WSPR Scheduler Files

Arduino LPF Switching using PLJ-096 Frequency Counter

The PLJ-096 is a sensitive frequency counter that can be easily connected to an Arduino. This tiny module has a 0.96” OLED display and a UART serial output that can be easily interfaced to a microcontroller. It operates from a 5V supply, which makes it easy to work with an Arduino. An external antenna can be connected – a short wire is all that is needed to pick up an RF signal. The PLJ 096OLED L1 can be used at HF (operating from 1kHz to 100MHz), although a PLJ 096OLED H1 model is also available for 20MHz to 2.7GHz).
The Arduino circuit does not need any external connection (eg. wifi or serial) once tested and running, apart from the 5V supply.

Note: this circuit shows how to switch an LPF according to band, however the Arduino app can also be used to switch antennas automatically according to frequency. 

This guide describes how to configure an Arduino Nano ESP32 to talk to the PLJ-096 to operate relay switched low pass filters such as the QRP Labs LPF filter kit. 

Wire up the Arduino and the PLJ-096 module and connection to the relay board supplied with the LPF filter kit as shown in the diagram. Connect a short wire to the frequency counter module antenna port. (note: don't forget to solder a 1M resistor between the positive side of the on/off switch of the PLJ-096 to gnd so that it switches it on when you power up the Arduino circuit).

Programming the Arduino
1.	If you haven’t already done so install the Arduino IDE (from https://www.arduino.cc/en/software)
2.	Connect the Arduino to your PC
3.	Find which COM port it is connected to (you can find this by entering the Device Manager and looking for Ports COM).
4.	Download the software from the “Antenna Switches” folder on Sourceforge at: https://sourceforge.net/projects/wspr-scheduler/files/Automatic_LPF_selector/
5.	Unzip it and then open the .ino file in the Arduino IDE.
6.	Under Tools select Board and then Boards Manager
7.	In the search bar enter: Arduino ESP32 – this should be installed – if not install it.
8.	In the IDE under Tools select Port
9.	You should see the Arduino Nano ESP32 here – so select it and make sure that it’s the one displayed at the top.
10.	Under Programmer in Tools – this should be “esptool”
11.	Once you have the connection to the board select the Upload button (the right facing arrow to the top left of the IDE)
The app will load to the Arduino.

Testing the LPF
It is best to test each filter in the LPF kit separately – please see the testing instructions provided by QRP Labs (or other filter manufacturer).
However, you can test the LPF filters before connecting them to the Arduino by connecting a signal generator to the input of each and monitoring the input and output on an oscilloscope or spectrum analyser. 
1.	Install each filter in the relevant socket on the LPF relay switching board. 
2.	Connect a 5V supply to the board.
3.	Connect a jumper temporarily between the switching input for each filter and ground as you monitor the input and output levels on each band.
There should be no noticeable difference between input and output levels. If there are then you may need to inspect the wiring. You may have the filters in the wrong socket or there may be some other wiring fault. Put a multimeter on the cathode of the switching diode of each relay as you conduct the test to make sure the right relay is switching – also listen for a click when they are being switched (though it is difficult to tell which one is being switched by sound). If the filters are being switched correctly you may need to make adjustments to the ferrite windings on the band that has the issue.

Testing of the Whole system

1.	Once you have tested the LPF filters connect them to the Arduino. 
2.	Once you have uploaded the Arduino sketch select Tools and Serial Monitor.
(or you can use Putty to connect to it)
3.	Ensure that the baud rate is set to 9600 (right hand side of the monitor)
4.	Key a nearby transmitter or connect a frequency generator to the input of the LPF board.
You should see the frequency register on the PLJ-096 display and also output from the Arduino serial monitor. The relays should also switch according to the frequency injected – indicating that a filter has been selected.
5.	To test properly connect a sig. gen. to the input of the LPF board or your TX on low power with the output of the LPF module connected to a dummy load.
6.	Then, as you change the band on your sig. gen or TX monitor pins D5 to D9 on the Arduino to make sure that they are switching correctly according to band. 
The output from one of D5-9 should read Low (0V) when selected – with the other pins remaining high.
You can also put a meter on the cathode of the corresponding diode on the switching board to make sure that it is getting the signal.

Full Test

7.	With a sig. gen. or TX source (on low power) on the input and a dummy load on the output of the LPF (not necessary if a sig. gen. is used) place an oscilloscope probe on the LPF input and the LPF output. 
8.	Monitor the input and output whilst changing frequency onto each ham band. 
If the system is working correctly the output should be about the same as the output and you should here the relays clicking on an off according to band.