This double tone generator uses a specific integrated circuit, having been found in a 1977 CMOS Cookbook. The transistors can be the BC548.
This circuit uses opposing MOSFETs operating on a 6 to 15 V input voltage. The circuit is from...
The sensor of this circuit is a photodiode. Equivalent operations can be used, and the instrument can be...
Charcoal microphones are rare today. In the figure we have a circuit that shows the use of one of...