The Batty Ultrasound Generator

Vincent Goffin

Last updated Sat Aug 19 12:12:32 EDT 2000

The Batty Circuit

Batty is a very simple circuit that generates ultrasound chirps. It uses the 3 oscillators that can be built from a single hex IC. A slow frequency oscillator controls the repetition rate for the call, approximately 1 a second. A faster oscillator controls the pulse rate within a call, approximately 10 pulses a second. The fastest frequency sets the sound tone within each pulse, anywhere from 40 to 80 khz, well into the ultrasound range.

The rest of the circuit is simply 2 AND operations that merge the three signals into one, and then a final amplification drives the ultrasound emitter.

Schematic diagram and parts list.

Recordings

through a division detector:

amplitude and frequency plots

division (3s, 23 kb mp3) two batty pulse trains recorded with a simple/belfry division detector.

through a heterodyne detector:

amplitude and frequency plots

heterodyne (8s, 63 kb mp3) four batty pulse trains heard through an sk207 heterodyne detector. Two pulse trains at one tuning frequency, and two more at another.

Notes

The Classic CMOS Oscillator:

The most interesting part of the circuit is the operation of the 3 oscillators. Each one is built from two inverters, thus three can be built from a single hex inverter IC. Why and how do they oscillate? The following link offers an explanation: CMOS oscillator. (They misstate, however, how the capacitor discharges: both loading and discharging go through the same resistor).

Design vs. Realization:

The expected time constants, based on component face values and CMOS logic thresholds (the 2.2) can be compared to the ones measured in the recordings above.

                        expected                 measured
                 ---------------------------     ---------
                 seconds/cycle    cycles/sec     cycles/sec
-----------------------------------------------------------
f1 = 2.2*R2*C1 =  0.0000185, 1/f1 = 54,500       52,800
f2 = 2.2*R4*C2 =  0.1232,    1/f2 =      8.1         12
f3 = 2.2*R6*C3 =  2.71,      1/f3 =      0.37         0.59

The numbers are in the ball park, even if it looks like the electrolytic capacitor face values may be off by a generous 50%

Testing and Calibration:

Having a known source of ultrasounds is of course very useful to check if (and how) a detector works. But in addition Batty can be used to calibrate a heterodyne detector. As the tuning dial reaches the chirp sound frequency, the detector's audible frequency will drop then climb again. The lowest audible frequency point of the dial corresponds to the chirp ultrasound frequency.