The purpose of this circuit is to create a pair of sine waves for testing stereo equipment. While it is useful for many types of audio test tone generation, the circuit is specialized for the purpose of aligning an FM Stereo Modulator, like the type used in low power FM stereo transmitters. Two tone outputs are available, the low tone has a secondary output that is 180 degrees out of phase with the primary output. The circuit is also handy for testing computer sound card inputs. source

This a simple circuit demonstrate the Tiny family high speed PWM capabilities to produce a sinewave output. It was also incorporated functions to play Piano Key Tones using sinewave waveform, as a smooth and clean sound as the ones produced by cords.The sinewaves produces are better in low frequencies. source

The tone generator (top) uses the 5589 chip and a DIP switch. You can actually hear the tones through the speaker. The bottom circuit uses the 8870 to decode a tone and display its associated number on the 7-segment LED. source

Although the oscillator using schmitt trigger gate is simple, it is hard to determine the accurate timing since its hysteresis thresholds depend on device characteristic.However, hysteresis thresholds are easily made by positive feedback.This is the hysteresis oscillator by normal inverter gates.source

The circuit presented here enables you to detect any object capable of reflecting the IR beam and moving in front of the IR LED photodetector pair up to a distance of about 12 cm from it. The circuit uses the commonly available telephony ICs such as dial-tone generator 91214B/91215B (IC1) and DTMF decoder CM8870 (IC2) in conjunction with infrared LED (IR LED1), photodiode D1..source