For this module, I used the diagram published in n ° 293 (April 2005) of Practical Electronics. Its operating principle is based on capacitive detection associated with a variable frequency oscillator.
Its operating principle is based on capacitive detection associated with a variable frequency oscillator. The sensor has on its upper face (the one intended to receive rainwater, etc. ) the fine scratches making up the capacitive detection probe of approximately 100pF. On the underside, you can see the tracks of the 42Ω heating resistor. According to the manufacturer's data, and at a DC voltage of 12V, it can dissipate a power of 3.5W, or a current of 292mA. A maximum temperature of 106 ° C on the surface can therefore be reached. In the center of these resistive tracks, there is a temperature sensor of the CTN type with a value of 1KΩ at 25 ° C. These characteristics are very interesting, especially in the event of low temperature, snow or even ice.
Two astable oscillators, one at fixed frequency ( NOR gates IC102C and IC102D ), the other at variable frequency ( NOR gates IC102A and IC102B ), will generate a modification of the pulse width shaped by the 3 NOR gates. IC103E, F and G. All that remains is to proceed with an integration, that is to say to convert this variable frequency into an exploitable voltage. This is done thanks to a cell composed of the diode D102, C106, P101 and R104. The resulting signal is then directed to the input of an operational amplifier ( AOP LM741 ), whose adjustment threshold is effected by means of the potentiometer P102, in a range of values going from 0 to 12V. A small relay ( RE101) can then be activated by pin 6, through the light emitting diode ( LED102 ) and resistor R105. The management of the probe heating is based on an identical principle. The CTN detector forms a dividing bridge with the adjustable P103 and controls the non-inverting input of the second AOP ( IC105 ) through resistor R106 . A second relay ( RL102 ) identical to the first is responsible for energizing or not the heating resistor integrated under the probe. The LED103 diode confirms the correct operation of this function. The adjustable P104 will allow you to choose a precise tipping point, and thereby limit the temperature reached.
The assembly is powered through the voltage regulator IC101 for 5V, 12V coming from a power supply circuit common to all the modules. For reasons of performance and beyond energy saving, I indeed opted for a power supply of type SMPS . This type of power supply operates with an efficiency of more than 80%, displays a minimum of losses and overheating, and offers protection against overvoltages, all thanks to high performance components. In addition, their operating range is wide (from 85 to 264V approximately, and at a frequency of 47-63Hz), which allows use in all latitudes!
Schematic diagram :
The assembly of the plate does not call for any particular remark, except that it is better to start by soldering the vias replacing the metallized holes ( to do this, use pieces of resistance legs, for example ... ) . Then solder the tulip supports ( or lyre according to your choice, but it is not recommended to directly solder the integrated circuits, because it is easier to unplug than to unsolder the component during a breakdown! ).
You have finished ? Well done ! Now remove all the integrated circuits, only the 5V regulator and the transistor should remain as active components.
Arm yourself with a voltmeter, and connect the + 12V to pin 3 of connector K101 (this is the fuse input ). As soon as this is done, the green Led should light up ( if this is not the case, unplug immediately, and check your connections, the polarity of the power supply, the absence of short circuits, etc. . ).
Test on the IC sockets for the presence of + 5V and / or + 12V on the supply tabs, namely:
|+ 5V||pine 14||pine 14|
|+ 12V||pin 7||pin 7|
|GND||pin 7||pin 7||pin 4||pin 4|
If you do not get this result, there is a problem with the PCB itself ... it will be reviewed!
Check earth continuity with an ohmmeter when de-energized. We are never safe from a micro-cut, perhaps due to too long exposure in iron perchloride ...
All this is absolutely necessary before proceeding with the final power up!
You have arrived at the end of this test, the next episode will be the development ...
Photos of the module :
The links below will allow you to download the diagram and the artwork in PDF format ( scale 1:00 )
Note : The version numbers may differ between the block diagram and the typons, and do not necessarily have a relationship between them. The files that I placed on my site are indeed the most successful versions.
This document was strongly inspired by the article "Intelligent rain sensor" published in the Practical Electronics of April 2005 (EP n ° 293)
The Practical Electronics article
Link to the Lextronic website