Topic: Measuring the electrical characteristics of the Davis Leaf Wetness sensor

[SLOweather]

I decided to see what I could learn about the Leaf Wetness sensor without destroying it.. I made a breakout cable from a surface mount modular jack and a 4 conductor pigtail, and inserted that between the sensor and the jack on the SIM board.

Green is ground.
Black is nominal 3VD power, on for a fraction of a second every 15 seconds or so.
Red is 3VDC from the sensor back to the SIM, presumably as afeedback indicator that there is a sensor plugged into the jack.
Yellow is proportional sensor voltage out, again, just for a fraction of a second every 15 seconds or so. Dry is ~0.04VDC, soaking wet is ~2VDC. Misting the sensor with a spray bottle of water raised the sensed voltage incrementally.

Now on to the fun stuff. I wasn't going to try to probe the gold sensor fingers once every 15 seconds to see what was going on, so I disconnected the sensor and brought it to the bench.

 Then, I fed it steady 3VDC from a bench supply, and put my venerable Fluke Scopemeter across the fingers. There are 2 sets of interlaced conductive gold plated fingers.



As I have surmised in the past here, there is an AC voltage across the grid, nominal 2.6 volt square wave at ~600 Hz.

So, buried under the potting at the top of the sensor is some sort of oscillator to convert the DC power to the 600 Hz square wave, some way to measure the impedance change (probably) across the grid as moisture collects on it, and some way to convert that impedance back into a proportional DC voltage for the SIM.

[SLOweather]

This just in... Another data point: I watched the AC voltage as I misted the sensor. The voltage is inversely proportional to the wetness. So it must be current limited with a resistor or something.

And, the pulsing of the supply voltage is important to the sensor output voltage process. I only got varying output by quickly switching the power supply on and off.

[miraculon]

It is good that they are using pulsed AC, since DC would cause plating effects that would erode the foil pattern eventually.

Greg H.

[Bashy]

It is good that they are using pulsed AC, since DC would cause plating effects that would erode the foil pattern eventually.

Greg H.

Yes, i had had eroding with a home made Rain Duration unit that is pretty much a LW sensor

Nice to see you having a play SLOweather, it makes things difficult when its £££ / $$$ worth of sensor :/

Should i expect schematics on the copied version anytime soon???  (tongue in cheek)

[SLOweather]

Should i expect schematics on the copied version anytime soon???  (tongue in cheek)

I'm hoping that the measurements I made will help one of the electronics whizzes here come up with a circuit.

OTOH, that potting is making me curious. For some reason, I always thought it was rubber, like on the temp/hum sensor. It's not, it's black poured rigid epoxy.

[SLOweather]

Been thinking... The soil moisture sensors require a similar AC excitation to avoid damage. I wonder if the leaf wetness circuitry is substantially similar or identical to that which is on the SIM board for the soil sensors... I clipped my butt set across a set of soil terminals. I got a blip about every 1:08 minutes, too short to hear a tone.

[dupreezd]

SLOweather, a couple of question regarding your measurements.

1. I see on your scope that the signal is  plus and minus 2.6v AC. (5.2V peak to peak). Was your scope ground connected to the green ground wire?

2. With the sensor completely dry, do you see the signal on both sides of the sensor grid or just one side?

Thanks

[SLOweather]

SLOweather, a couple of question regarding your measurements.

1. I see on your scope that the signal is  plus and minus 2.6v AC. (5.2V peak to peak). Was your scope ground connected to the green ground wire?

No. It was across the 2 sides of the interlaced grid fingers. And, as noted, that was dry. The voltage drops as the grid becomes wetted/misted.

2. With the sensor completely dry, do you see the signal on both sides of the sensor grid or just one side?

See above. I remounted it a couple of hours ago. I'll have to bring it in again and do a little more testing. As I recall from a previous less involved probing of the sensor but forgot to mention here, there was no direct connection (as measured with an ohmmeter), between any of the wires and either side of the grid.

[dupreezd]

This circuit combines two basic circuits together, an oscillator and an amplifier/buffer.

A 12V center tapped DC power supply is used to provide +6v and -6v to power the 741 op amp.This makes it easier to bias. +6v is also used to power the 555 timer.

The 555 is setup as a free running oscillator. Calculated frequency with the component value should produce 20Hz. Feeds alternating pulses to one side of the electrodes of the sensor.

On the other side, the diode, resistor and capacitor rectifies and smoothes the AC voltage, feeding the 741, producing a DC voltage

A dry sensor will produce an output of 0V and ~3V when completely wet.

Please check and comment if this is workable. Yep, the voltages used are a bit funky, but this might be a starting point to tweak the design.
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[kobuki]

Tangentially related, but nevertheless an interesting read regarding the same technology - it might give more ideas. I'm not associated with them in any way but was thinking of getting one or 2. More docs.