Ronja is the author’s dog, a beagle-mongrel, who seems increasingly often to need to be called to heel either with a shout or with a whistle. And so the idea came about for an electronic dog whistle that could produce two alternating high-frequency tones. A design like this has several advantages over conventional whistles or calling.
• You can continue to carry on a conversation with your friends without having to stop to whistle or call to your dog.
• Using high frequencies means that the whistle sound is barely audible to (especially older) humans and so is less annoying to other people than conventional whistles or calls. As is well known, dogs have rather better hearing than we do and can hear frequencies of up to 40 kHz.
• The two alternating pitches mean that the dog can more easily distinguish it from other whistles.
The dog whistle is constructed from two standard 555 timer ICs (or a single 556 IC), both wired as astable multivibrators. The first 555 oscillates at around 1.5 Hz and modulates the frequency of the second, which thus switches between two different frequencies every 0.7 seconds or so. The output of the second 555 is connected to a piezo sounder. If the volume from the sounder used is insufficient, a small transistor amplifier can be added between it and the output of the second 555. The circuit draws current only when activated by pressing S1. An optional green LED indicates that the circuit is functioning. When S2 is pressed the output frequencies are reduced, making them more audible to human ears for test purposes.
R1, R2 and C1 set the frequency of astable multivibrator IC1. Diode D1 ensures that the output is a symmetrical square wave, by making C1 charge only via R1 and discharge only via R2.
Turning to IC2, where there is no diode in the circuit, capacitor C2 is charged via R3 and R4 and discharged only via R4. With C2 = 22nF the 555 oscillates at about 10 kHz; with S2 pressed, and hence C3 in parallel with C2, this falls to about 1.8 kHz. Changing C2 to 10 nF results in an even higher frequency (about 22 kHz), which can only be heard by dogs and certain other animals. Setting C2 to 15 nF gives an output frequency of about 15 kHz. IC1 modulates the frequency of IC2 via R5. The green LED D2 is connected to the output of IC1 via a series resistor and thus flashes at the modulation frequency. The output from the piezo sounder at 10 kHz (C2 = 22 nF) should be loud enough to verify by ear. If desired, a more efficient piezo horn tweeter can be used instead.