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.
Using the four amplifiers of an LM324 this circuit was found in the June 1996 issue of Electronics Now...
This simple timer can be used in many applications such as egg timer. The LED glows up when S1 is...
In the figure we have a simple FM tuner using a tunnel diode. Can be adapted to other frequency ranges....