Bat sound detection circuit

The frequency of the "click" sound (actual cracking sound) emitted by bats ranges from 12kHz to 15QkHz, 20 to 100 times per second, and the specific value depends on its species.

There are two ways to convert the ultrasonic waves emitted by bats into audible sounds. The first method is to compress the ultrasonic spectrum of 20kHz-100kHz into the audible frequency range (for example, convert it into 2kHz~10kHz). The second method is to mix the signal generated by the oscillator with the signal emitted by the bat. The frequency of the mixed output signal is the sum and difference of the two input signal frequencies. The sum frequency cannot be heard, but the difference frequency can be heard.

The circuit shown in the figure is the second solution.

First, the bat call is obtained by using the microphone (piezoelectric high-frequency speaker) X1, and amplified by the three operational amplifiers ICIB, ICIC and ICID of the four-op-amp TS9241N. The amplifier gain is adjusted by the potentiometer P1. The amplified signal enters the NAND gate IC2B for shaping and outputs a pure square wave. IC2A and surrounding components form an adjustable frequency oscillator. Its output is mixed with the output of IC2B in the NAND gate IC2C, and the difference frequency signal of the two input frequencies is generated at the output end of IC2C and sent to the earphone (X2). IC2D inverts the signal at the other end of the earphone, thereby doubling the volume of the earphone. This circuit based on the superheterodyne principle is very sensitive to the acoustic feedback between the microphone (X1) and the speaker (X2), which may cause a unique screaming sound. Using earphones can reduce acoustic feedback. Another way is to increase the current limiting resistor Rll to 1MΩ. Or put the microphone X2 into the tubular body to reduce feedback, that is, use a directional microphone.

Rubbing dry hands in a quiet environment generates ultrasonic waves, and uses their higher harmonics to adjust the circuit.

Connect the power supply and plug in the earphones, make sure that P1 and P2 are set to the maximum resistance value, then the volume and oscillation frequency are both at the lowest value, adjust the sensitivity potentiometer P1 until the higher frequency harmonics can be clearly heard.

Since birds and crickets can also emit ultrasonic waves, a microphone must be installed at the flying bat, and then "click" or "click" noise will be heard. If the result is not ideal, the potentiometer P2 can be adjusted appropriately.

The detector can also be used to listen for other ultrasonic sounds! For example, it can be used to find the location of a leak in a bicycle tire, because air flowing through a narrow opening will also produce ultrasonic sounds.