Electronic Dog Whistle
idea: Elex Team
Ultrasonic dog whistles are used in situations where the owner would like to give commands to the dog, and that will only be heard by the dog – for example for training purposes.
The circuit described here can, however, also be used in ‘reverse’: to scare off aggressive, unleashed dogs – this is because the circuit generates a very loud sound that nearby dogs definitely find unpleasant.
The circuit is simplicity itself, see Figure 1.
Transistors T3 and T4 together form a classical astable multivibrator (call it a free-running oscillator) with a frequency of about 21 kHz (with the component values as indicated). Because of the completely symmetric design, the circuit generates a nice symmetric square wave. Diodes D1 and D2 prevent the circuit from generating a lot of audible noise in addition to the desired ultrasonic square wave.
The 21-kHz square wave is substantially amplified by the two push-pull output stages (T1/T2 and T5/T6) and drives the piezo tweeter LS1.
The original design used a Monacor type KSN1001A (or equivalent) for the tweeter; if you look around on the Internet you will find plenty of ‘modern’ variants.
The circuit was designed to be powered from a 9V ‘block’ battery (PP3 / 6LR22). Finally, a word of warning: because of the large sound volume produced you should not use the circuit in the close proximity of sensitive dog ears – unless you are attacked, of course.
And naturally you can experiment to your heart's content with this circuit – with different component values and a normal loudspeaker you could also build a bicycle siren or whatnot!