This optical theremin was found in a 90s Hands-On Electronics. The basic frequency is given by C1 and the output is made in a speaker through a small output transformer.
The National Semiconductor LM675 operational amplifier can provide up to 3 A of output current. In the...
We found this circuit in a 1969 Mullard documentation. The same configuration appears elsewhere in...
Found in a PE Hobbyst Handbook of 1991, this circuit was originally proposed to time photographic...