I made a device using an XR-2206 function generator, a couple capacitors, a couple resistors and a 30V power supply. my coil was about 1000 ft of 22 gauge magnet wire with a diameter of about 20cm it sits nicely on my head I call it my "Enlightenment Halo" lol. I use a variable resistor to change the frequency for 1.5hz - 22hz. I do not notice any difference in my mood or level of wakefulness and I do not have a EEG machine to check for any entrainment. but it puts out near perfect sine waves. I cost me $10 CDN to construct.