Rain Sensors: Detector In progress

Requirements

There are two requirements for the rain detector:

An alarm is sounded when rain is detected.
The alarm must have an on/off switch so we can switch it on only when needed, for example, when there is washing drying outside.

Design

The rain detector comprises an off the shelf sensor pad containing two conductive tracks that are not connected to each other. Water falling on the plate bridges the tracks and lowers the resistance of the pad. A MH series interface module is connected to the sensor pad and produces both an analogue signal whose voltage depends on the resistance of the sensor pad and a digital signal that goes from high to low when the sensor pad resistance reaches a certain threshold value. A potentiometer is used to adjust the threshold. Both items were purchased as a single package along with a very short DuPont cable to connect them:

Rain detector sensor pad and interface module — Figure 1: Sensor pad and inteface module

The output of the interface module will eventually be used to trigger an alarm. This has not yet been designed.

Construction

A housing for the rain detector was designed in TinkerCAD and 3D printed on a Bambu A1 using Amazon Basics orange PLA. The housing includes two lugs to enable the unit to be screwed to a wooden base. Brass M3 heat-set threaded inserts were fitted to each corner of the housing to enable the rain sensor to be screwed in place. A hole was included in the base of the housing to permit water to escape.

The following photo shows the housing after the heat-set inserts were added:

Rain detector housing — Figure 2: Detector housing

The rain sensor pad was attached to the housing with nylon M3 crosshead bolts:

Rain detector sensor pad fitted to the housing — Figure 3: Sensor pad fitted to housing

After a rethink it was decided that it would be better to mount the detector at an angle to enable rain water to drain from the detector more effeciently. At first it was planned to fasten the housing to a triangular shaped piece of wood that would in turn be fastened to the level wooden base. Instead of this piece of wood an base for the housing was designed that had a 22.5° angled top. The base has two pairs of lugs, one pair to be bolted to the existing lugs on the housing with the second pair being used to fasten the base to the wooded baseboard.

The base was designed in TinkerCAD and 3D printed using the same printer as above, but this time using Bambu Labs basic black PLA. The base was printed with thicker walls than the housing for extra strength. The change of PLA was only made because I'd run out of the orange PLA used for the housing.

Detector base unit as printed, with and without supports — Figure 4: Detector base as printed, with and without supports

The base was then attached to the housing using 2 × M4 nuts, washers and 27mm bolts. The 27mm M4 bolts were too long, but were all that were in stock. They were cut down using a slitting disk mounted on a multi-tool. After the bolts were shortened the nuts were fixed in place with superglue. The housing was then blackened using black permanent marker pens. The following photos show the assembly before and after blackening.

Detector base attached to housing. — Figure 5: Base and housing assembled.

That's as far as things have progressed as of 2015-02-15.

STL files

STL files that can be sliced to produce the 3D printed components can be downloaded from the cahamo/rain-sensors repository on GitHub.

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