This circuit has the intensity of the signals controlled by the light that falls on the sensor. Note that it is not a frequency-controlled oscillator. Equivalent transistors can be used.
Found in a 90s Silicon Chip documentation this circuit is intended to protect a speaker system in...
The circuit shown is a reference of -10 V with a maximum current of 50 mA obtained from the Linear...
Found in a 1980s American documentation, this circuit can use a SCR and a general-purpose transistor....