Sensor filtering and hysteresis: stop threshold chatter

Use valid-sample filtering and separate switching thresholds without hiding sensor failures.

A sensor reading close to a threshold may alternate between both sides of it. If a distance is 39.8 cm and then 40.2 cm, a single 40 cm threshold can repeatedly change an indicator. First check the target, wiring and supply. Filtering is useful after the basic measurement works; it cannot repair an unsuitable sensor or bad connection.

A short median filter can reject isolated spikes. Keep the most recent three or five valid readings, sort a copy and choose the middle value. An odd window length avoids ambiguity. Do not insert failed ultrasonic echoes as zeros. That would push the median toward a false nearby object.

Track sample freshness separately from the filter. If valid readings stop for a chosen interval, mark the output invalid and tell the user. Clear or rebuild the window when recovering after a long gap. Keeping old good values indefinitely hides a unplugged sensor.

A moving average smooths noise but lags behind real changes. Increasing the window usually increases that lag. For a classroom distance indicator, compare a raw value and a filtered value while moving a target slowly and then quickly. Choose a small window that improves stability without concealing the change you want to show.

Hysteresis uses different thresholds depending on the existing state. For example, enter a nearby state below 35 cm, but leave it only above 45 cm. Between those points, retain the previous state. The 10 cm band prevents chatter; it also changes the meaning of the indicator, so explain it in the demo.

Keep measurement, validation, filtering and output decisions as separate steps. Print the raw value, valid flag, filtered value and resulting state at a controlled rate. This makes it easier to discover whether a wrong indicator comes from the sensor, the filter or the switching rule.

Test stationary readings near each boundary, a rapid approaching target and complete signal loss. Use a table of expected results. Hysteresis reduces unnecessary switching but is not a substitute for safe control hardware. Related build: Ultrasonic Parking Indicator on RoboXol. Reference for timing and measurement limits: https://cdn.sparkfun.com/datasheets/Sensors/Proximity/HCSR04.pdf