The infinity bug is connected across the phone-line of a distant phone in parallel mode. It uses surface-mount resistors, capacitors, transistors and diodes. It gets operating current from the phone line (leech device), no external power supply is required.
To use the Bug, the distant phone is rung. After having a conversation with the person at the other end, they will replace the receiver. At this point you whistle into the receiver and the Infinity Bug will pick up the line. The high-gain amplifier in the Infinity Bug will pick up the audio at the place where it is located. A timing circuit in the device will cancel after 3 minutes. To open the Bug you need to whistle again. This can be repeated any number of times. If the phone is picked up, it cancels the Bug.
Project Firmware, Schematic and Documentation