We found this circuit in the 1957 Raytheon transistor application manual. The transistor can be a BC558 and the common 6 V relay, just by using this voltage in the supply.
This circuit with the operational amplifier CA3130 was found in an English publication from 1977....
This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose...
This circuit was found in an English magazine from 1978, but it is current and the common 2N3055 can...