Video delay line rain sensor

[dr pepper]

Just playing around, I have some Quartz video delay lines, I was thinking if I was to seal the transducer end with epoxy and expose the glass to the weather, could the thing be used to detect rain?
The process as I get it uses sound waves, the transducers bounce a sound wave around the glass, picking it up 1 video line later.
So if the thing got drops of water on it the wave would be deflected and or attenuated, a circuit to detect time delay & amplitude would then be able to sense this.
I guess one issue would be to seal the transducer end up without messing up the operation of it, maybe replace the transducers for laser & photo transistor (making sure the laser is aimed at the ground).
Any comments on this?

 

[betwixt]

I haven't seen a quartz delay line in years!
You would have to experiment but my guess is it wouldn't work. Almost all the delay is from reflections internal to the slab of quartz so the external influence of a relatively small water droplet would probably be insignificant. You would have to send a burst of a few cycles at a few MHz and note perturbation in the output waveform due to premature, delayed or attenuated passage.

A simple conduction test, funnel and weight sensor (or paddle wheel) or even optical reflection would be far easier to implement.

Brian.

 

[dr pepper]

I have a tipping bucket gauge, but its not quick enough, the observatory would be soaked by the time the bucket tipped.

I was thinking maybe the quartz would reflect light in the same way it reflects sound waves, maybe not.

Probably too flimsy anyways, I have a sheet of 10mm clear poly, I'll cut a chuck out of it & polish up the edges, not sure how I'll do that, maybe see if theres a vid on youtube.

 

[betwixt]

I have not tried it myself but another solution might be a capacitive sensor, either a charge detecting type (= detecting the addition mass of a water droplet) or a coupling type. Interleaved fingers on a PCB with one set driven and the other sensed would show increased output if a droplet increased the dielectric coupling between them. It might even work with the fingers on the 'dry' side of the PCB.

Brian.

 

[dr pepper]

Agreed,
I have ordered a cheapo soil moisture sensor just to play with, it has the same principle 2 electrodes measuring the capacitance of the soil, a 555 generates a few hundered kc, and an integrator generates a dc o/p, the more moisture the better the coupling and higher dc o/p.
I'll reverse engineer the moisture sensor & see if i can come up with something that will sense rain, maybe a copper clad pcb as a capacitive sensor with a thin sheet of plastic over the top.
I'm trying to avoid using loads of power with a heater to speed things up, I'll try having the sensor at an angle, at most a vibrator motor to shake water off.

 

[d123]

Hi,

Like the soil moisture sensor, circuits called water-level detectors and lie detectors (all the same thing) might be cheap, low-power solutions to sense a drop of water closing a circuit between two electrodes/tracks on a bit of PCB, as you are describing.

An oscillator made from e.g. CD/HC/etc. 4049 or 4069 ICs can be used that is (presumably) lower power than a 555.

--- Updated Sep 25, 2020 ---

Hi,

Simulated this ages ago. Maybe only viable in sim-world. Maybe it's absolute rubbish. Idea behind bidirectional signal is so that electrodes don't corrode. LEDs wouldn't be there for a water level sensing circuit...

 

[dr pepper]

I have done something similar to that to detect water level, however this time I want to avoid any electrical connection due to corrosion, Ok AC isnt going to promote electrolysis, however in the uk even stainless left outside goes dull, so I want to go capacitive sensing with the electrodes sealed away from the elements.
My circuit has an osc just like the upper section, only the signal Vfa goes to one side of a sense capacitor, and the other side of the capacitor goes through a 1n4148 and a cap to ground, to integrate the o/p level, water on the sensor increases the capacitance and more energy gets coupled through and hence the o/p voltage goes up.
This then goes to an analogue in on a processor whereby if the voltage exceeds a setpoint the processor says its raining, it might even be feasible to have a long delay integrator in software to compensate for contamination on the sensor.
The soil moisture sensor is just a couple of tracks on a pcb sealed up with solder resist.

 

[betwixt]

I too have done a similar thing for liquid detection but in my case it could ignite so the voltage had to be very low. I did it using an MCU driving alternating probes but instead of reading back a direct voltage I tied a capacitor between the probes and took two measurements from the UN-driven probe with a fixed time interval between them. The capacitor and series resistors to the MCU resulted in almost an AC sine wave across the probes and the difference between the two measurements indicated a change in conduction between them or an imbalance of loading between them. Essentially it used the change or imbalance of discharge curves to sense the liquid. The beauty of this method is it is inherently tolerant of component values and self calibrating but it does involve some additional circuitry around the probes.

Brian.

 

[dr pepper]

Sounds almost like a liquid version of a lvdt or resolver.

 

[betwixt]

Not sure the technology is quite the same as lvdt but it could detect 1M between the probes or 50pF from one probe or the other to ground. It was a safety device that cut the AC power instantly if any liquid leakage was detected. I can't reveal the exact design or MCU software without shooting you.

Brian.