So lets just see the efforts we did / need to do to achieve my long lived goal of a simple wireless link.(I know its re-inventing the wheel, but anyways I want to do it )
Let us define the wireless link :
a) A transmitter
b) A receiver
c) Carrier frequency
d) Type of modulation
The purpose of this wireless link is simply act a a one-way wireless switch.
It should be built be readily available components and be simple to make .
Let us focus what experience we have in each of these.
a) Transmitter -
Circuit which is able to generate a carrier frequency and able to modulate it in accordance with the message we want to send.
We just want to transmit on/off .At the bare minimum what can we have?
Just the carrier ? Carrier present is ON, not present is OFF.
Well, what is a carrier? Some sort of high frequency waveform.
Does it need to be a sine wave? Yes. Square wave will sputter all over the spectrum.
Anyways if we have a oscillator and antenna it should suffice.
And we have already built a couple of fm transmitters.
From our experience, we have some experience making the coil.And from the net we had also some experience making a frequency detector which works well as a cap and inductance measurement system provided we have some precise reference cap. This helps us in knowing that indeed our oscillator is functional and at the right frequency.
Also we had built a simple rf power meter(which is just a rectifying diode pumping into a cap across which is a led.)This one also helped in kowing that the oscillator was functional.
The problem is that, I did not consolidate the efforts into pcb - breadboard to pcb is must so that I can use it later too.
b) Receiver-
We don't have much experience here.
The only simple one which I had ever built was the one available on swagatam majumdars webpage.
It was pretty stupid. Just an non-inverting opamp configuration with an antenna and output connected to an led.
If any mobile was in vicinity and a call was placed, the led would blink.
I did get most parts for the sommerset single transistor radio.but never actually built it.
So there is some efforts required to be put in this area.
c)Carrier frequency-
We tried only the FM transmitters i.e 89-108MHz .
Seen some easy AM tramitters working in few 1000khz range just by toggling a pin on the microcontroller.
I have recently read that there are many regulations regarding the freq band actually available for free use.
One such band of interest is lower ISM band.In this for freq <30MHz, the radio wave can be reflected off the ionosphere and infact via multiple reflections from ocean and ionosphere, it might be possible to communicate with someone across the world.This is called DX-ing.
d)Type of modulation-
We we have done just freq that.Infact it was a VCO and the v was dependent of the output from condenser mike.I think it worked this way.
*PS- we have used ready made rf tx-rx for robotics projects,which were 433MHz , PSK and ASK based.