This circuit was found in a documentation of the 80s, having didactic purpose. For R1 values between 10k to 1M can be used. Transistors are general purpose types.
This circuit was originally used in a 1989 documentation presence simulator. The circuit activates...
This old circuit uses an operational one from the 70s, but it is still useful in current projects. The...
We found this circuit in a Popular Electronics from the 80s. The circuit is based on the LM3909, a low...