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LoRa Location Tracker Part 4: Transmitter (Hardware)

In the last chapter, we looked at the server part. Then in this chapter, I will start discussing the transmitter. As the transmitter is designed for the elderly to wear, hence, the size I should make it as small as possible. Also, a Lithium polymer battery is needed to make the transmitter wearable. Therefore, in this chapter, the main target of the transmitter is to make it as small as possible and make it wearable.

The LoRa communication in this system is supported by the Adafruit RFM69W LoRa module. The Adafruit module has an SX1276 LoRa based module with an SPI interface. It enables the data transmission of this module up to a 2 km line of sight with simple wire antennas or up to 20 km with directional antennas (i.e., wide coverage area).

For the MCU, I chose the Arduino Nano 33 IOT, which is small enough for me and has a built-in Wi-fi module that I can use in the receiver part.

Then, to recharge the battery, I used a Microchip MCP73832 IC for charging the Lithium polymer battery. Microchip MCP73832 IC is a typical Li-Ion Battery Charger, the charging current is between 15 mA and 500mA. 

As the MCU needed a 5V input voltage, but the battery is only 3.7V. Hence, I used a Texas Instruments LMR62421XMFE/NOPB IC to a boost converter, and step up the voltage from 3.7V to 5V to the microcontroller.  The input voltage range of the Texas Instruments LMR62421XMFE/NOPB IC is between 2.7V and 5.5V, the output voltage is up to 24V, and the switch current is up to 2.1A.

The power consumption of the transmitter is 0.22W and it can operate for around 20 hours with a 3.7V Lithium-polymer battery. Note that the power consumption of LoRa is low compared with Wi-fi (0.37W).

Circuit under test

Note that the size of the shell is small so that a user can wear it as a necklace and the design of the shell supports the ON/OFF button and the socket to charge the battery inside.

The function of a LoRa transmitter is simple: it repeatedly sends signals with its user ID (an integer) to all the LoRa receivers after a specified period (e.g., five seconds) so that the system can locate the position of the transmitter through the RSSI values of signals received by the receivers. If there is more than one user in a system, they may take turns transmitting signals.


Circuit in an enclosure

Boost up voltage circuit

Below, the first image shows the battery voltage and shows the measured voltage is 3.579 V. and the following image shows the MCU input voltage, the input voltage is 4.91 V. Hence, the boost up voltage circuit has worked.

Battery Voltage of 3.579V

MCU input voltage of 4.91V

Charging circuit

Below shows the charging of the LoRa transmitter. A red LED lights up next to the Micro USB port to indicate that the Li-polymer battery is successfully charged.

Charging the LoRa transmitter

Battery level circuit

The below table shows the battery level circuit of the LoRa transmitter. Fig. a shows when the battery is fully charged, the voltage is 3.7 V and both red and green LEDs are on. Fig. b shows when the voltage decreases to 3.0 V (normal operation level), the red LED is on and the green LED is off. When the voltage drops to 2.4V (see Fig. c), the red LED is still on and the green LED is on to show that it is “low battery”, When the voltage drops to 2.3V (see Fig. d), the green LED is off and later (see Fig. e), when voltage drops to 2.2 V (the power is not enough from the microcontroller), the red LED is still on but the MCU is off.

Battery level circuit

Shell design

Shell design

Also, I have designed a 3D printing shell for the LoRa transmitter. It has a sliding cover to protect the PCB, and the LoRa antenna is also placed inside the case. The PCB is turned upside down, better performance for the LoRa module. However, I haven’t printed it out, and I only found a plastic box to carry it.

In the next chapter, I will focus on the programming part and introduce how the transmitter works in this system.


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