I'm building an ultrasonic drill, and I've already collected or machined all of the components. Since I'm very inexperienced in electronics, I'm planning to simply buy an ultrasonic driver (power source) rather than build one myself. The one I'm interested in is here: http://www.ebay.com/itm/110V-100W-28KHz-ultrasonic-cleaner-power-driver-board-/120972831045 Once completed, the ultrasonic drill with its 28 KHz piezoelectric transducer will NOT resonate at exactly 28 KHz. It may be off by a little, and the transducer itself is rated +/- 0.5 KHz. Since impedance can increase by two orders of magnitude if the drill is driven by just +/- 0.25 KHz away from the optimal, I really need to fine tune the frequency. Someone on another forum told me that I can measure impedance with a multimeter (again, I'm very inexperienced), and they also told me that if the power source is driven by an oscillator then it probably has something he called a "trim pot" that I can apparently tap into. Does the product above look like it has a trim pot? If so, what is the name of the device that will let me tune the frequency between say, 27-29 KHz?