There might be other things wrong with your design, but one that jumps out of the figure in your question is the
Voltage Regulator step down to 3 V.
You don't need a "voltage regulator" per se - you simply need an interface to deliver either 0V or ~3.3V to the GPIO from the
coin line. There are several ways to do this, the two most obvious are:
resistive voltage divider
The voltage divider may be easier, but it depends on having a known and consistent voltage produced from the
coin line. You have not told us what that voltage is, and without that the resistor values cannot be determined.
The zener diode interface replaces one of the resistors with a zener diode. The breakdown voltage (aka zener voltage) should be ~3.3V; a 1N5988B is one part that will provide this voltage.
Schematically, here's how they look:
simulate this circuit – Schematic created using CircuitLab
For the voltage divider option, the values of R1 & R2 may be calculated using the voltage divider equations.
For example if Vcoin line is a 12 volt signal, these values should work for R1 and R2:
R1 = 1 kOhm
R2 = ((1,000 * 12 V)/(3.3 V)) - 1,000 = 2.6 Kohm
For the zener diode option, the calculation is simpler; we must set the zener current to a reasonable value (~10 mA in this case). Ohm's Law advises:
R3 = (Vcoin line - 3.3 V)/.01 A
Again assuming Vcoin line is a 12 volt signal:
R3 = 870 Ohms