This interesting circuit found in American documentation generates a busy signal, without you having to take the phone off the hook if you do not want to answer. The components used are common.
The trigger element of this circuit is a neon lamp. The SCR admits an equivalent. We found it in...
This bistable with transconductance operational amplifier was found in early National...
This 1965 circuit consists of a small inverter for an ultraviolet lamp. The circuit is from a...