Oil change computer (CB6373E)

This electrochemical circuit monitors the conductivity of the oil indicating when to change it. The circuit is from an 1967 American documentation.

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TTL Optical Interface (CB16022E)
In the figure we have a Texas Instruments circuit for TTL interfacing using discrete transistors. The two...
Remote control tunnel transmitter (CB23885E)
This circuit was found in Radio Electronics magazine from June 1963. The range is small and the...
50W 40 Meters RF Amplifier (CB17683E)
The project presented in a 2009 documentation is intended for radio amateurs operating in the...