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■ What is it for?
Under this name, or sometimes even under more esoteric designations, devices for repelling "pests" (rats, mice, but also various vermin) have been marketed for some time. Without wishing to enter into a polemic over the actual or supposed effectiveness of these devices, it must be said that they are sold at a price that is often not commensurate with their "electronic" content. If you have been tempted by the trial of one of them, here is the economic way par excellence. Indeed, our assembly generates the same types of signals, and must therefore have the same efficiency on the said critters ...

■ The schema
All specialists will tell you that ultrasound has a certain repellent effect on rats, mice and other "joyous-set" nature, but that this effect is constant only if the frequency of ultrasound varies. Indeed, it would appear that these animals adapt to a constant frequency signal, which, effective as it may be at the beginning, becomes ineffective after a few months. Our assembly thus generates an ultrasonic signal whose frequency varies slowly but continuously between 35 and 60 kHz approximately. The scheme used to achieve this is very simple and calls for a single integrated circuit; In this case a 556, which is, let us recall, a double 555.
One half of this circuit, located in the upper part of the figure, is mounted in an astable oscillator and the passive elements are calculated in such a way that it can oscillate from approximately 35 to 60 kHz. The variation in frequency is obtained by applying a slowly variable voltage to the tab 11, which is none other than the modulation input of the conventional 555, which is rarely used and is almost always connected to the ground via a capacitor.
This slowly variable voltage is obtained with the second half of the 556, also mounted as an astable multivibrator, but at very low frequency, this time. This frequency is adjustable, for a better efficiency, thanks to the potentiometer P1.
The transistor T1 serves as an impedance buffer between the capacitor C3 and the modulation input 11. Indeed, as saw teeth are needed, the signal at the output of half of 556 from below, but directly on the timing capacitor.
An LED placed in the collector of this transistor makes it possible to check the operation of this part of the assembly. Moreover, in order to test the entirety of the latter, it is sufficient to install the strap Test II then operates between 3 and 6 kHz, frequencies perfectly audible and very effective to scare humans! The power supply is of the most classic and does not even call for a regulation, perfectly useless here.

■ Realization
Our PCB supports
The whole of the components, including the transformer, because, due to the low overall consumption, a molded printed circuit model can be used. The assembly does not present any difficulty and works from the last welding performed.
Attention however, the tweeter used output must be a piezoelectric model and not an electrodynamic tweeter. To make the difference, know that the ohmic resistance of the piezoelectric tweeters is infinite measured at the ohmmeter, whereas for the electrodynamic models it is close to their impedance (4, 8 or 160).
To check the correct operation, put the test strap in place. You will then have an idea of ​​what the rats and other rodents will hear, but a decade above, of course.

Nomenclature of components
IC1: 556
T1: BC 557,558,559 - LED: red LED - D1, D2: 1N4004
Resistors 1/2 or 1 / 4W 5%
R1, R3: 1kR - R2: 22 kR - R4: 22 R - R5: 15kR - R6: 1.8 kR
C1: 1,000 / MF 25 V chemical radial
C2, C5: .10 nF ceramic or mylar
C3: 10 MF 25 V chemical radial
C4: 1 nF ceramic
P1: potentiometer adjustable 100 kR, horizontal - TA: transformer 220 V, 2x9V, 2,5 or 3VA - HP: piezo tweeter -Fuse: Fuse T 20 of 0.25 A timed

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