aFSK modulator using a 555 timer

A fsk modulator with frequencies in the audio range (hence audio FSK) built using a 555 timer chip and a 2n3904 npn transistor used for switching between 2 resistor values (and by that way changing the output frequency of the timer). The ear phones are driven by an op amp configured as a voltage follower (lm324).

[ehvway]

I have made an encoder years ago. Also with the 555 timer chip. But decoding is the problem because the parts aren't easy to get.