The circuit shown in the figure below is from the 1993 Linear Technology Applications. The operational ones used are uncommon and the use of equivalents can be analyzed.
This interesting circuit of an ancient American publication makes the pattern of an exclamation mark...
This receiver we found in a 1992 documentation operates in the 27 MHz band, triggering a relay...
We found this circuit in a publication of 1990. With the changes it can be mounted easily today....