This receiver was found in an article in Electronics Now magazine in March 1994. The coils are critical, as shown in the figure. Listening is through headphones.
This filter that rejects determines frequency (notch) was found in a Popular Electronics of 1993....
Found in an Elektor from the 90's, this double door-bell drive circuit switches a relay by...
Using a 555 this circuit was found in old documentation, but it is still valid. | Clique na imagem...