With this circuit it is possible to modulate pulses from a pulsating input signal. The PNP transistor can be the BC558 and the power must be supplied with 5 V.
This TTL circuit for digital signals up to a few dozen megahertz was found in old documentation, but...
This circuit with an operational amplifier was found in the magazine Radio Electronics of...
This detector is from the 1990 Motorola Dataar Databook, but can be adapted to work with more...