This circuit generates short pulses. The transistor can be a 2N2222 or BC548 and the frequency is given by the resistors and the capacitor that can be changed.
I found this circuit in a Popular Electronics from November 1999. The magazine is no longer...
This circuit was found in a 1970s documentation. The integrated circuit used is uncommon today....
This filter was found in Exar documentation from the 1980s. The circuit has an attenuation of 60 dB at the...