10 Output Shift Registers
By this stage I'm nearing the point where pretty much everything needed to connect a call over two phones is in place; on the source phone I can detect OH (and potentially send a dial tone) as well as detect a dial pulse train. On the destination phone I can send it ringing current to operate the bells and detect OH if the call is answered. What I'm going to need next are some outputs to activate the relays to bridge and route calls, send ringing current and operate the dialing tones. So it pretty obvious I'm going to need many more outputs than the ones available on the Arduino. To get round this I'm going to use a couple of daisy chained SN74HC595's shift registers to give me 16 outputs for the price of three on the Arduino, as shown below. I end up with an output map something like this... #define C595SourceRly1 1 // 1 to 8 will operate the relay matrix #define C595SourceRly2 2 #define C595SourceRly3 4 #define C595SourceRly4 8 #define C595DestRly1 16 #define ...