Microcontroller alternatives for wireless circuits

[Khaba]

Hello everybody, actually I'm doing a final year project on a selective wireless public address system. The selective part is the problem, because the system should be able to select the receivers(up to 4) the user wishes to address. Rule out infrared because the receivers are in different classrooms. The transmitter has already been built and has a range of 500m

 

[ADWSystems]

So are we to assume the receivers are on 4 different frequencies then? If not and they are are receiving on the same frequency, then all 4 receivers will detect the signal at the same time; thus there needs to be another transmission (signal) that indicates which receiver the transmitted signal is meant for.

 

[birdman0_o]

You can give each receiver a unique serial code, and for a signal to be sent, that code needs to be matched.

 

[Khaba]

actually the four receivers are meant to receive at the same frequency, but my difficulty lies in setting up a circuit that can `wirelessly´ switch any of them or enable any of them receive the transmitted frequency.

 

[Sceadwian]

That's exactly how the FRS/GMRS system does it. What you do is always listen on the receive side, but have a circuit that disables the speakers, listen for DTMF tones, when it detect a tone it turns the loudspeaker on until a separate tone is received to turn it off.

 

[ADWSystems]

I can come up with two options. Either send a parallel signal on another frequency to activate the appropriate receiver. Or send a specific signal at a given interval (every 60 seconds for example) that will keep a specific receiver enabled. After a timeout period of not receiving the "enable signal" (80-90 seconds) the receiver stops broadcasting whatever it is receiving until the enable signal is received again.

 

[Khaba]

Thanks for your ideas, they're very vital. Do you then know any circuit or IC config that will detect a specific frequency and activate a relay or enable the receiver for which that parallel signal or DTMF will be sent

 

[Sceadwian]

I can't recommend specific IC's but a couple DTMF decoder chips and some basic logic would work.
Normally your RF receiver would always be receiving and feeding it's output into the DTMF decoders, when the first one received the tone it was waiting for it would turn on the loud speaker. When the second DTMF decoder received it's signal it would turn off the loud speaker, reset the first DTMF decoder, and then reactivate itself. Kind of like two SPST tone activated switches in series.