R1 may be 10k and R2 100k. The transistor can be the BC548 and instead of the small lamp we can use a series LED with a 1k resistor. D1 is germanium and C1 is 10 nF. C2 can be 47 nF.
This circuit was found in old documentation. The frequency depends on R1 and C1. For the...
For low-frequency signals, this circuit transforms a single oscilloscope into a dual-trace. The...
Using the 555 this circuit was found in a Popular Electronics from the 90's. The circuit directly...