The circuit can be laid out on the protoboard as shown in the photograph. I chose to use 8 pin sockets for the 555s, but they are not necessary.

(The connector numbers are not that clear on the PNG, but can be seen in Fritzing.

The pigtail DIN6 male connector can be plugged into the computer's keyboard input to provide 5 V :

• pin 4 -- 5V
• pin 4 -- GND

The MR350PS4 cable and male DIN6 connector pin out is,

The female DIN6 connects 1 PPS, 5V, and GND from the MR350PS4 to the protoboard, and RX and TX from the MR350PS4 to the DB9.

The DB9 connects GND to the protoboard, and receives from the protoboard the stretched 1 PPS signal on DCD, pin 1.

The time constant for the pulse stretcher is given by T ~= 1.1 R C,.

• For the 1 PPS LED it is T = 1.1 10^4 10^-6 = 11 ms
• For the 1 PPS signal it is T = 1.1 220 10^-6 = 242 us

I chose to add a couple of header pins so I could easily connect a scope and see the stretched 1 PPS.

With a 5V supply, the pulse amplitude should be about 4.5V, which nicely exceeds the RS232 spec minimum.