This is an introduction to the three ultrasonic circuits that you will find on this website in our menu.
In the realm of electronic surveillance, there is no stone that goes unturned. In exotic, unusual, and unorthodox methods of trying to figure out new ways, new methods and new engineering out of the box, to invade someone’s privacy. There are the old standard ways, usually the telephone tap or the concealed rf transmitter on premise, or even a wire – Tape Recorder worn by and informant, or a body wire – transmitter worn by whoever may be wearing it with someone off in the distance monitoring and/or recording it, repeaters and etc.
There are three ultrasonic circuits published on this site. My Ultrasonic Voice Transmitter, My Ultrasonic Receiver Circuit, and My Ultrasonic Transponder Circuit.
I scanned the original schematics, which were crude, and using Microsoft paint, replaced a lot of the part numbers that were unreadable in the original schematic.
current drain @ 9 volts = 25 ma (.025 amps). freq stable from 9.0 to 6.0 battery falloff. pulse hgt
feeding U.T. is 5.0 v @ 40 khz. On time on the positive pulse is 12 us and off time is 12 us. rise
time on the leading edge is 1 us. Basically what you have here is that audio is picked up in the area the unit is placed from the microphone, amplified and then fed to the chip. That audio is mixed within a 40 kHz frequency. That ultrasonic frequency is sent out by the ultrasonic transducer and intercepted and demoulated by our ultrasonic receiver shown elsewhere in our pages. For best transmission clarity, simply align it by adjusting the 5K Trim Pot, setting it to 40 kHz while having a friend listening in with our ultrasonic receiver around 100 feet away until he receives quality audio.