Wemos and reed switch example

The reed switch is an electrical switch operated by an applied magnetic field. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied. The switch may be actuated by a coil, making a reed relay,or by bringing a magnet near to the switch. Once the magnet is pulled away from the switch, the reed switch will go back to its original position.

An example of a reed switch’s application is to detect the opening of a door, when used as a proximity switch for a burglar alarm.



Here is a module that is part of a sensor kit which is sold for use with the Arduino


In this case we have a schematic of the module, so if you wanted to build your own one, you would need the parts in the schematic below



This is for the module above

A0  ->   Not used in this example
G    -> Wemos Mini Gnd
+    -> Wemos Mini 3v3
D0  -> Wemos Mini D8 (you could use an digital pin)



Very basic example, when the reed switch is open Wemos Mini pin 8 will be low (0). Place a magnet near the reed switch and it should close which is a high and pin 8 will be high (1)

If you open the serial monitor you will see this

const int digital = D8;
void setup()
  pinMode(digital, INPUT);
void loop()



Under $1 for one of these modules, so again won’t hurt the wallet too much
AliExpress.com Product – Reed switch Module for Arduino UNO


  1. This appears to be a universal board meant for optical sensors – with a reed switch in place of the phototransistor.
    The pot looks like it’s set up as a voltage divider in order to set the voltage level at which the phototransistor trips the comparator (+) leg, thus setting the optical sensitivity.
    The 10K input resistor in series with the phototransistor makes the other voltage divider for the (-) leg.
    The capacitor in the feedback loop (-) damps any noise on the input leg. This will probably debounce the reed switch just fine.
    With the right thermistor, this could work as a temp switch as well.


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