🚀 go-pugleaf

In article <1993Jan20.104438.5447@ucc.su.OZ.AU>, andy@extro.ucc.su.OZ.AU (Andrew Miehs) writes:
> G'day,
> 
> I'm not much in the know about electronics (but I'm learning) and so I'd 
> appreciate anybody's help in designing the following:
> 
> I need a circuit that reacts to a certain high-frequency sound, in order to
> activiate another circuit. Is that clear? In other words, when a microphone
> picks up the sound wave from a nearby source, it turns some auxilary circuit
> on and it stays on until the sound wave ceases.
> 
> Please e-mail me with any suggestions or references.

Look at the schematic (Howard Sams, etc.) of a TV made sometime before the
late '70s.  Old TVs used remote controls that used unmodulated ultrasonics:
just provide enough energy at the right frequency and the function would
be triggered.

The circuits were pretty simple: a piezo microphone followed by a preamp
followed by bandpass filters (L-C) followed by actuators/drivers for the
solenoids or motors.  Frequencies used were in the range 20-40kHz.

These old remotes were very prone to false triggering, as there are many
sources of ultrasonics: electomechanical-bell telephones, jangling keys,
squeaky garage doors would often trigger these things.

Because of the false triggering problem it makes much more sense to use
some sort of modulation and encoding, as is done with modern IR remotes.

> 
> Malcolm Ryan
> 
> (care of Andrew Miehs - email andy@extro.ucc.su.OZ.AU)
> 
> -- 
> +-------------------------------+---------------------------------------------+
> | Andrew Miehs                  |                                             |
> | Mail: andy@extro.ucc.su.oz.au |       $$$ THIS SPACE FOR RENT $$$           |
> | NB:   Don't call me andy!     |                                             |