We found this circuit in a Philips documentation from 1965. The circuit uses a common LDR sensor and the transistors are general purpose types. TR2 can be a BD135.
The figure shows how to implement an alarm buzzer in an LM3914 to warn about crossing the scale...
This circuit can act as a shield being controlled by a microcontroller by pin 1 or 2. The...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...