There is a lot
wrong with the original article
. The parts list specifies "1 x Small Rain Sensor" but provides semi-useless instructions on how to make one using interleaved aluminum strips, presumably aluminum foil, cut out and glued to a "Bakelite or Mica board." The picture shown on the website features a soldered right-angled pair of connector pins attached to the Small Rain Sensor board. Clearly, this is an etched and tin-plated circuit board, conventionally manufactured, with no indication of how to obtain one. If one is gluing interleaved wires to a bare circuit board substrate, the obvious choice is to use bare copper wires, which can be easily soldered, instead of aluminum foil, which cannot be soldered... at least not as well-soldered as shown in the picture.
It doesn't end there. The website goes on to try to describe Working of the Circuit
: "The 555 timer is configured in Astable Mode. As the Reset pin of the 555 Timer IC is given positive voltage, it becomes active and we will get a Pulse signal at the output pin 3 of the 555 Timer IC. This will turn ON the buzzer and the alarm is activated." There is No indication in the parts list of what the "1 x Buzzer (or Speaker – 8Ω)" might be, but as @Audioguru
pointed out in his post #18, as well as earlier posts, a typical piezo sounder resonates at somewhat less than 5kHz and will produce NO output at the roughly 1kHz frequency produced by timing components R6 (Ra
), R7 (Rb
), and C3 (C) as described in the Texas Instruments datasheet
and my post #17. Of course an 8Ω loudspeaker, even (or especially) one as puny as the speaker shown by the original poster
, would reproduce a 1kHz square wave... except its low impedance would overwhelm a 555
timer, as @Audioguru
pointed out in his post #8.
Conclusions? This circuit does not work as described. It was probably never built and tested as described. The author does mention that C1 eventually charges up, turning off Q3 and stopping the oscillations from the 555
timer, but if the "Small Rain Sensor" is still conducting, Q2 remains conducting, and C1 has no discharge path. Also, C1 DOES NOT charge (mainly) through the huge resistance of 470kΩ presented by R4. It mainly charges through R5 (1.3kΩ), the forward-biased diode D1 (1N4007), and the emitter-base junction of transistor Q3.
Recommendations: Add a current-limiting resistor in series with the 8Ω speaker. Try about 1kΩ and work down from there until sufficient audio volume is achieved or the 555
timer burns out, whichever comes first. Or maybe just add another transistor to drive the loudspeaker... Fugeddabout using a piezo buzzer without circuit modifications, which I am sure someone here on Electronics Point would be willing to provide. So, all you EP gurus, jump right in here and help @laurencelovesphysics
get his rain detector working. I would breadboard a working version of this, but my wife's HoneyDew jar is full of work slips that have nothing at all do do with electronics. <sigh>