This circuit is from a documentation on the MC1539 from Motorola and may, however, be implemented with equivalent operations. The transistor also supports equivalent.
This 80's circuit uses an integrated amplifier of the era, but nothing prevents it from being...
This simple circuit for an audio link uses a lamp, which has a very poor frequency response, but...
Relays from 20 mA to 200 mA can be driven by this sensitive circuit.6 or 12 V relays can be used...