This circuit is a MIDI interface between drum pads and computer or hardware sequencer. Using this circuit you can hit pads with drumsticks and store the MIDI data in real time.This circuit is based around the PIC16C84 microcontroller which scans the inputs on RB0 to RB7 and when a high is detected the software transmits the equivalent note-on ( and then the MIDI note-off after a short delay) for the required drum sound on the MIDI output on RA0. source

This circuit automatically records all incoming and outgoing phone conversations with an ordinary cassette or tape recorder. The circuit below distills the audio signal from the phone line and automatically starts/stops the cassette recorder.source

This circuit allows you to construct a electronic switch which will automatically turn on your tape recorder when you pick up the handset of your telephone.So that your tape recorder must have both a MIC socket and a REMOTE socket on it so that circuit can plug into the recorder and control it.This circuit will allow you to record both sides of your telephone conversations.source

Telephone call Voice Changer is voice manipulation device specially intended for props 9V Battery operation.The voice effect can be obtained by means of some audio computer programs, a few correspondents required a stand-alone device, featuring microphone input and line or loudspeaker outputs.This design fulfills these requirements by means of a variable gain microphone preamplifier built around LM358 .source

This Metronome use 12V micro battery which can be used to obtain a higher output power and more compactness.This mini metronome gives a linear scaled output and 40 to 208 beats per minute. source