LoraBatBox: Difference between revisions

Latest revision as of 12:11, 28 July 2022

Project LoraBatBox

Monitoring bat boxes with LoRa
Status	Completed
Contact	bertrik
Last Update	2022-07-28

This page is an idea for adding LoRa capability to a bat box.

A popular kind of bat box is a flat wooden box with an opening on the bottom and mounted to a tree trunk or building. Bats can use these to sleep during the day (they hunt at night for insects). Some kinds of bats can even use a bat box to raise their young.

To check if a bat box is in use by bats, you can walk up to the tree that the box is mounted on and shine a light into it to see if there are any bats there. But I think it would be nice to investigate if this can be done automatically, without having to go to the individual box and without disturbing the bats with a light!

Update: Consider this ready-built sensor: https://www.antratek.nl/lorawan-motion-sensor It appears it already does what this project was planned to do. Senses motion in a 50x50x20 mm box with battery compartment, built-in LoRaWAN antenna, low-power design. Also reports internal temperature and battery level. This sensor is supported on http://cayenne.mydevices.com

Apparently KPN got this idea now too, using their own network: https://www.overons.kpn/nl/nieuws/2021/innovatieve-toepassing-op-lora-meet-aanwezigheid-beschermde-diersoorten-en-geeft-boost-aan-verduurzaming-woningsector

How to migrate this particular sensor from TTNv2 to TTNv3: https://www.thethingsnetwork.org/forum/t/migrating-tabs-browan-sensors-remote-reboot-to-trigger-otaa/44461

Introduction

The basic idea is to mount some piece of electronics in or on the bat box that monitors if there are any bats in there, by monitoring movement within the bat box. Periodically (say once an hour), detection of movement is broadcast using a LoRaWAN radio transmitter to a central place.

Hardware design

PIR movement detection

To sense the presence of bats inside the bat box, I plan to use a PIR (passive infrared) detector module which can detect movement of warm bodies. A PIR typically has a low current requirement (say < 0.1 mA), so it can probably be turned on most of the time. Sleeping bats will likely not trigger the movement detection, but that's OK I think, knowing when they enter or leave the bat box is probably enough info.

PIR sensor investigation

I ordered a couple of different mini PIR sensors, so I could do some measurements on which PIR sensor is the 'best', meaning good sensitivity and low quiescent current.

The table below shows some of the properties I measured, including the supply current. Some sensors use a little more when activated, but this was so little that I didn't write it down separately. Also, all sensors I bought seem to have an internal regulator, so every one was fed with 5V, which is then down-regulated to some lower voltage on board the PIR module. This can be a problem because I plan to use a single lithium cell so I have only about 3.2V available.

Price	Claimed current	Measured current	Onboard regulator	Remarks
USD 1.99	< 0.1mA	15 uA	HT7530-1 (3.0V)	probably best candidate to investigate further: probably 3.3V compatible + very low idle current. See also this page, explaining it contains an AS312 PIR sensor, with the electronics on the board basically only doing some voltage regulation
USD 2.19	< 50uA	1 mA	7144 (4.4V)	probably worst candidate: high idle current, weird pitch (2mm?) connector, not 3.3V compatible!
USD 1.75	<60uA	75 uA	7133 (3.3V)	aka HC-SR505, it looks like it misses a some part connecting to the OUT pin!

In relation to being 3.3V compatible, here's a guide on modifying a PIR module for 3.3V.

LoRa / The-Things-Network

I think the LoRa radio technology is perfectly suited as a transmission medium for sending the bat box occupancy data. LoRa has a larger range than similar radio technologies, has relative immunity to narrow-band interference and allows for low-power transmissions. There are already several LoRa gateways provided by the-things-network (TTN) that can receive and forward the data for free (no subscription needed, fair-use policy). If the LoRa network is down for some reason, no harm is done to the bats, possibly just a few days worth of data lost, and I think I can come up with a solution to that.

Microcontroller

I've been considering several boards:

Heltec lora32 v2, familiar platform (ESP32), has a display, has low-power support in hardware (Vext can be switched off), has a 32 kHz crystal to use with the onboard RTC, however not sure if I can use that with Arduino software
Heltec Cubecell board, should also be very low power, recently got platformio support
stm board, currently not investigating

LiFePO4 Battery

To power the electronics, I plan to use a LiFePO4 battery. A battery of this type has a voltage of around 3.2V, so it is capable of running the electronics directly, without a voltage regulator. It also has a quite large capacity (mAh per gram) and is relatively safe (doesn't catch fire easily, unlike some other lithium battery types).

Alternatively, just use a standard 18650 lithium cell and put a diode in series to drop the voltage by about 0.6V.

Operation

The basic idea is to indicate whether movement was detected with one minute resolution. If movement is detected by the PIR, the processor is woken up, it notes the movement in internal memory, goes back to sleep.

At some larger interval, the accumulated data is uploaded, possibly with redundancy to allow for the occasional missed message.

time synchronisation algorithm

It would be nice for the node to know the time, so it can accurately report on time periods where movement was detected.

The LoRaWAN specification has the MAC commands DevTimeReq and DevTimeAns, however they are not supported by TTN yet. Using them even might give interoperability problems.

To keep the time, we can use the (RTC) onboard the processor/board. The plan is to synchronize the time using the following algorithm:

in an uplink transmission from the bat box to the network, there is a field containing the UTC time (seconds since 1970/1/1) from the point of view of the bat box
upon reception (a few seconds later), the network application compares the time field sent by the bat box with the actual time
if the difference is beyond some threshold (say plus/minus 30 seconds), it prepares a down-link transmission with the required offset to correct the time
when the bat box receives the down-link transmission, it updates its clock with the correction offset.

So, for example if a bat box powers up with no knowledge of the time at all, it'll start at UTC time 0. The network application will note a huge time error and prepare a down-link message with the correct offset. When the node receives the down-link message, it applies the offset to its clock and so updates itself to the correct time. It doesn't really matter how much time there is between an uplink and a downlink or when the downlink message is scheduled exactly, since the network sends a time *difference*, not an absolute time. Downlink messages are only sent when there is a large time error, so they should be relatively infrequent, reducing the network load.

message format

All messages in big endian format.

Uplink (device to network):

first 4 bytes: timestamp in seconds (since 1970/1/1)
variable number of bytes: one bit per minute (1 = movement, 0 = no movement) in the past.

Downlink (network to device):

4 bytes: time correction value (signed, seconds), positive value means that the device should move its time forward
4 bytes: time correction sequence number (to prevent duplicate correction), devices should only accept increasing sequence number

For redundancy, messages can be "overlapping", e.g. send 30 bits presence data (for the past 30 minutes), every 10 minutes.

When sending 1 message every 10 minutes, each message contains 4 byte timestamp + 4 byte movement = 8 bytes:

Using SF9 this fits within the TTN FUP, total 26.64 seconds per day.
Using SF10 exceeds the FUP by a factor 2 approximately.
Using SF11 exceeds the FUP by a factor 4 approximately.

Software

Arduino firmware: https://github.com/bertrik/lorabatbox
backend Java application: https://github.com/bertrik/motionsensorbackend

External references

Future ideas

Challenges and ideas to be implemented in the future:

find some way to keep the electronics safe from weather, e.g. wrap it all in a kind of silicone pouch
use the built-in RTC of the ESP32, use ESP32 low-power sleep

@@ Line 3: / Line 3: @@
     |Omschrijving=Monitoring bat boxes with LoRa
     |Picture=lorabatbox.jpg
-    |Status=Initializing
+    |Status=Completed
     |Contact=bertrik
 }}
@@ Line 15: / Line 15: @@
 To check if a bat box is in use by bats, you can walk up to the tree that the box is mounted on and shine a light into it to see if there are any bats there.
 But I think it would be nice to investigate if this can be done automatically, without having to go to the individual box and without disturbing the bats with a light!
+Update:
+Consider this ready-built sensor: https://www.antratek.nl/lorawan-motion-sensor
+It appears it already does what this project was planned to do. Senses motion in a 50x50x20 mm box with battery compartment, built-in LoRaWAN antenna, low-power design. Also reports internal temperature and battery level.
+This sensor is supported on http://cayenne.mydevices.com
+Apparently KPN got this idea now too, using their own network:
+https://www.overons.kpn/nl/nieuws/2021/innovatieve-toepassing-op-lora-meet-aanwezigheid-beschermde-diersoorten-en-geeft-boost-aan-verduurzaming-woningsector
+How to migrate this particular sensor from TTNv2 to TTNv3:
+https://www.thethingsnetwork.org/forum/t/migrating-tabs-browan-sensors-remote-reboot-to-trigger-otaa/44461
 == Introduction ==
-The basic idea is to mount some piece of electronics in or on the bat box that monitors if there are any bats in there.
+The basic idea is to mount some piece of electronics in or on the bat box that monitors if there are any bats in there, by monitoring movement within the bat box.
-Periodically (say once a day), the presence of bats is broadcast using a LoRa (long range) radio transmitter to a central place (e.g. the internet).
+Periodically (say once an hour), detection of movement is broadcast using a LoRaWAN radio transmitter to a central place.
-== Design choices ==
+== Hardware design ==
 [[File:pir-sensor.png|left|100px]]
@@ Line 55: / Line 67: @@
 [[File:HTB1PemDJpXXXXXHXpXXq6xXFXXXM.jpg|right|200px]]
-=== STM32 low-power microcontroller ===
+=== Microcontroller ===
-As the main processor, I plan to use an [https://en.wikipedia.org/wiki/STM32#STM32_F1 STM32 processor board]:
+I've been considering several boards:
-* already voltage compatible (3.3V) with an RFM95 LoRa transceiver
+* Heltec lora32 v2, familiar platform (ESP32), has a display, has low-power support in hardware (Vext can be switched off), has a 32 kHz crystal to use with the onboard RTC, however not sure if I can use that with Arduino software
-* low-cost, processor board can be found for about E2,- each
+* Heltec Cubecell board, should also be very low power, recently got platformio support
-* apparently has good low-power performance and sleep modes
+* stm board, currently not investigating
-* has a 32 kHz crystal on board, so it can run a real-time-clock (RTC)
 [[File:Lifepo4.jpg|right|100px]]
 === LiFePO4 Battery ===
 To power the electronics, I plan to use a [https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery LiFePO4 battery].
 A battery of this type has a voltage of around 3.2V, so it is capable of running the electronics directly, without a voltage regulator.
 It also has a quite large capacity (mAh per gram) and is relatively safe (doesn't catch fire easily, unlike some other lithium battery types).
+Alternatively, just use a standard 18650 lithium cell and put a diode in series to drop the voltage by about 0.6V.
 == Operation ==
-The basic idea is to indicate whether movement was detected within intervals of 15 minutes.
+The basic idea is to indicate whether movement was detected with one minute resolution.
-So each day the electronics see 96 such intervals (24 hours, 4 intervals per hour).
+If movement is detected by the PIR, the processor is woken up, it notes the movement in internal memory, goes back to sleep.
-Encoded using 1 bit per interval, we can send the result of one day in just 12 bytes (excluding overhead from the LoRaWAN protocol, about 13 bytes and our own overhead, e.g. a time stamp).
+At some larger interval, the accumulated data is uploaded, possibly with redundancy to allow for the occasional missed message.
-If movement is detected by the PIR, the processor is woken up, it notes the movement in internal memory and turns off the PIR so it can sleep for the remainder of the 15 minute interval.
+=== time synchronisation algorithm ===
-This way, we conserve the maximum amount of energy in the battery and make it last as long as possible.
+It would be nice for the node to know the time, so it can accurately report on time periods where movement was detected.
-(idea: use a solar panel to recharge).
-Once a day, at some random time, the bat box sends all the data collected so far to the network application connected to the TTN.
+The LoRaWAN specification has the MAC commands DevTimeReq and DevTimeAns, however they are not supported by TTN yet.
+Using them even might give interoperability problems.
-=== time synchronisation algorithm ===
+To keep the time, we can use the (RTC) onboard the processor/board.
-To do all of this, the LoraBatBox needs to know the time of course.
+The plan is to synchronize the time using the following algorithm:
-To keep the time, we can use the (RTC) onboard the STM32 processor.
+* in an uplink transmission from the bat box to the network, there is a field containing the UTC time (seconds since 1970/1/1) from the point of view of the bat box
-I plan to synchronize the time using the following algorithm:
-* in every "up-link" transmission from the bat box to the network, there is a field containing the UTC time (seconds since 1970/1/1) from the point of view of the bat box
 * upon reception (a few seconds later), the network application compares the time field sent by the bat box with the actual time
-* if the difference is beyond some threshold (say 2 minutes), it prepares a down-link transmission with the required offset to correct the time
+* if the difference is beyond some threshold (say plus/minus 30 seconds), it prepares a down-link transmission with the required offset to correct the time
 * when the bat box receives the down-link transmission, it updates its clock with the correction offset.
 So, for example if a bat box powers up with no knowledge of the time at all, it'll start at UTC time 0.
 The network application will note a huge time error and prepare a down-link message with the correct offset.
-When the bat box receives the down-link message, it applies the offset to its clock and so updates itself to the correct time.
+When the node receives the down-link message, it applies the offset to its clock and so updates itself to the correct time.
-It doesn't really matter how much time there is between an up-link and a down-link or when the down-link message is scheduled exactly, since the network sends a time *difference*, not an absolute time.
+It doesn't really matter how much time there is between an uplink and a downlink or when the downlink message is scheduled exactly, since the network sends a time *difference*, not an absolute time.
-Down-link messages are only sent when there is a large time error, so they should be relatively infrequent, reducing the network load on the TTN.
+Downlink messages are only sent when there is a large time error, so they should be relatively infrequent, reducing the network load.
+=== message format ===
+All messages in big endian format.
+Uplink (device to network):
+* first 4 bytes: timestamp in seconds (since 1970/1/1)
+* variable number of bytes: one bit per minute (1 = movement, 0 = no movement) in the past.
+Downlink (network to device):
+* 4 bytes: time correction value (signed, seconds), positive value means that the device should move its time forward
+* 4 bytes: time correction sequence number (to prevent duplicate correction), devices should only accept increasing sequence number
+For redundancy, messages can be "overlapping", e.g. send 30 bits presence data (for the past 30 minutes), every 10 minutes.
+When sending 1 message every 10 minutes, each message contains 4 byte timestamp + 4 byte movement = 8 bytes:
+* Using SF9 this fits within the TTN FUP, total 26.64 seconds per day.
+* Using SF10 exceeds the FUP by a factor 2 approximately.
+* Using SF11 exceeds the FUP by a factor 4 approximately.
+== Software ==
+* Arduino firmware: https://github.com/bertrik/lorabatbox
+* backend Java application: https://github.com/bertrik/motionsensorbackend
-== Links ==
+== External references ==
 * https://developer.mbed.org/teams/Semtech/code/LoRaWAN-demo-76/
+* https://www.thethingsnetwork.org/labs/story/a-cheap-stm32-arduino-node
 == Future ideas ==
 Challenges and ideas to be implemented in the future:
-* port the arduino lmic stack to the STM32 processor:
-** the current code uses a call to fdev_setup to bind printf-like debug statements to the serial port. This doesn't appear to exist in the STM32 arduino libraries, I'm working adding this to STM32 too
-** make the radio interface work over SPI, I'm confident this can work
 * find some way to keep the electronics safe from weather, e.g. wrap it all in a kind of silicone pouch
-* consider adding some kind of test signal, to verify the behaviour of the PIR, perhaps light a bulb once a day for a few seconds?
+* use the built-in RTC of the ESP32, use ESP32 low-power sleep
-* measure other properties of the bat box, like battery state (voltage), temperature and relative humidity. Once the whole data chain is set up, this should be easy.
-* create a nice user front-end that shows the received data in easy to understand map
-* add a solar panel to recharge the battery

LoraBatBox: Difference between revisions

Latest revision as of 12:11, 28 July 2022

Contents

Introduction