All about hysteresis

1/****** Hysteresis ******/
2
3/* Threshold and hysteresis are the two
4   constants which determine the system
5   behaviour. Try to change them and see
6   what happens. Their values must be
7   between 0 and 1023 */
8
9const int threshold=500;
10const int hysteresis=0;
11
12#define inputPin A0
13
14void setup()
15{
16  pinMode(LED_BUILTIN, OUTPUT);
17}
18
19void loop()
20{
21/* Read the analog input */
22  int V=analogRead(inputPin);
23
24/* If the analog input is larger than the
25   threshold, turn the LED on */
26  if(V>=threshold)
27    digitalWrite(LED_BUILTIN, HIGH);
28
29/* If the analog input is smaller than the
30   threshold minus the hysteresis, turn the
31   LED off */
32  if(V<threshold-hysteresis)
33    digitalWrite(LED_BUILTIN, LOW);
34
35/* And if the analog input is between the
36   threshold minus the hysteresis and the
37   threshold, do NOTHING, the LED keeps its
38   previous state */
39
40  delay(50); /* to let you see the LED blink
41                when the hysteresis is 0 and
42                the analog input is near the
43                threshold */
44}

1/****** Auto_lighting ******/
2
3/* Threshold and hysteresis are the two
4   constants which determine the system
5   behaviour. Try to change them and see
6   what happens. Their values must be
7   between 0 and 1023 */
8
9const int threshold=500;
10const int hysteresis=100;
11
12#define LDRPin A0
13#define ONPin 2
14#define OFFPin 10
15
16void setup()
17{
18  pinMode(LED_BUILTIN, OUTPUT);
19  pinMode(ONPin, INPUT_PULLUP);
20  pinMode(OFFPin, INPUT_PULLUP);
21}
22
23void loop()
24{
25/* If the 3 position switch is not in AUTO
26   position, control the lamp according to
27   the switch position */
28  if(digitalRead(ONPin)==LOW)
29    digitalWrite(LED_BUILTIN, HIGH);
30  else if(digitalRead(OFFPin)==LOW)
31    digitalWrite(LED_BUILTIN, LOW);
32
33/* If the switch is in AUTO position, the LDR
34   controls the lamp according to threshold
35   and hysteresis */
36  else
37  {
38/* Read the analog input */
39    int V=analogRead(LDRPin);
40
41/* If the analog input is larger than the
42   threshold, turn the LED on */
43    if(V>=threshold)
44      digitalWrite(LED_BUILTIN, HIGH);
45
46/* If the analog input is smaller than the
47   threshold minus the hysteresis, turn the
48   LED off */
49    if(V<threshold-hysteresis)
50      digitalWrite(LED_BUILTIN, LOW);
51
52/* And if the analog input is between the
53   threshold minus the hysteresis and the
54   threshold, do NOTHING, the LED keeps its
55   previous state */
56  }
57  delay(50); /* to let you see the LED blink
58                when the hysteresis is 0 and
59                the analog input is near the
60                threshold */
61}