Is it time to mow the grass?

Experts,

I want to make a sensor & reflector use light levels to determine if grass has grown high enough to mow, based on how little light can make it back to the sensor.

  • So imagine a retroreflector (hard or tape/sticker) mounted low on a tree/post/whatever.
  • A bright, directional light source, like a red led (example).
  • A CdS sensor (light sensor).

Operation:
Periodically, the sensor would kick on, and a few pulses from the laser diode would fire, and the level returned (to be differentiated from ambient) recorded. What is interesting/convenient is that the sender/sensor and reflector need not be the same height over the lawn. This makes mounting options more flexible, as the lower to the ground a μC housing and sensor are, the more prone they are to interference and other environmental nasties.

Theory:
Over time, one would figure that less light would be returned as more blades of grass got in the way. By comparing the levels with grass height, it should be possible to tell when a good time to mow would be.

Problem solved:
Mowing too often, or not frequently enough, as rain & sun do not operate based on a calendar. "Every 2 weeks" doesn't take into account positive or negative growing conditions.

Right now, everything is easy, except for a cheap retroreflective target for the LED laser diode to point at...

TIA!

P.S. For those who don't know, "Retroreflective" does not mean 80s fashion. It means that like prismatic reflectors on things like the moon, a light shone on it from any direction will be returned to that same direction. This is different from a normal mirror, where the reflective light needs to be from a specific direction. Some call this a cat's eye effect.

Modulated IR (38kHz) and a 3-pin receiver seems more appropriate outdoors than a laser and LDR.
Aiming an IR LED is also much easier than a laser.
See this thread.
What sort of distance are we talking about (twice the distance for a reflector setup).
Leo..

Manufacturers who make industrial optics (like Banner), sell those (corner reflectors).

A cheap safety reflector will generally serve.

With a 50mm reflector, the grass detection near the reflector can be 50mm off.
You don't have that problem with an IR LED and 3-pin receiver at opposite ends.
Leo..

Wawa:
With a 50mm reflector, the grass detection near the reflector can be 50mm off.

Black paint! :grinning:

How about a microphone? When the grass gets long enough for it to "tickle" the microphone, its time to mow.

-jim lee

Paul__B:
A cheap safety reflector will generally serve.

This is exactly what I tried, and it works! For less than $1, and they are actually relatively sturdy. They need to be, as they are the end mounted low to the ground, where a wed-whacker would hit it.
Great idea!

Wawa:
Modulated IR (38kHz) and a 3-pin receiver seems more appropriate outdoors than a laser and LDR.
Aiming an IR LED is also much easier than a laser.
See this thread.
What sort of distance are we talking about (twice the distance for a reflector setup).
Leo..

Leo,
The distances are environment-dependent. If there is a tree 20' away, I need to use it. I have a solar panel to augment the battery (small panel), so if not on the south side of building, or at least gets decent sunlight, putting a unit on the north side, or in permanent shade, will impact the solar panel effectiveness. It will all be dependent on a place on the building that is within a distance of a reflector's location/radius (still to be tested).
I see adding an RTC to make sure readings are taken at night, along. Perhaps 2-4 readings, averaged, as grass moves. So it is more important that the readings be made than the solar panel be used.
Only testing will tell what that distance should be.

jimLee:
How about a microphone? When the grass gets long enough for it to "tickle" the microphone, its time to mow.
-jim lee

Jim,
This is a great idea. There are some areas for poor readings, like the unit being on a building with no grass under it. Also, playing the negatives, the grass would need to move at the same time as the sampling. But yeah, the 'tickling' would be loud on a tiny microphone... Maybe as a secondary sensor... It wouldn't add any cost...
Hmmm... :slight_smile:

Wawa:
With a 50mm reflector, the grass detection near the reflector can be 50mm off.
You don't have that problem with an IR LED and 3-pin receiver at opposite ends.
Leo..

Leo,
Luckily, all readings are relative. Each location would be different. But firstly, I cannot run underground wiring to the 'other' sensor. Electrically, the unit has to be self-contained. And with a retroreflector, both IR and laser-pointer LED can be used simultaneously, though with a slim power budget, just having WiFi is costly...

I can see considerable difficulty discerning grass "tickling the microphone" from every other noise source. :astonished:

holesflow:
I want to make a sensor & reflector use light levels to determine if grass has grown high enough to mow, based on how little light can make it back to the sensor.

Will this be a chindogu?

Funny!
But no. The device will have the usual environmental sensors and well. And it isn't to alert/signal the people watching the grass grow (like you need a device to do that for you), but rather services or companies hired to mow your lawn.

Will this be a chindogu?