This circuit is from an American 80's documentation. The circuit indicates when the voltage of a 9 V battery drops below 7 V. The transistors can be BC548.
This amplifier for hearing aid was found in a 1968 documentation using an integrated of the time....
This transmitter was obtained in a 1953 publication. The original transistor was made of...
The integrated circuits used are from SGS, as we obtained this configuration from the 1982 company...