FSK encoder for recording 500 bits per second to casette tape. I think the frequencies are good enough, but I'm not sure. With the higher frequency below 5kHz, a hi-fi tape deck is probably not required. (Not that I know where you'd find a lo-fi tape recorder any more! :)
~ Operation:
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The 555 oscillator varies its frequency depending on its control input. This input goes to an internal resistor chain, so a single external resistor suffices to adjust the (CMOS) input to a suitable range.
For the tape, I want equal width high and low periods. The flip-flop provides this by halving the frequency from the 555.
The voltage divider adjusts the level for a line input and the capacitor blocks DC.
The 20kR resistor represents the tape recorder's line input. It's pessimistically low; the nominal input impedance for a line input is 50kR.
