Advice for temperature sensors

Forum 2005-2010 (read only) Hardware Interfacing
#1

Hello everyone. I am working to build an Arduino to do a few things for an electric vehicle that I converted. (mysmartev.com if you want to see it). Anyway the question I have is how best to monitor temperature for low cost and minimal wiring. I will be controlling a heater for 4 zones and I would also like to read ambient. So I will need 5 temperature sensors and the range of interest is to ~0-80F. Also one more for a high temperature limit somewhere in the range of 150-200F. (not decided yet)

It looks like the LM35 is ~$4 each. Maybe there is a better place to buy them?

I also found this gizmo (sorry for the format, but I can not post the url until I have other messages) mpja. c o m /prodinfo.asp?number=16370+ME for the same price. This would require a bit of work to figure out the sensor, but then I would also have the LCD to play with.

Can anyone recommend durable hardware to measure temperature and where to purchase in USA?

Thanks in advance.

Nathan.

#2

A cheaper solution is the MCP9700A, also available as a TO92. Uses the Arduino's power (3.3 or 5v) and gives you 10mV/C from -40C to 120C. Only $0.35 in the States. There is a 7901 which reads in Fahrenheit.

#3

Looks like the MCP9700A and the LM35 are roughly the same device, except for supply voltage. The LM34 is the F version of the LM35. Whichever you can find the cheapest will probably work.

Use twisted pair or shielded conductors if the sensors will be more than a few inches from the board. And you may have to compensate in software for voltage loss due to wire length.

In the USA, off the top of my head I use:

www.mouser.com
www.newark.com
www.jameco.com

And a few others I can’t remember. :wink:

Also look into setting the analog input reference voltage to lower than 5V on the Arduino. It will give you better resolution. One of the built-in options is 1.1V, which translates to a full scale input maximum of 110 deg. The default analog ref is 5V, which would be 500 deg.

#4

Just remember the LM34 shows 0v at 0C, while the 9700 shows +500mV at 0C.

Which means the 9700 can actually go as low as -40C! Although anything below under -25C, current through the thermister will actually warm it by 2 degrees!

Here's a video of a version I made with the Fahrenheit sensor: http://www.youtube.com/watch?v=vvnZzCHOTRQ

#5

Thanks to everyone for the advice so far.

I will be doing remote temperature sensing and it looks like the MCP9700A might need a capacitor at the device. This could be a hassle.

What about RTDs or thermistors? They look rather easy to implement.

Also can anyone comment as to the failuremode of the lm* or RTDs or thermistors? I have a bit of experience with thermocouples in industry and I know that when they usually fail an make an open circuit. This is easy to detect and then go into a fail safe mode. How about with the above devices?

#6

I hate thermistors!!

I've used the MF52 from canterm cause they were cheap: http://www.cantherm.com/products/thermistors/mf52.html

The forumla to clacuate the temperature based on the resistance is super ugly: http://www.cantherm.com/products/thermistors/rt_characteristics.html

Plus we noticed that they weren't very responsive. The bead had to be very close to the temperature you were trying to sense, and then the item would have to be there for a good 20 seconds for it to sense the temperature (We had the thermistor on a robot that was supposed to go through a course and sense little candles and trigger an alarm when it did).

However, if you are trying to measure the temperature of a room in general this is not a big deal.

#7

I will be doing remote temperature sensing and it looks like the MCP9700A might need a capacitor at the device. This could be a hassle.

The 0.1uF Cbypass is to suppress noise picked up on the wire, and should be on the processor side.

Response rate of the MCP is virtually immediate.

#8