistor is also inserted into that very same analog input
2b. The other end of the resistor is inserted into the 5v output

3. The value of this resistor is based upon the resistance range of the sensor within the probable operating range, determined using the handy calculator you linked to

The electrical logic is that of a voltage divider, pretty well described here about half way down under pull-up divider?

Clearly this should be a WIKI item. Also, starting an open wiki page where users can put the known values of sensors they've investigated would be useful. I suspect there is a HUGE knowledge base for such things in the MegaSquirt community?

Erik Torgeson
Workin on RaceTech Motorsports Charmaro

rharris19
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Joined: Mon Jan 07, 2013 10:40 pm
Location: Houston, TX

Post by rharris19 »

The input for the fuel gauge on the miata is a resistance signal that does not have a pull up in line before the gauge. The gauge itself does it. My question is that since I want to still have a fuel gauge in the car and it needs just a resistance signal without the reference voltage, could I split the signal line coming form the fuel level sender and install a diode inline going to the Race Capture so that it doesn't send the 5V signal back to the gauge cluster?

brentp
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Post by brentp »

If the gauge has the pullup built in, then you should be able to piggy back right onto the gauge, no diode needed. Here's what you should do:

When empty, read the voltage reading on the gauge. Use your meter to measure voltage between the gauge sense wire and ground. Record this number.

Now fill the tank. read record the voltage number again.

If both recorded voltages are less than 5v, you're golden.

Go into Race Analyzer and configure your analog channel to have a mapped reading.

For the first column:
put the empty tank voltage in the voltage cell.
put 0 (zero percent) in the scaled value cell.

For the 2nd column:
put the full tank voltage in the voltage cell
put 100 (100%) in the scaled value cell

for the 3rd, 4th, 5th columns repeat the value you put in the 2nd column

Save / write the configuration to RaceCapture/Pro.

Now you have that channel in RaceCapture/Pro scaled from 0 - 100% fuel level.

Wow, we need this documented in the wiki. This would be a great article.
Brent Picasso
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brentp
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Post by brentp »

Here's the beginning of wiring documentation, with a diagram on connecting resistance type sensors.

http://autosportlabs.net/index.php?titl ... ors#Wiring
Brent Picasso
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