transducerultrasound
I have a 40 kHz ultrasonic transducer that transmits a pulse. The pulse is generated by switching a phase shift oscillator on and off with a NE555 timer. For some reason the sender is making a faint audible noise. This is the first time I am noticing it. I thought the sender might be damaged but the pulse at the receiver seems fine although there is more coupling at the receiver than before.
Any idea what might be wrong?
Update:
Using larger resistors for the oscillator solved the problem.
Recently, I fixed an ultrasonic cleaning device (f = 40 kHz). The circuit consists of two high voltage bipolar transistor feeding a resonant converter. The piezo actuator is connected to the secondary of the converter's transformer. I have strong reason to believe that the circuit is of the Royer resonant converter type because it has two bipolar transistors, see this answer for more info.
There's another self-oscillating schematic with just one transistor in Sam Goldwasser's Sci.Electronics.Repair FAQ
Some clever techniques allow you to build the ciruit in a way that it catches onto the resonant frequency of the piezo acutuator, maximizing the output amplitude just right.
For your circuit, it would be a good starting point to check whether the triac has the job to turn the rest of the circuit on or off or if it is used to modify the power using a phase angle control technique. The rest of the circuit around the two bipolar transistors will very likely be some sort of resonant oscillator or converter stage.
Use caution. The entire thing is likely connected to the mains supply, featuring high and dangerous voltages with reference to earth ground (= your body).
Related, with a schematic: Replacement Transistor for Ultrasound Cleaner
Try these linear amplifiers made by Apex. They are designed specifically for ultrasound aplications.
Best Answer
It looks like the pulse repetition rate is about 30 ms. This is based roughly on the 348kohm and 100nF in the 555 circuit. The pulse width is in the region of 200 us. I assume the main oscillator is running at 40 kHz and 40kHz has a cycle time of 25 us.
In a perfect world exactly 8 cycles of 40kHz would be gated by each pulse but, it's not going to work that way. It is likely to be a fractional number of cycles that are gated and this is going to cause a low frequency audible blip that evolves with time as things move into and out of sync.
I don't think it's a problem but you might be better served deriving a pulse from the 40 kHz and gating the 40 kHz to dump an exact number of cycles onto the transducer. You could even arrange it to start at a zero cross. I'm sure this wouldn't produce as much audible noise but then again, on the other hand is it such a big deal. After all, if it performs its main task and the noise isn't annoying then let it be.