The circuit below is suggested by National Semiconductor in technical documentation on the LM3909. The circuit is powered by a single battery and has a very low consumption.
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...