Found in a Philips' 1968 documentation, this circuit is slightly different from the previous one. The components are also from the era, and modern equivalents can be used.
This simple circuit can make use of any general-purpose transistor. The power comes from the...
This circuit that appears in similar versions on this site was found in a Popular Electronics from the...
This alternating voltage voltmeter was found in a 1967 magazine. The magazine no longer exists, but...