Which magnetic door/window switch for battery powered projects?

[maddhin]

Hi,
I am new to MySensors and planning to build battery powered window/door sensors based on a "raw" ATMEGA328P with 2xAA batteries.

I am opting for the usual, cheap white Ali door switches (e.g. https://www.aliexpress.com/item/32255861885.html?spm=a2g0o.productlist.0.0.29ec3a95TEE6We&algo_pvid=48234541-eff1-4f39-96ee-284b1da18796&algo_expid=48234541-eff1-4f39-96ee-284b1da18796-1&btsid=2100bde316181377232332753e08a1&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_).

But the thing confusing me is the "Normally Closed"/NC and "normally open" /NO.

In my understanding NC means that the circuit is closed = current can flow. So for a battery project I think it is best to use NO switches, which only connect when the door/window is opened and therefore wake up the sensor to send a message.

But, NO switches seem to be hard to come by at Ali.

Am I missing something or do I misunderstand something? What would be the best setup for a battery powered project?

[Richard_Ske]

If you put a large resistor (>100K) between the pin you want to use for sensing and Vcc (battery voltage) then link one of the wires of the sensor to gnd and the other wire to the sense pin the circuit can then check for a change.

So if the window is shut then the sense pin will be Vcc if it’s open then it will be 0V

I hope that helps

[skywatch]

@maddhin You need to look for "changeover reed relay" - they are cheap and allow you to reverse the logic in the door/window sketch and get a much longer battery life.

I connect a 5.6Mohm from input pin to ground and then via the relay contacts that are OPEN when in magnetic field to VCC (3V).
This way the input is always at ground until the door/window opens and then only a small current flows into the input and via that big resistance to ground.

These relays are glass and can be fragile, so handle with care and make something to put them in to protect them.

https://www.aliexpress.com/item/32831085928.html

[maddhin]

@skywatch said in Which magnetic door/window switch for battery powered projects?:

@maddhin You need to look for "changeover reed relay" - they are cheap and allow you to reverse the logic in the door/window sketch and get a much longer battery life.

Awesome, thank you for this. I came across those switches but I would find it troublesome to install them if I can find ready-to-install sets for similar price.

I found a useful answer over at stackexchange, where they recommend your and @Richard_Ske's solution as well: connect Vcc with a large resistor (3.3MOhm) and input pin and input pin/resistor with switch and grd. According to that post, the power consumption is not much higher than a NO setup.

Would that be about right? You write "much longer battery life" for NO setup. If there is a large difference in power consumption, I would go and try to find NO switches, which exist, but are significantly more expensive (relatively speaking). It is very hard for me to judge this as my skills are limited. I would love to operate a bunch of SlimNodes with 2 x AA batteries.

[skywatch]

@maddhin I am running with 2xAAA and so far so good - been 8 weeks and still at or above 3V.

As for the relays, you could put them in any small plastic tube (ballpoint pen or similar) or even in a white plastic drinking straw, cut to length and seal the ends with silicone. Maybe add some plastic or wood (toothpicks glued together) for strength....Just a suggestion to keep costs down.....If you have access to a 3D printer then you can custom make something......

[maddhin]

@skywatch awesome! Once all my parts arrive, I'll test as well! I'm planning a letterbox sensor, so usage will be minimal (1-3 events a day), hence my concern with the power consumption NO vs. NC. But if battery life (out of 2 x AA) is 2 years+, it shall be ok
I guess, I just need to build and test...
One issue I'll be facing is that the cable between ATMEGA328P and the sensor will be 50-60cm. I read that this could cause issues with the large resistor method:(

[skywatch]

@maddhin You attach the 'large resistor' on the ATmega board from ground to input pin used for interrupt. Then connect the same input pin to 3V via the reed relay switch contacts that are OPEN when the door/window/letterbox is in the untripped (closed) position. If you use the sketch on here you need to reverse the tripped/untripped (open/closed) as the basic sketch is negative triggered and this way it is positive triggered -

I doubt that you will have issues with the cable at such short distance, but if you do just drop down to the next lowest value resistor and try again until you have the desired result. I assume you know about removal of components (or track cutting) and bootloading to get max battery life?

I am sure you will work it out but if not post again and I will always try and help you!

[monte]

Honeywell SM35.

[maddhin]

@skywatch said in Which magnetic door/window switch for battery powered projects?:

@maddhin You attach the 'large resistor' on the ATmega board from ground to input pin used for interrupt. Then connect the same input pin to 3V via the reed relay switch contacts that are OPEN when the door/window/letterbox is in the untripped (closed) position. If you use the sketch on here you need to reverse the tripped/untripped (open/closed) as the basic sketch is negative triggered and this way it is positive triggered -

sorry, it took me a while to get back to this project as finally all parts for my slim nodes arrived and I first had to tinker with the temp sensor:)

The NC door switch is working now - although I did connect the large resistor with VCC and pin and connected the switch with pin and GND. The other way around didn't seem to work for me.

Here is my sketch for other people having similar issue or for comments (improvements always welcome!).

/**
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2015 Sensnology AB
 * Full contributor list: https://github.com/mysensors/Arduino/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
*******************************
 * 
 * DESCRIPTION
 *
 * Simple binary switch example 
 * Connect button or door/window reed switch between 
 * digitial I/O pin 3 (BUTTON_PIN below) and GND.
 * http://www.mysensors.org/build/binary
 * 
 * Connect normally closed (NC) door sensor:
 * VCC <--> 3.3M ohm resistor <--> pin D3
 * GND <--> NC door sensor <--> pin D3
 * REVISION HISTORY
 * v1.0: Example at https://www.mysensors.org/build/binary
 * v1.01: Maddhin
 * v1.03: first working version
 * 
 */ 
 

#define SKETCH_NAME "DoorSen_NC"
#define SKETCH_VERSION "1.03"        //<--------- UPDATE version nummer!!!

// Enable debug prints
#define MY_DEBUG                   //<--------- switch DEBUG mode on/off

// Enable REPORT_BATTERY_LEVEL to measure battery level and send changes to gateway
#define REPORT_BATTERY_LEVEL

// Enable and select radio type attached 
#define MY_RADIO_RF24
//#define MY_RADIO_RFM69
//#define MY_RS485

#define MY_NODE_ID 101
//#define MY_PARENT_NODE_ID 0
//#define MY_PARENT_NODE_IS_STATIC
//#define MY_RF24_PA_LEVEL RF24_PA_MAX

#include <MySensors.h>  
//#include <Bounce2.h>  

//#define CHILD_ID_HUM  0
//#define CHILD_ID_TEMP 1
#define CHILD_ID_VOLT 2

//Door sensor
#define CHILD_ID_DOOR 3
#define BUTTON_PIN  3  // Arduino Digital I/O pin for button/reed switch
#define ONE_DAY_SLEEP_TIME  86400000  // report battery status each day at least once
//Bounce debouncer = Bounce(); 
int oldValue=-1;
// Change to V_LIGHT if you use S_LIGHT in presentation below
MyMessage msg(CHILD_ID_DOOR,V_TRIPPED);


// Sleep time between sensor updates (in milliseconds)
static const uint64_t UPDATE_INTERVAL = 5*60000; // x * 1 min
//for debugging:
//static const uint64_t UPDATE_INTERVAL = 5000;

/*#include "Adafruit_HTU21DF.h"
Adafruit_HTU21DF sensor = Adafruit_HTU21DF();

// Set this offset if the sensor has a permanent small offset to the real temperatures
#define SENSOR_TEMP_OFFSET 0
*/

#ifdef REPORT_BATTERY_LEVEL
#include <Vcc.h>
static uint8_t oldBatteryPcnt = 200;  // Initialize to 200 to assure first time value will be sent.
const float VccMin        = 1.8;      // Minimum expected Vcc level, in Volts: Brownout at 1.8V    -> 0%
const float VccMax        = 2.0*1.6;  // Maximum expected Vcc level, in Volts: 2xAA fresh Alkaline -> 100%
const float VccCorrection = 1.0;      // Measured Vcc by multimeter divided by reported Vcc
static Vcc vcc(VccCorrection); 
#endif

#ifdef MY_DEBUG
 #define DEBUG_PRINT(x)    Serial.print (x)
 #define DEBUG_PRINTDEC(x) Serial.print (x, DEC)
 #define DEBUG_PRINTLN(x)  Serial.println (x)
#else
 #define DEBUG_PRINT(x)
 #define DEBUG_PRINTDEC(x)
 #define DEBUG_PRINTLN(x)
#endif



void setup()
{
  
  /*// Temp Hum Sensor
  while (!sensor.begin())
  {
    DEBUG_PRINTLN(F("Sensor not detected!"));
    while(1);
    delay(5000);
  }*/

  //Door Sensor
  // Setup the button
  pinMode(BUTTON_PIN,INPUT);
  // Activate internal pull-up
  digitalWrite(BUTTON_PIN,HIGH);
  
  /*// After setting up the button, setup debouncer
  debouncer.attach(BUTTON_PIN);
  debouncer.interval(5);
  */
}

void presentation()  
{ 
  // Send the sketch version information to the gateway and Controller
  DEBUG_PRINT("SKETCH NAME: "); 
  DEBUG_PRINT(SKETCH_NAME);
  DEBUG_PRINT(", VERSION: ");
  DEBUG_PRINTLN(SKETCH_VERSION); 
  
  sendSketchInfo(SKETCH_NAME, SKETCH_VERSION);
  
  // Present sensors as children to gateway
  //present(CHILD_ID_HUM, S_HUM,   "Humidity");
  //present(CHILD_ID_TEMP, S_TEMP, "Temperature");
  present(CHILD_ID_VOLT, S_MULTIMETER, "Voltage");
  // Register binary input sensor to gw (they will be created as child devices)
  // You can use S_DOOR, S_MOTION or S_LIGHT here depending on your usage. 
  // If S_LIGHT is used, remember to update variable type you send in. See "msg" above.
  present(CHILD_ID_DOOR, S_DOOR);  
}

void loop()      
{  
  /*// Read temperature & humidity from sensor.
  const float temperature = float( sensor.readTemperature() );
  const float humidity    = float( sensor.readHumidity() );

  DEBUG_PRINT(F("Temp "));
  DEBUG_PRINT(temperature);
  DEBUG_PRINT(F("\tHum "));
  DEBUG_PRINTLN(humidity);

  static MyMessage msgHum( CHILD_ID_HUM,  V_HUM );
  static MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

  send(msgTemp.set(temperature, 1));
  send(msgHum.set(humidity, 1));
*/

  //Door sensor
  /* debouncer.update();
  // Get the update value
  int value = debouncer.read();
  */
  int value = digitalRead(BUTTON_PIN);
  //DEBUG_PRINT("digitalRead PIN: ");
  //DEBUG_PRINTLN(value);
  wait(50); // "debouncing"
  //if (value != oldValue) {
  if (value != oldValue) { 
     // Send in the new value
     DEBUG_PRINT(F("DOOR SENSOR: "));
     DEBUG_PRINTLN(value==HIGH ? 1 : 0);
     send(msg.set(value==HIGH ? 1 : 0));
     oldValue = value;
  }

#ifdef REPORT_BATTERY_LEVEL
  const uint8_t batteryPcnt = static_cast<uint8_t>(0.5 + vcc.Read_Perc(VccMin, VccMax));
  
  static MyMessage MsgVolt(CHILD_ID_VOLT, V_VOLTAGE);  // Node voltage

  DEBUG_PRINT(F("Vbat "));
  DEBUG_PRINT(vcc.Read_Volts());
  DEBUG_PRINT(F("\tPerc "));
  DEBUG_PRINTLN(batteryPcnt);


  // Battery readout should only go down. So report only when new value is smaller than previous one.
  if ( batteryPcnt < oldBatteryPcnt )
  {
      send(MsgVolt.set(vcc.Read_Volts(),2));
      sendBatteryLevel(batteryPcnt);
      oldBatteryPcnt = batteryPcnt;
  }
  #endif

  // Sleep until next update to save energy
  //sleep(UPDATE_INTERVAL); 
  sleep(BUTTON_PIN-2, CHANGE, ONE_DAY_SLEEP_TIME);
}

Now, I'll look into adding a second door switch onto the same 328P and hope I have a neat letterbox sensor up and running soon...

Power consumption I will have to monitor though. But I don't really know how to measure the sleep / send current. I guess I should build some INA219 based monitor or similar. I guess, for the moment, I just "hope" the setup is good and I'll see how long the batteries will last. After running for ~24h and having triggered a bunch of times, the voltage didn't seem to have changed, so I guess that is good news...

Thanks for your help, guys!