Linx

How to Use the LC Series LINX Modules

by Lee Rumsey

These are basic instructions on hooking up the modules. This document is split into five nifty sections:

For data sheets on the LINX modules, see this web site: http://www.linxtechnologies.com. Click on the RF Modules link. Then click on the Manuals item under the LC Series heading. READ through the LC Series data sheets to get an understanding of their function. This will reduce the likelihood of errors.

Necessary Parts

TX board (1) Smaller PC board; has TXM-XXX-LC chip on it
RX board (1) Larger PC board; has RXM-XXX-LC chip on it
Antenna (2) Either whips or helical antennas
RG-174 50W coax cable Available from ECE Store; for connecting antennas
22 ga hookup wire Should be laying around the 445 lab
390 W, 1/4 W resistor (1) Needed for 5V operation

Before we start

  1. Learn to solder. You will be making some delicate connections, so practice if you don't know how. Also be sure to use solder sparingly- big blobs will likely result in damage or malfunctions.
  2. You should have the parts listed above. Get them from a TA.
  3. Make sure the transmitter and receiver are running on the same frequency. Do this by checking the model number on the surface mount package. For example, a 418 MHz TX module will be labeled TXM-418-LC, and a 315 MHz RX module will be RXM-315-LC.

Transmitter Assembly

The LINX transmitter is the smaller of the two modules. It runs on between +2.7 and 5.2 VDC. We will use a +5 VDC power supply, so that the data input can be a TTL level signal. DO NOT apply a voltage greater than Vcc on the data input pin!

Step 1. Acclimate yourself with the circuit we are building:
schematic

Step 2. Place the TX board with chip side facing UP. The writing on the chip should be right side up, with a little wire coming off the top. This wire is GROUND. A '1' or a small dot should be visible on the lower left-hand corner of the chip. GROUND is also connected to the center bus strip on the board.

Step 3. Solder the 390 W resistor from pin 4 (lower right-hand corner) to ground. This sets the chip to accept 5 VDC. NOTE: make all connections on the top of the board. DON'T feed any leads through the holes. There are connections on the other side!

Step 4. Carefully solder a 3-inch wire to pin 2. This will be the DATA input. DO NOT apply a voltage greater than Vcc on the data input pin!

Step 5. Carefully solder a 3-inch wire to pin 7. This will be the VCC power input. REMEMBER: +5 VDC ONLY on this pin!

Step 6a. If you have a HELICAL COIL antenna, solder a short (2 inch) length of coax cable to pin 5 and GROUND. The inner conductor is the RF signal, and the outer shield goes to GROUND. Now solder the helical coil to the other end of the coax to form a magnetic loop. (You may need to extend the shield connection with a separate wire.)

Step 6b. If you have a WHIP antenna, solder the coax cable directly to pin 5 and GROUND. The inner conductor is the RF signal, and the outer shield goes to GROUND.

Here is the completed TX board assembly:
board


Receiver Assembly

The LINX receiver is the larger of the two modules. It runs on between +4 and 5.2 VDC. We will use a +5 VDC power supply. GROUND is the center vertical strip on the solder side.

Step 1. Acclimate yourself with the circuit we are building:
circuit

Step 2. Place the RX board with chip side facing DOWN, with the solder side UP. Orient the board so that one wire comes off the top, while another comes off the left of the board. The top wire is the power connection; the left wire is ground. Pin 1 of the chip is actually at the lower left with this orientation; it is identified on the chip by a '1' or a dot.

Step 3. Carefully solder a 3-inch wire to pin 5 (top left solder pad connected to the chip). This will be the DATA output.

Step 4a. If you have a HELICAL COIL antenna, solder a short (2 inch) length of coax cable to pin 1 and GROUND. The inner conductor is the RF signal, and the outer shield goes to GROUND. Now solder the helical coil to the other end of the coax to form a magnetic loop. (You may need to extend the shield connection with a separate wire.)

Step 4b. If you have a WHIP antenna, solder the coax cable directly to pin 1 and GROUND. The inner conductor is the RF signal, and the outer shield goes to GROUND.

Step 5. IMPORTANT! If there is a connection to pin 10 on the Linx chip, carefully desolder it, leaving the others intact. It is misconnected.

Here is the completed RX board assembly:
complete


Testing and Operation

Now we'll see if these modules work. If you are working with 418 MHz chips, then a reference receiver - transmitter pair are available for testing. Otherwise, you better hope you made the right connections? we don't have anything to test the 315 MHz RX/TX chips, except maybe another functioning pair of modules.

Step 1. TESTING THE TRANSMITTER. Refer to the schematic for the TX module we just assembled. Connect +5 VDC to the power lead, and ground the GROUND lead.

Step 2. Turn on or plug in the power. Use +5 VDC only! If anything is heating up, UNPLUG the module and check your connections!

Step 3. Set up a function generator the make a 1 kHz TTL compatible square wave (i.e. it swings between 0 and +5V only!) Connect this signal to the data input lead from the module. If anything is heating up, UNPLUG the module and check your connections!

Step 4. Plug the data output of the reference receiver into an oscilloscope. ASK A TA FOR ASSISTANCE! If all is well, a 1 kHz square wave should appear on the screen. You are done; wire your module into the rest of your circuit.

NOTE: For a flaky power supply, a bypass capacitor (10 uF electrolytic) may be necessary between power and ground on YOUR board.

Step 5. TESTING THE RECEIVER. Refer to the schematic for the RX module we just assembled. Connect +5 VDC to the power lead, and ground the GROUND lead

Step 6. Turn on or plug in the power. Use +5 VDC only! If anything is heating up, UNPLUG the module and check your connections!

Step 7. Set up a function generator the make a 1 kHz TTL compatible square wave (i.e. it swings between 0 and +5V only!) Connect this signal to the data input on the reference transmitter. ASK A TA FOR ASSISTANCE! If anything is heating up, UNPLUG the module and check your connections!

Step 8. Connect the data output of your receiver module into an oscilloscope. If all is well, a 1 kHz square wave should appear on the screen. You are done; wire your module into the rest of your circuit.

THE END.

Covert Communication Device

Ahmad Abuisneineh, Srivardhan Sajja, Braeden Smith

Covert Communication Device

Featured Project

**Partners (seeking one additional partner)**: Braeden Smith (braeden2), Srivardhan Sajja (sajja3)

**Problem**: We imagine this product would have a primary use in military/law enforcement application -- especially in dangerous, high risk missions. During a house raid or other sensitive mission, maintaining a quiet profile and also having good situational awareness is essential. That mean's that normal two way radios can't work. And alternatives, like in-ear radios act as outside->in communication only and also reduce the ability to hear your surroundings.

**Solution**: We would provide a series of small pocketable devices with long battery that would use LoRa radios to provide a range of 1-5 miles. They would be rechargeable and have a single recessed soft-touch button that would allow someone to find it inside of pockets and tap it easily. The taps would be sent in real-time to all other devices, where they would be translated into silent but noticeable vibrations. (Every device can obviously TX/RX).

Essentially a team could use a set of predetermined signals or even morse code, to quickly and without loss of situational awareness communicate movements/instructions to others who are not within line-of-sight.

The following we would not consider part of the basic requirements for success, but additional goals if we are ahead of schedule:

We could also imagine a base-station which would allow someone using a computer to type simple text that would be sent out as morse code or other predetermined patterns. Additionally this base station would be able to record and monitor the traffic over the LoRa channels (including sender).

**Solutions Components**:

- **Charging and power systems**: the device would have a single USB-C/Microusb port that would connect to charging circuitry for the small Lithium-ion battery (150-500mAh). This USB port would also connect to the MCU. The subsystem would also be responsible to dropping the lion (3.7-4.2V to a stable 3.3V logic level). and providing power to the vibration motor.

- **RF Communications**: we would rely on externally produced RF transceivers that we would integrate into our PCB -- DLP-RFS1280, https://www.sparkfun.com/products/16871, https://www.adafruit.com/product/3073, .

-**Vibration**: We would have to research and source durable quiet, vibration motors that might even be adjustable in intensity

- **MCU**: We are likely to use the STM32 series of MCU's. We need it to communicate with the transceiver (probably SPI) and also control the vibration motor (by driving some transistor). The packets that we send would need to be encrypted (probably with AES). We would also need it to communicate to a host computer for programming via the same port.

- **Structural**: For this prototype, we'd imagine that a simple 3d printed case would be appropriate. We'd have to design something small and relatively ergonomic. We would have a single recessed location for the soft-touch button, that'd be easy to find by feel.

**Basic criterion for success:** We have at least two wireless devices that can reliably and quickly transfer button-presses to vibrations on the other device. It should operate at at *least* 1km LOS. It should be programmable + chargeable via USB. It should also be relatively compact in size and quiet to use.

**Additional Success Criterion:** we would have a separate, 3rd device that can stay permanently connected to a computer. It would provide some software that would be able to send and receive from the LoRa radio, especially ASCII -> morse code.