Wireless control over a long distance with the LoRa modem

A Practical Telemetry System

To demonstrate the utility of the LoRa modem module, I packaged two Raspberry Pis into waterproof plastic boxes. One Pi was set up as the transmitter running the switch application, with two control switches and an LED mounted externally (Figure 5). Toggling either switch causes a packet to be sent to the receiver. The LED flashes when the transmitter receives an ACK from the receiver.

Figure 5: The switch module.

The second Pi runs the relay application and has a small module with two relays [11] wired and mounted internally, and a single LED mounted externally that flashes on reception of a valid packet from the transmitter (Figure 6).

Figure 6: The relay module.

This is pretty much the simplest application of LoRa. You can configure a lot more, such as an example spreading factor, channel frequency, and bandwidth that allow you to tailor the radio link to a specific situation. I was able to outfit both modules to run on solar-charged battery packs (Figure 7).

Figure 7: Switch and relay modules powered from solar-charged battery packs, ready for deployment.


The software for this system builds on the test software described earlier. The complete code for both switch and relay applications, together with a makefile are available for download from my GitHub page [12]. Only the end-user parts of the code are listed here; for brevity, I have omitted error-reporting code.

Listing 1 shows the switch module. This module sets up the required GPIO pins and then enters a loop looking for pin changes from the external switches. In detecting a change, it sends a message to the receiver to turn on or off the associated relay.

Listing 1

The Switch

01 #include "SX1272.h"
02 #include <stdio.h>
03 #include <syslog.h>
05 int main ()
06 {
07         // set up the LoRa modem
08         sx1272.ON();
09     sx1272.setMode(4);
10         sx1272.setHeaderON();
11         sx1272.setChannel(CH_17_868);
12         sx1272.setCRC_ON();
13         sx1272.setPower('M');
14         sx1272.setNodeAddress(3);
16         // set up GPIO: 2 inputs and one output
17         bcm2835_gpio_fsel(RPI_V2_GPIO_P1_18,BCM2835_GPIO_FSEL_INPT);
18         bcm2835_gpio_set_pud(RPI_V2_GPIO_P1_18,BCM2835_GPIO_PUD_UP);
19         bcm2835_gpio_fsel(RPI_V2_GPIO_P1_08,          BCM2835_GPIO_FSEL_INPT);
20         bcm2835_gpio_set_pud(RPI_V2_GPIO_P1_08,          BCM2835_GPIO_PUD_UP);
21         bcm2835_gpio_fsel(RPI_V2_GPIO_P1_16,BCM2835_GPIO_FSEL_OUTP);
23         // get the current switch states
24         uint8_t oldLev_08=bcm2835_gpio_lev(RPI_V2_GPIO_P1_08);
25         uint8_t oldLev_18=bcm2835_gpio_lev(RPI_V2_GPIO_P1_18);
27         while(1)
28         {
29                 // get the new states
30                 uint8_t lev_08=bcm2835_gpio_lev(RPI_V2_GPIO_P1_08);
31                 uint8_t lev_18=bcm2835_gpio_lev(RPI_V2_GPIO_P1_18);
33                 //if state is different, send message and update
34                 if(lev_08 != oldLev_08)
35                 {
36                         char *msg=(char*)(!lev_08?"ON0":"OFF0");
37                         syslog(LOG_USER,msg);
38                 uint8_t e = sx1272.sendPacketTimeoutACKRetries(8, msg);
40                         if(e==0)
41                         {
42                                 // flash the LED
43                                 bcm2835_gpio_set(RPI_V2_GPIO_P1_16);
44                                 bcm2835_delay(500);
45                                 bcm2835_gpio_clr(RPI_V2_GPIO_P1_16);
46                                 // update rthe state
47                                 oldLev_08=lev_08;
48                         }
49                 }
51                 //if state is different, send message and update
52                 if(lev_18 != oldLev_18)
53                 {
54                         char *msg = (char*)(!lev_18?"ON1":"OFF1");
55                         syslog(LOG_USER,msg);
56                 uint8_t e=sx1272.sendPacketTimeoutACKRetries(8, msg);
58                         if(e==0)
59                         {
60                                 // flash the LED
61                                 bcm2835_gpio_set(RPI_V2_GPIO_P1_16);
62                                 bcm2835_delay(500);
63                                 bcm2835_gpio_clr(RPI_V2_GPIO_P1_16);
64                                 // update rthe state
65                                 oldLev_18=lev_18;
66                         }
67                 }
69                 // wait a bit & repeat
70                 bcm2835_delay(500);
71         }
72 }

Listing 2 shows the relay module, which sets up the required GPIO pins and then enters a loop listening for messages from the switch module. In detecting a message, it turns on or off the associated relay.

Listing 2

The Relay

01 #include "SX1272.h"
02 #include <stdio.h>
03 #include <string.h>
05 int main ()
06 {
07         // set up the LoRa modem
08         sx1272.ON();
09     sx1272.setMode(4);
10         sx1272.setHeaderON();
11         sx1272.setChannel(CH_17_868);
12         sx1272.setCRC_ON();
13         sx1272.setPower('M');
14         sx1272.setNodeAddress(8);
16         // set up GPIO: 3 outputs
17         bcm2835_gpio_fsel(RPI_V2_GPIO_P1_26,BCM2835_GPIO_FSEL_OUTP);
18         bcm2835_gpio_fsel(RPI_V2_GPIO_P1_18,BCM2835_GPIO_FSEL_OUTP);
19         bcm2835_gpio_fsel(RPI_V2_GPIO_P1_16,BCM2835_GPIO_FSEL_OUTP);
21         // turn both relays off
22         bcm2835_gpio_clr(RPI_V2_GPIO_P1_18);
23         bcm2835_gpio_clr(RPI_V2_GPIO_P1_26);
25         while(1)
26         {
27                 // wait for an incoming message and sens an ACK
28         if (sx1272.receivePacketTimeoutACK(10000) == 0)
29         {
30                         // get the packet data and act on it
31             char *msg = (char *)sx1272.packet_received.data;
32             printf("%s\n", msg);
33             sx1272.getRSSI();
35             if(strcmp("ON0", msg)==0)
36             {
37                         bcm2835_gpio_set(RPI_V2_GPIO_P1_18);
38             }
40             if(strcmp("OFF0", msg)==0)
41             {
42                         bcm2835_gpio_clr(                          RPI_V2_GPIO_P1_18);
43             }
45             if(strcmp("ON1", msg)==0)
46             {
47                         bcm2835_gpio_set(RPI_V2_GPIO_P1_26);
48             }
50             if(strcmp("OFF1", msg)==0)
51             {
52                         bcm2835_gpio_clr(RPI_V2_GPIO_P1_26);
53             }
55                         // flash the LED
56                         bcm2835_gpio_set(RPI_V2_GPIO_P1_16);
57                         bcm2835_delay(500);
58                         bcm2835_gpio_clr(RPI_V2_GPIO_P1_16);
59                 }
60         }
61 }

Next Steps

For further development, I should point out that LoRa data flow is truly bidirectional. There's nothing to stop the remote node, in this case the relay node, from sending unsolicited packets to the switch node. So any node can be both a sender and a receiver. It is also possible to send broadcast packets to any node listening on the same channel, as well as to send unacknowledged packets, similar to TCP/IP's UDP packets. A complete, freely available WAN implementation called LoRaWAN [13] even turns a bunch of LoRa nodes and a gateway node into a true network, with semantics very similar to TCP/IP.

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