Waterproof a LM35 Temperature Sensor

Here is a instructable to waterproof a LM35 for use on a tethered ROV using a automobile 12V battery as a power source. This came out of a need for the MATE ROV Competition. The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius (Centigrade) temperature. Here is the datasheet - http://www.national.com/ds.cgi/LM/LM35.pdf

Step 1 Cat-5 used for the tether

I used cat-5 cable for the sensor as it will shield the sensor signals on the twisted pair (and I had a spool sitting around.) It will also easily attach to the tether.

I used six wires (three pair) for the sensor (Vs+, Vout, Gnd). I twisted colored pairs together to give me the three conductors.

Comments

[qark123]

Hi, would you please explain the function of the stabilizing resistor in more detail? I understand that there is no current flow through it, so it doesn't change the temperature value, but what does it do?

[solarmax]

Check out the datasheet at http://cache.national.com/ds/LM/LM35.pdf . Figure 2 shows the resistor from Vout to Ground.

[JohnTom]

Fig 2. shows the resistor connected from Vout to -Vs. This is not Ground, but a negative bias voltage to allow you to use the full range of -55C to +150C Your probe will measure +2C to +150C The resistor is not necessary in your circuit.

[etscheid]

solarmax, I think you meant Figure 3 of the LM35.pdf (& series, like you have it shown in the pic). JohnTom, I believe the series resistor is good to have there.

qark123, if you do DC analysis, there's practically 0 current. But, there is current in transient analysis. The problem is that a long cable can have a lot of capacitance, and when the little lm35 needs to change the Vout, it has to charge or discharge that capacitance to change the voltage. This series resistor will help limit how much current it drives at once. Another other (& maybe more important?) reason for an external resistance (instead of the device just having a larger output resistance of it's own), is in case there's any transient reflections from driving into a transmission line like a cable. The reflected voltages can cause a potentially harmful (to the lm35 or receiving device) voltage level. Finally, like solarmax referred to by saying 'stabilize', since the R slows the driver down, avoiding/minimizing reflections in the cable, the sampling won't see an incorrect voltage due to reflections either.

Anyway, you can see that avoiding transmission line effects is a good thing. The net effect is that a larger R like this will make it a tiny bit slower for a new Vout voltage to show up since it's being slowed by the resistor. But, you'd probably have to have a bigger R &/or much bigger capacitive load before the slower responsiveness was noticeable.

[dr.knowitall]

actually, 16.7mV would result in 1.67C thus giving us a reading of 35F.

you convert mV to V by dividing mV by 1000. which would give us .0167V.

the LM35 is .01V/C, so we then divide our .0167V by the .01 and we get 1.67V/C then you can convert to F from there.

[skillett]

maxim/dallas has way cool CHEAP parasite power temp sensors in TO-92 cases and you can get FREE samples. link:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2815

They also have a bunch of software to read from them. they can be tagged with XML. if needed. you can put a bunch of them on a wire and they can tell temps for each one due to a 64 bit serial number. you can cobble together a serial interface for a couple of bucks instructions here:

http://martybugs.net/electronics/tempsensor/hardware.cgi

there is a opensource app called digitemp for/linux/winDOZE
link here :
http://www.digitemp.com/software.shtml

enjoy,
skillett

[daleth]

Wow—these look absolutely perfect. Since 2.5 years have elapsed since this comment, is there an Arduino library for these yet? ;)

[solarmax]

Thanks! I'll check those out.

[skillett]

PS. The DS18S20 can be programmed to set off an alarm at a specific temp too. It does centigrade AND farenhight

[mooseo]

This is definitely slick, but may be overkill in many situations.

I've had good luck just sticking them in adhesive-lined heat shrink tubing. Heating that will seal around the cable and then I crimp the outer end of the tubing with a pair of pliers while it is still soft. The plastic won't have the same temperature conductivity as the copper, but if you aren't looking for fast response, it's OK.

Another thing I've done is use the 8-pin SMT version of the LM35 and for the resistor. They end up being a smaller package that slips into the heat shrink. Don't drink coffee before you solder up the leads.

[handidad]

An easy method of sealing the squished end of the 1/4" copper tube would be to solder it. But do this before inserting the temp sensor.

[LinuxH4x0r]

Could you give more info about the sensor and how to use it with a voltmeter? Thanks Btw, great ible

[solarmax]

The LM35 (and LM34 for F temps) takes a voltage in (from 4Vdc to 30Vdc) on the on the left lead (see the photo above) and ground on the right lead. Then, it adjusts the output voltage of the center lead (referenced to ground) to whatever the temperature is (10.0 mV/°C) - from 0C to 50C in the case of this little guy. Very ease to work with...but, I'm glad I bought extras. I broke one of the leads while manipulating the sensor during this procedure. Best thing is the voltage input range of this can be a button-cell or a car battery...output is consistent.

BTW, some guys have taken the output of this sensor (10.0 mV/°C) and multiplied it (using a LM324 op-amp) to give a 0-5V output. Then taken that and plugged it into an analog input of a robot controller. http://tinyurl.com/62lvoc (.pdf)

[LinuxH4x0r]

Thanks for the info. I might buy some of these.

[solarmax]

You're welcome.
BTW - $1.69 USD at Jameco - http://tinyurl.com/64zjkt

[LinuxH4x0r]

I might just get some free ones from ti Thanks though

[joejoerowley]

Cool! Very Nice!

[Ora]

I had no idea that there were such simple temperature sensors, this a very well documented instructable, and has inspired me to use these in an upcoming project or two.