# temperature sensing using 2 wires

#### Dollarday

Joined Jan 25, 2012
32
Hi everyone

I would like to know, do you know of any temperature sensors which you can use to measure multiple points, but only requires 2 wires to function? (In other words, you only require 2 core cable, but can measure temperature at mutliple points)

One such IC is the Maxim DS18S20... Are there any other IC's that can accomplish this? (Which are preferably much cheaper?)

Regards,
Dollarday

#### ErnieM

Joined Apr 24, 2011
8,183
There are none. A temperature sensor can only sense the temperature at the place where it is, excepting of infra red sensors that sense the light emitted from a distant object and infer the temperature.

There are some 1-wire (that also need a ground), many voltage devices (2 or 3 wires), and also several I2C devices (4 wires); I2C has the advantage of those same 4 wires may service multiple sensors.

#### spinnaker

Joined Oct 29, 2009
7,835
There are none. A temperature sensor can only sense the temperature at the place where it is, excepting of infra red sensors that sense the light emitted from a distant object and infer the temperature.

There are some 1-wire (that also need a ground), many voltage devices (2 or 3 wires), and also several I2C devices (4 wires); I2C has the advantage of those same 4 wires may service multiple sensors.
I'm confused. You say there are none then you say there are some. One wire devices like Dallas DS18B20 can be used with 2 wires. One ground one sense / parasitic power.

But I am also confused because the op is already aware of those sensors.

#### ErnieM

Joined Apr 24, 2011
8,183
I'm confused. You say there are none then you say there are some.
When I said there are none I am referring to the OP's request for "temperature sensors which you can use to measure multiple points"

There are many that measure a single point given a simple interface.

#### spinnaker

Joined Oct 29, 2009
7,835
When I said there are none I am referring to the OP's request for "temperature sensors which you can use to measure multiple points"

There are many that measure a single point given a simple interface.

The one wire sensors from Dallas can measure multiple points. Each device has a 64 bit unique serial code. Not sure of the limitation (if any) but several can be multidropped on the same pair of wires.

The code to parse the serial numbers is a bit hard to understand but not too awful bad. I have a library in C that I need to drag out of the closet for an upcoming project.

#### MrChips

Joined Oct 2, 2009
24,385
As spinnaker says, there are 1-wire sensors that actually use two wires, signal and ground.
I have built such devices using simple microcontrollers all networked across one wire.

#### Dollarday

Joined Jan 25, 2012
32
Ah, I understand where the confusion is. Let me rephrase: Are there any temperature sensors in which a number of these sensors can be placed on multiple points along a cable which you can use to measure multiple points, but only requires 2 wires to function?

In other words, you only require 2 core cable, but can measure temperature at mutliple points by connecting multiple sensors to the cable at each point where you would like to measure the temperature.

#### MrChips

Joined Oct 2, 2009
24,385

#### Jaguarjoe

Joined Apr 7, 2010
767
If you are measuring the average temp, you can string some LM335Z's in series. If you want minimum temp, put them in parallel. These are 2 wire devices. GND and signal. You'll need to supply a ~6.2k current limiting resistor.

#### spinnaker

Joined Oct 29, 2009
7,835
Ah, I understand where the confusion is. Let me rephrase: Are there any temperature sensors in which a number of these sensors can be placed on multiple points along a cable which you can use to measure multiple points, but only requires 2 wires to function?

In other words, you only require 2 core cable, but can measure temperature at mutliple points by connecting multiple sensors to the cable at each point where you would like to measure the temperature.
I answered you long ago. It is called One wire and Dallas is one of a number of companies that manufactures such devices. See above.

#### Dollarday

Joined Jan 25, 2012
32
Hi Spinnaker, I refer to my original question:
"One such IC is the Maxim DS18S20... Are there any other IC's that can accomplish this? (Which are preferably much cheaper?)"

This is in fact the same one wire temperature sensor from Dallas, which is a subsidiary of Maxim-IC.

The DS1820 is a one wire sensor from Dallas, I'm looking for a similar IC which is cheaper.

#### spinnaker

Joined Oct 29, 2009
7,835
Hi Spinnaker, I refer to my original question:
"One such IC is the Maxim DS18S20... Are there any other IC's that can accomplish this? (Which are preferably much cheaper?)"

This is in fact the same one wire temperature sensor from Dallas, which is a subsidiary of Maxim-IC.

The DS1820 is a one wire sensor from Dallas, I'm looking for a similar IC which is cheaper.
No idea. You will have to search.