I have Casio CDP 120 digital piano it has one output socket for the headphones, USB cable and socket for Pedal. I want to use the metronome and play the piano through one set of headphones for late night practice any suggestions. Is there an adapter cable etc anyone could recommend. Look forward to hearing from you. Take care. Keep safe and blessings to you
Simple and stupid option: use 2 sets of headphones. You will hear metronome sound even when its headphones are not directly over ears.
Simple, stupid and somewhat technically advanced option: get 2 extension cords for headphones, cut them and connect the piano sound to one ear and the metronome sound to the other ear. You'll lose stereo effect, but it is overrated anyway and profoundly worthless for "late night practice".
2.1: You can as well mix both signals to both ears. You'll need 4x 33ohm, 0.5W hole-thru resistors and some more insulating tape.
Use mixer. But you'll get extra clutter that is not really worth for a metronome only.
Use light-based metronome.