Getting circuit input on a computer
You can buy USB "bridge" ICs (or better, modules) that simulate a serial UART with control lines at the "user end" and connect to USB. You can in some cases just use the control lines to send high/low binary data or send or receive RS232 serial. 

This is an example of a device that provides serial connection to USB 

PL2303HX USB to TTL RS232 COM UART Module Serial Cable Adapter for Arduino

Similar - here you can see the insides - which are probably similar
Cp2102 USB 2.0 to UART TTL 6pin Module Serial Converter Adapter Blue Silver N3

And here are many more
Many give you serial access only but some add control leads.

This one has DTR

Some/many DO have the extra signals on onboard pads but these may not be identified.
This one has them silk screened on.

WARNING - note that the correspondence of the top and bottom legends may be confusing. You would expect that the bottom view resulted from rotating the PCBA 180 degrees along the top edge.
The "Gnd" marking at the top left of the top view (probably) relates to the first through-hole next to SU2 and not to the pinned end connector. 

From here