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--- projects:anemometer [2023/10/02 18:23] – [Calculating wind speed] jattie
+++ projects:anemometer [2024/02/18 19:54] (current) – [Important notes about reed switches] jattie
@@ Line 29: / Line 29: @@
 Therefore, with a rotation speed of 60 RPM, the resulting wind speed would be approximately 7.307 meters per second.
+===== interrupt-driven counter =====
+To create an interrupt-driven counter on a Raspberry Pi using GPIO pins, you can follow these steps:
+==== Import the required libraries ====
+<code python>
+import RPi.GPIO as GPIO
+import time
+</code>
+==== Set up the GPIO mode and initialize the counter variable ====
+```python
+GPIO.setmode(GPIO.BCM)
+counter = 0
+```
+==== Set up the GPIO pin for input and configure the interrupt ====
+<code python>
+GPIO.setup(pin_number, GPIO.IN, pull_up_down=GPIO.PUD_UP)
+GPIO.add_event_detect(pin_number, GPIO.FALLING, callback=increment_counter)
+</cpde>
+**Note:** Replace ''pin_number'' with the GPIO pin number you want to use.
+==== Define the callback function to increment the counter ====
+<code python>
+def increment_counter(channel):
+    global counter
+    counter += 1
+</code>
+==== Create a loop to keep the program running ====
+<code python>
+try:
+    while True:
+        time.sleep(1)
+except KeyboardInterrupt:
+    GPIO.cleanup()
+</code>
+This code sets up the GPIO pin as an input with a pull-up resistor and detects falling edge interrupts. When the interrupt is triggered, the ''increment_counter'' function is called, which increments the counter variable. The program then enters an infinite loop, waiting for interrupts.
+Remember to replace ''pin_number'' with the desired GPIO pin number for your application.
+===== Important notes about reed switches =====
+The  sensitivity of a REED switch is characterized by the value of the magnetic field which is required to
+change its state. The magnetic field is expressed in Ampere-turns  (or ampere/meter). In order to close a normally open switch, for instance, a certain number of Ampere-turns, called **CAT**, are required (**c**losing **A**mpere-**t**urns). To open the same switch the magnetic field must be reduced down to an **o**pening value in **A**mpere-**t**urns **OAT**).
+<WRAP center round tip 60%>
+OAT<CAT is always true
+</WRAP>
+Order of magnitude OAT-CAT :
+^For standard switches^^
+|CAT |12 and 50 Ampere-turns|
+|OAT |5 and 30 Ampere-turns|.
+^For standard or mercury wetted switches^^
+|CAT |35 and 150 Ampere-turns|
+|OAT |10 and 80|
+<WRAP center round important 60%>
+NOTE : if the leads on a switch are cut, the CAT and  OAT values will increase
+Bending modifies the CAT and OAT values of a switch
+Cutting the leads on a REED switch has the effect of increasing the CAT and OAT ratings in a significant manner.
+</WRAP>
+The figure below illustrates this variation.
+{{:projects:screenshot_2024-02-18_195106.png?400|}}
+===== Reed Reference Documents =====
+  * {{:projects:ap-interen.pdf|}}
+  * {{:projects:notes_reed_switch_modifications.pdf|}}