LoraBatBox

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

A bat box is a flat wooden box with an opening on the bottom often mounted on a tree. Bats can use these to sleep during the day (they hunt at night for insects). To a bat, a bat box emulates a roost in a tree. Some kinds of bats can even use a bat box to raise their young.

To check if a bat box is in use by a bat, you can walk up to the box during the day 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 also be done automatically!

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. Periodically (say once a day), the presence of bats is broadcast using a LoRa transmitter forwarding it to a LoRa gateway from the-things-network (TTN), eventually collecting all data in a central place.

Design choices

To sense the presence of bats, I plan to use a PIR (passive infrared) sensor to sense movement inside the bat box. A PIR typically has a low current requirement (say 70 uA), so it can probably be turned on all the time. Sleeping bats will like not trigger the movement detection, but that's OK I think, knowing when they enter or leave the bat box is probably enough info.

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 (with a fair-use policy). If the TTN 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.

As the main processor, I plan to use an STM32 processor:

  • already voltage compatible (3.3V) with an RFM95 LoRa transceiver, perhaps I can power it directly from a LiFePO4 cell
  • low-cost, processor board can be found for about E2,- each
  • apparently has good low-power performance and sleep modes
  • has a 32 kHz crystal on board, so it can run an RTC

To power the electronics, I plan to use a LiFePO4 battery. A battery of this type has a voltage of around 3.3V, 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).

Operation

The basic idea is to indicate whether movement was detected within an interval of 15 minutes. So each day the electronics see 96 such intervals (24 hours, 4 intervals per hour). Encoded using 1 bit per interval, we can send the result of one day in just 12 bytes (excluding overhead from the LoRaWAN protocol).

If movement is detected, the electronics can just note this in internal memory, turn off the PIR and go to sleep for the rest of the 15 minute interval. This way, we conserve the maximum amount of energy in the battery and make it last as long as possible. (idea: use a solar panel to recharge).

Once a day, at some random time, the bat box sends all the bits collected so far to the network application connected to the TTN.

To do all of this, the LoraBatBox needs to know the time. To keep the time, there is a real-time-clock (RTC) onboard the STM32 processor (which also consumes very low power). I plan to synchronize the time using the following algorithm:

  • in every "uplink" transmission from the bat box to the network, there is a field containing the UTC time from the point of view of the bat box (seconds since 1970/1/1)
  • upon reception, the network application compares the time field from the bat box with the actual time
  • if the difference is beyond some threshold (say 2 minutes), it prepares a downlink transmission with the required offset to correct the time

So 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 downlink message with the correct offset. When the bat box receives the downlink 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, since the network sends a time *difference*.

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