Monday, April 2, 2018

Stand-alone Simulated Analog Meter updated 4/27/18

Now that I have the latest QQ article finished, I have a little more time for some of my other projects.  One of them is working on the graphic display for the uBITX, I came up with a simple simulated analog meter that worked quite well.  Only problem is with the Raduino, board there is only one available analog pin. Thinking about ways to get around this I looked at some of the inexpensive I2C ADC baords available on e-Bay.  That would do for the uBITX, but I also wanted something I could use with the BITX 40 and a couple of other projects I have been thinking about doing.  I started to wonder how hard it would be to build a stand alone simulated analog meter with several input channels and control lines to select what values are being read and displayed. I wanted it to be small, and simple to construct.  Looking around I found aYou-Tube video showing a Arduino Nano based VU meter using one of the small .96" OLED displays.  Since I had several of those displays' from another project I thought I would try something based on it.

 The video comments had links to the source code and instructions for building the meter.  I thought this would make the basis for a nice little meter that could be added to almost any transceiver.

Thinking about what values I would like to display, I came up with three basic items.  A S-meter when in receive, and a power output display when in transmit.  In transmit, I would also like to have the capability of measuring VSWR.  Thinking about the switching functions required for this I will need one control line that monitors  transmit/receive, this can come from the PTT or key line in the transceiver.  Then  I use a second control line to select either power or VSWR when the T/R line is in transmit.  Another control line can do the same for the S-meter or some other display when in receive.  Since this is based on a VU meter, I will use that for the secondary function in receive.  Now looking at the signal lines I need to measure, they are the AGC line for S-meter, audio signal for VU meter.  And in transmit, the forward and reverse power levels will take care of power and a computed VSWR reading.

I can easily get the 4 analog and digital lines I need from my favorite Arduino for embedding in systems, the Pro-mini.  I like it because of its smaller size since it does not have the USB interface built in like the Nano.  This also means that it takes a little less current than the Nano.  Looking at the physical layout of pins on the Pro-mini, I can get 4 digital, and 4 analog pins in a row that I can use for my control and input signals. 

 I have some pre-made 8 wire cables with a connector, and matching jack that fits the pin spacing on the Pro-mini.  And since I only need 4 wires for I2C OLED display, it was easy to solder a short cable directly from the Pro-mini to a 4 pin connector for the display.
I will be able to supply 5 volts to power everything through the programming pins at the end of the Pro-mini.  If needed I can build a small board with a 5 v regulator and possibly some filtering, so I can run everything off the 12 volts in the transceiver.

After everything was wired up, I loaded the VU-Meter sketch and was very happy with the response of the meter.  Since the display is basically monochrome, it only takes a 1 kilobyte image file to completely redraw the screen.  This allows for a very fast update, and a realistic looking needle movement. The OLED display I had has a two color effect, that is created by two different colors of plastic in its construction.  I have ordered several additional displays that have only the blue overlay. Following the procedure mentioned in the original article, I drew a set of the meter faces I would need for the basic functions of the meter.
 For the power meter function, I decided on a maximum reading of about 15 watts.  The uBITX is rated at about 10 watts on 12 volts, and a little bit more if a higher supply voltage is used on the final amplifier stage.
 Not sure what I plan on using for the S-meter, for now I will probably just rectify the audio output of the preamplifier stage and read that directly.  Later I will see about some sort of an AGC circuit that I can use for the S-meter reading.   The display function I wrote for all except the original VU reading, have provisions for using simple constants to set the full scale movement of the needle, and any DC offset in the circuit at the being measured.

The VSWR display goes up to about 1:3.8 which should be more than adequate for most use.  I think I can use the same directional coupler, without its amplifier that I have in my QRP power/SWR meter. Since I am measuring both forward and reverse power, I can compute the VSWR without having to worry about the power output. 

As far as the signal levels being measured, I can take anything up to 5 volts without having to add in a adjustment pot or voltage divider.  The software should be able to manage full scale and offset values with simple defined constants.

I still have one extra control signal I can use for selecting the display function.  Now what else can I do with this display? Possibly an audio frequency spectrum display, so I can join those on 40M with their SDR rigs that are quick to let everyone know exactly how too wide your signal is.  Lets see what I can come up with, stay tuned.

UPDATE  4/4/18
Finished up the software to select the desired display screen, and modified one of the Audio spectrum analyzer sketches I found on line to fit in the program space available.  It works, but after playing with it, I don't know how useful it would be.  With the memory available, it is only possible to do a FFT that will cover to about 5 KHz. , and I would probably have to add a preamp to get enough voltage for a usable display.  
But It works and feeding a fairly large tone in from a signal generator, I could get a reliable display of the spectrum of that tone.  Trying it on a normal SSB signal did not give anything I found to be usable.  I guess if you were doing CW and had a narrow audio filter in line, you could use the FFT display to see what other signals are around you.  Here is a photo of the response with a 1200Hz. signal from a signal generator. 

UPDATE  4/4/27
I spent a little time working on a directional coupler to measure forward and reflected power for the Power/SWR display functions.
I went with the same basic design I had used in my SWR meter, without the amplifier.  This should give decent sensitivity with normal output levels on the uBITX.  

I need to test it with one of my other transmitters to see what power range I have and if I need to do anything to scale the values before I feed them to the pro-mini ADC.  
Here is a link to the software so far if anyone is interested in playing with one of their own.