Introduction

The reed switch is used in detecting if the door is closed or opened. However the MCU (and maybe for almost all MCU) I used, only accpets the high to low transaction external interrupt for waking up the MCU.

Hence a simple circuit is needed to allow triggering high to low interrupt in both door closing and door opening. This is basically a form or NOT logic circuit using transistor.

Circuit

A simple circuit is designed:

Here the 1Ohm resistor is the reed switch in the state of closing. we could see that on the collector pin of NPN transistor, the voltage is high.

Now when we open the reed switch:

The collector pin voltage is low

How to use

The base pin is connect to one GPIO (allowing external interrupt) on MCU to wake up the MCU when reed becomes from open to close.

The collector pin is connect to another GPIO on MCU to wake up the MCU when reed becomes from close to open.

Power consumption

In most time, reed is closed (door is closed), the power consumption is 3uA. While if reed is open, the power consumption is 5.36uA.