This circuit was found in a 1968 Motorola documentation. The circuit can be implemented with virtually any photo transistor and general-purpose transistor.
I found this circuit on a December 1976 Radio Elettronica. The Italian magazine no longer exists, but...
This circuit was found in the February 1993 Radio Electronics magazine. The frequency depends on...
A 25 mVpp applied to the input of this circuit produces in the output a 1 Vpp signal. The...