This amplified metronome was found in a 90s Popular Electronics. The circuit is powered by a 9 V battery but will also work with 6 V of 4 standard batteries.
We found this voltage multiplier circuit in a Radio Electronics magazine from December 1991. The...
The figure shows how to use a CMOS amplifier as a crystal-controlled oscillator. The circuit is...
This simple circuit does not use active components and uses a small speaker as a microphone. The...