Say you're aiming at up to 3 digits...what happens if I put in 3 coins,
or 2 coins when I still have time left. Does it "max" to 9:59?? Does it
just max the minutes (e.g. to 9:23 if I had 2:23 remaining). Since
you're doing what sounds like a whole board replacement, is the
display on that board? If the display isn't can it's board be altered to
accommodate 4 digits? Must it display as minutes and seconds, or
would 300 seconds per quarter do???
Why? I'm thinking about the idea of 2 counters (mm:ss). In this
format, you'll need to monitor the binary codes from the counters
for each segment of the timer, through a multiple or sets of OR
gates (so when they are all low <out of time>, the output will be a
low <presuming the relay is grounded and has it's positive turned
on/off>). The counters would have to be up/down (only the minutes
would if the seconds stay the same <re utilizing 4 digits [who would
put 20 quarters in at once?], and if only 3 digits <re maxing at
9:59> both counters would have to be up/down (MUCH more
complex to max the seconds to 59!!!). If it has to stay 3 digits, how
about keeping the seconds the same and ripping off the patron? (if
they have 7:01 left a quarted would raise it to 9:01...).
One circuit would be the :59 countdown, that when it reaches
zero, if the minutes timer is zero, it would stop and trip the relay. If
the minutes weren't zero, it would count one down on that timer.
Since you would also be using the minute counter to add time, the
up/down control on it would need to always be down. But, when a
coin is inserted, the up/down would need to go to up, and a circuit
would send 3 clock pulses in (to add three minutes) and then turn
the minute counter back to down.
So, you're looking at three aspects...a second counter that if at
zero, will continually monitor the minutes counter (if it's
zero, hold the relay on, but if not will not hold the relay on and will
start counting down from 59, which will also trip the minutes
counter to count down one (whenever the seconds go from :00 to
:59). The clock input is disabled by the ORs that monitor ALL digits.
The second aspect is the minutes counter, it's down controlled as
previously specified, and it's up controlled by the quarter circuit.
This is quite complex, but if you give details re my questions re
how it is desired to operate (rollup of the seconds - yuk!, maximized
count, etc.) and the limitations of your display board, I (and/or
someone else) might be able to help you out. Of course, this is if
you can solder, make protoboards and most importantly,
understand clocks, binary code, LED drivers & general electronics...