Super cheap short range transmitters already exist which don't require an audio frequency listening device (garage door openers, wireless doorbells, X10 type wi

Super cheap short range transmitters already exist which don't require an audio frequency listening device (garage door openers, wireless doorbells, X10 type wireless controllers). The only thing missing to provide the same functionality for those devices as Echo could be built with a cheap SDR. The smoke alarm and doorbells are exceptions (and I guess there are several other good exceptions: clothes washer, clothes dryer, dishwasher, coffee pot) but I think I'd prefer a purpose-built listener with a high pass or notch filter. The Echo may have some unintentional funny failure modes, such as when a doorbell or other trigger sound plays on a TV commercial.