The transistor used in this circuit is uncommon today. AC124 or equivalent such as AC188K and 2N2431 can be used. The circuit is from the 80's and makes use of transformers.
With the configuration shown in the figure, we have the relay being activated when the microphone picks...
This circuit dates to 1999, when most vehicles did not yet have electronic ignition and used...
This circuit is from a 1968 documentation. It generates a non-symmetrical signal with the waveform...