This is actually a pretty interesting problem. The more I think about it, the more I think a hardware-software solution, using the computer itself, would be the best way to tackle this. You could use a separate microprocessor, but why? with a much more powerful computer sitting there.
In fact, you could reduce the coin-operated device to one simple function:
Tell the computer that someone put a coin in me.
Think about it: assuming the computer is up and running at all times, and monitoring the coin device, all it needs to know is when someone puts a coin in.
Based on its current state, it then decides what to do:
- If the machine is currently inactive (nobody's using it), start a new user session (log user on, etc.) and issue X minutes of time to the user
- If a user is active, add X minutes of time to their account
When the time expires, the user gets logged off and the computer is locked up. (As a courtesy to the user, it would be nice to alert them before this happens.)
Whoops, I forgot about your 3-digit display you said you wanted. Well, you could just chuck that, and use the computer itself to display the remaining time, in a corner of the screen, say.
So all your coin device needs to do is to send a signal over a port (serial, USB). The hardware could be extremely simple.
This also assumes minimal security. What kind of security requirements do you have? In other words, are you concerned about people trying to hack your system to steal time on it? Will it be running unattended, in a public place?
Anyhow, those are my current ideas.