Frequency Counter: "Hardware Keys" Schematic
This is basically 4 nearly identical debounced switches. The 2 resistors and one capacity at the input of each Schmitt trigger are used for debouncing the switches. The output is then fed into a D-type flip-flop which uses feedback to change the state (LOW to HIGH, HIGH to LOW) each time the button is pressed. Each key has a LED indicator, two with simple LEDs and two with bidirectional 2-color LEDs.
A simple "diode logic" circuit is used for the "any key pressed" signal (which is not used for the right-top "hold" key) to be able to make the logic board aware that buttons were pressed.
IC3E forms a simple reset logic which makes sure that all the flip-flops are in proper state when the device is switched on. The time constant is determined by R8 and C7.