Found in a 1964 Electronic Design, this circuit uses a common LDR and transistors from that time. The relay is a high sensitivity type.
We found this circuit in the Linear Applications Handbook of the National Semiconductor of 1994....
This tone-modulated transmitter was found in recent English documentation. The circuit operates in the 27...
This very low current circuit was found in the April 1995 issue of Electronics Now magazine. The...