Contact "bounce" is the physical bouncing and interference of metal switch contacts as they come to a closed state, causing spurious pulses which will be interpreted by the counter as multiple events. Perhaps the simplest way to mitigate the effects of contact bounce is to use a passive low-pass filter (RC integrator) circuit followed by a Schmitt trigger gate. Using a serial-in/serial-out shift register is a more sophisticated approach, but requires more components (including a clock signal generator).

Notes:
Contact bounce can be a big problem in counter circuits getting their input from mechanical switches. Learning how to detect and eliminate this problem is a very practical exercise for electronics technicians.

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