On-Off: the last step for the "radiobaliza".
In the first part of the today lecture we made a little brief about what we made up to know: the Pierce oscillator implemented with a bipolar transistor, the tank circuit and the cheap quartz oscillator.
In the last class we stopped talking about how can we interrupt our transmission to implemented a very simple modulation: On-Off.
The first idea that we would have is to switch on and off the voltage source, this is not working fine because of the presence of the condensator electrolytic that have a slow discharge period.
The other possible solution (the one that we decide to implement) is placing the transistor out of the active region, the most easy way is to insert a controlled generator (on-off) with a value of Vcc in series with the R_e, resistance of the emitter. In this way we can alter periodically the polarization network and therefore stop the transmission.
If we want to turn on and off the voltage source we need a component that generate a square wave, because this is a very well-know and common problem there is a IC that makes this works, its called 555. To control the period of on and off we only have to add two resistance and a condesator.
We went to the laboratory, we built the circuit, we added and antenna and we were surprising how our signal is well received by Josè Maria with a commercial radio receiver.
In the last minute of the lesson we talked about how connecting in a boat (and also in a air-plane) the antenna: it's a good engineering problem!
The next step??? We want to build up a receiver for this frequency and look how all the chain tx - rx works fine!
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