Marten Repeller inaudible 90 dB
Figure 1 shows the modest schematic of our ultrasonic sound generator. The heart of the circuit is formed by — and how could it be otherwise — a microcontroller, in this case a small one: the ATtiny25-20. But let’s start from the beginning: the power supply. The input voltage, in the range from 9 to 12 VDC (derived from a 9-V battery, a line power adapter or a car battery), enters through the two-way header K1 and continues via on/off switch SW1 to a well-known, low-drop voltage regulator, the LP2950 in its 5-V variant. Capacitors C1 and C2 are part of the standard configuration and ensure the stability of the output voltage. The microcontroller is powered from the regulated 5-V output voltage that is generated by this regulator; for the output stage this is not necessary of course, for this the ‘raw’ battery voltage is used.
IC2 is the heart of the generator — an ATtiny25 with an absolute minimum of ancillary components. Trimpot P1 serves for setting the output frequency (with a range of about 20 kHz to about 43 kHz). The firmware has been developed such that the generator supplies an intermittent signal. We have done this to prevent the battery from being drained too quickly, and to avoid the scared animals from becoming habituated. We chose to generate a burst of roughly 1 second every 10 seconds. Since we cannot hear whether the circuit is active (that is, makes noise), we have added LED1 (in combination with a series resistor R2), which makes to operation of the circuit visible.
The output of the microcontroller (Pin 6) is nowhere near capable of delivering enough current to drive a loudspeaker oscillodirectly, so for this purpose we have added a driver stage around T1 (the well-known MOSFET BS170).
In order to send the amplified output signal from the controller (bursts of a frequency that is (far) above our range of hearing into the world with a substantial amount of decibels, it is best to use a piezo horn tweeter with a high efficiency. After some searching and experimenting we selected the MPT-001 from Monacor (Figure 2). Although this tweeter is not specifically intended for ultrasonic applications, it combines a reasonable efficiency with modest dimensions and, above all, a pleasant price of not even a tenner. In any case, good enough for our purpose. But if you happen to have another U/S tweeter or would prefer to use a different model, then go ahead! In this aspect the circuit invites experimenting.