E)
This circuit uses a reversible double contact relay to obtain the trigger lock with the unijunction pulse. The relay must be of the sensitive type with at least 150 ohms of coil.
The circuit below shows how to use an LM139 comparator or series equivalent as an amplifier. The...
We found this receiver circuit for modulated light in a 90s Popular Electronics. Equivalent audio...
The circuit was found in the February 1992 Radio Electronics magazine. The scale can be changed by...