Aspirated air temperature sensor for a Polytunnel

In the UK air temperature is normally measured in a passively cooled Stevenson screen. The louvred design of the screen allows air to flow around the thermometer. The trouble with a polytunnel is there is no wind at all, as a result the sun heats the sensor up and without airflow you don’t know by how much.

By running a computer fan driven off a solar panel I can move enough air past the sensor to exchange the heated air from the sun shining on the sensor. For the sensor I use the standard Chinese supplied DS18B20 encapsulated in a stainless steel tube

Dallas DS18B20 epoxied in a stainless tube housing, from a Chinese Ebay supplier
Dallas DS18B20 epoxied in a stainless tube housing, from a Chinese Ebay supplier

The sensor is housed in a 6cm piece of white plastic waste pipe

sensor mounter in centre of white waste pipe
sensor mounted in centre of white waste pipe

The fan is mounted at the top of the pipe, designed to pull in air from below; this way the sensor is not heated by air passing the fan motor, and the airflow works with the natural tendency of warm air to rise. I’ve tried to keep the airflow as unimpeded as possible.

side view - the flange for the fan is made from a piece of wood glued to the pipe
side view – the flange for the fan is made from a piece of wood glued to the pipe

Looking at the results there is a difference of a few degrees

the difference opens up a few degrees at high temperature

between the aspirated sensor and another sensor mounted on the outside of the plastic tube. They track at low temperatures but not when the sun is shining – the difference here is about 6 degrees, even in March, before the vernal equinox. It is remarkable just how much the air temperature swings – 27 degrees on a couple of days which still have hazy sun.

Sensor mounted in polytunnel
Sensor mounted in polytunnel

Weatherproofing the sensor is easier in a polytunnel because as well as the wind not blowing, it also doesn’t rain. I can use a cheaper indoor solar panel, the one I used is a 12V 1.5W unit, Maplin L58BF bought on sale for about £6, not the £20 they seem to be charging for it. even £6 is a little dear! I extracted the flashing blue LED and series diode to maximise the power available to the motor. This also charges the battery of the temperature sensor dual unit, which reports back to the collecting station using Ciseco’s XRF every 10 minutes.

Solar panel schematic
Solar panel schematic

The computer fan was a 12V brushless unit but I run it at about 7V, we’re not after blowing a gale through the tube. It will start at 5V. The Zener is there to limit overcharging of the 4.8V NiMH battery pack in the electronics to about 4mA. It only reports every 10mins so this is enough. The 1N4148 diode stops the battery discharging back through the fan and solar panel in the night. I should really measure what the leakage current of that Zener is 😉

I used a PIC 16F628A driving a Ciseco XRF to send the temperature data from two sensors back. Nowadays I would use the Ciseco RFu which includes an Arduino and low-power standby mods to make this cheaper.

Other implementations

This is a nice weatherproof design – I can’t work out if I missed a trick with using just one plastic tube rather than a coaxial design. Lots more ideas here.

Postscript (July 20 2015)

five months of data
five months of data

This rig works reasonably well; if power were available I’d run the fan all the time in daylight for a more rigorous result on summer cloudy days. The biggest problem in a polytunnel is that they are shockingly dusty places, and you have to sponge the dust of off the solar panel every month or so.

The Texas LP2950 oscillates as an adjustable regulator

Nowhere in the datasheet does Texas tell you “hey use this fixed regulator as an adjustable”. However, I’m used to being being able to do that with the venerable 78XX series – indeed Texas tell you that you can do that with the 78L05 datasheet in Fig 14.

Adjustable 78l05. bear in mind the shocking Iq of 3mA that'll stand you up an extra 3V if resistor R2 is 1k, keep 'em low...
Adjustable 78l05. Bear in mind the shocking Iq of 3mA that’ll stand you up an extra 3V if resistor R2 is 1k, keep ’em low…

Given that there’s an adjustable variant of the LP2950 that appears on the same datasheet (the LP2951) I laid out a PCB and being the lazy sort I am I assumed that since I was using a load of these parts in their 3.3V KY5033 variant, where I wanted an 8V stabilised voltage for an audio mic amp sourced off a 12V supply I can simply do the LM317 trick, drop in a couple of resistors from the output to ground and the ground pin to real ground, job done.

what I planned...
what I planned…

For this I made R1 6k8 and R2 10k.I expected an output voltage of 3.3+3.3/6800*10,000=8.2V or near enough. I screwed up labelling the o/p 10V, mistakes happen…

What does that look like then?

Oy vey, about 4V of massive oscillation (I’m using 10x probes). At least it’s centred on the right value-ish. Let’s take that output capacitor out

1503_lp2950_openLooking good, only 1V of oscillation, now at 370kHz or thereabouts.

So if you come here from Google wanting to know why the LP2950 doesn’t work as an adjustable reg, now you know. There is a tiny clue in the datasheet in the ground current variation

LP2950 variation in ground current with output current

which varies by two orders of magnitude with a load current variation of 1000. This will be impressed upon R2, varying the target voltage – as more current charges the capacitor the target voltage will rise, then ease off as it is charged, making a handy relaxation oscillator.

There’s another clue that the output cap can give interesting results in this line


which actually specifies a ESR range, rather than less is better

No criticism of Texas’ product implied – these are great little fixed voltage regs with a low quiescent current and are my goto device for running 3.3V devices off a 5V rail because of that superb dropout voltage of 600mV max, across the entire range of load current and -40 to 125°C which is easily in spec off a 4.75V min 78L05. It’s just one less thing to worry about. Im future I won’t be a doofus and try and use one where a LM317L is called for 😉