Friday, October 27, 2017

AD8307 power meter QQ article additional build notes update 11/12/17


I just received the latest issue of QRP Quarterly with the first in a series of articles I am writing on using a building block approach for designing and building simple Arduino based test equipment. These articles mostly cover projects I have previously featured in my blog posts, and that I have received requests for additional information on.  I am going to post additional build information on this blog, and in a dropbox folder listed in the article. 

https://www.dropbox.com/sh/ooubfjxdvjao8by/AABe6N-vTZjaMSgUAG0jeGSca?dl=0

 I will be making available the sketch, along with the Cad and Gerber files for the circuit boards. I will also have .PDF files of the copper layers that can be used to make your own boards using the "toner transfer".  For those who do not want to make their own boards, I have a limited number available that I will mail to US locations, contact me directly at duwayne@kv4qb.us for pricing.  For those who would like to order multiple boards  or are outside of the US I have created a shared project at PCBWay.com.  The direct URL for the project is  https://www.pcbway.com/project/shareproject/W42368ASJ6_as8307_pwr_meter_brd__V_2.html 
 They have very reasonable prices for boards this size, and offer several methods of shipping depending on how quickly you want them.  Using HK post I usually get the boards in between two to three weeks, DHL gets them to you in less than a week but is more expensive.



Updated Arduino Display building block
A couple notes on the Arduino display building block.  When  I did a revised board layout to make it easier to use with a 3D printed case,there were a couple changes that are different from the schematic in the article.  The only display pins that must be connected to a specific Arduino output line are CLK (13), and SDA(11),  All  the other pins can be connected to any other Arduino outputs, as long as they are defined in the sketch.  I have moved the LED on the display to pin 9 on the Arduino.  This is a PWM pin and can be used to control the display brightness if desired. If you use this option, you can change the value of R1 to a smaller value such as 47R.  On the probe style board I used pin 7 instead of 9 to make layout a little easier.
R2 and R3 are a voltage divider used to measure the supply voltage.  The values are chosen to minimize current usage, and the values used must be defined in the sketch to correctly calculate the supply voltage input to the Arduino.  The board also has  provision for a optional reverse voltage diode.  If used uncomment the line of code in the sketch that adds this to the measured value.  The .7 volts is probably close enough to use, but you could measure the voltage drop across it and use that value instead.

Looking at the schematic for the AD8307 log amp, you will see that I use small molded RF chokes for connections to the Arduino.  In most circuits I have seen a small value resistor,  usually around 4.7 to 10 ohms is used instead.  I found that since I am using the 5 volt output of the Arduino to power both the AD8307 and the TFT display, I could get a lower noise floor with the RF chokes.  You could use the resistor instead if you want. Also when I did the board layout I only put in one 100nF bypass capacitor on the AD8307.  With good shielding of the AD8307 circuitry it did not make any difference I could see.

Here are larger parts placement diagrams for the top and bottom of the boards. 



Notice that I install all the through hole components on the bottom side of the board.  This allows the Arduino pro-mini to be installed in a 24 pin low profile IC socket if desired.  If you do I have found it works better to install the longer header strip pins  down through the Arduino and then clip off the shorter pins on top of the header strip.
 I usually just solder it to the board without a socket instead.

The placement of  SMD parts on the top layer for the AD8307 circuitry is the same for both the Probe  and  Altoids tin style boards.

 I hope to add some additional notes on the software later this weekend.

UPDATE 11/12/17

I have been fighting the small 1.44" 128x128 TFT display used in this project.  Received a order I had placed a while ago, and the ones I received were a different revision than I had used before.  I know there were two different versions and had the sketches setup so I could use either version.  Now there is another new version and I can not get it working with either version of driver I had used before.  I have tried several different configurations with no success.  Along the way I found a Arduino forum that covers this type display
 https://forum.arduino.cc/index.php?topic=260605.0
It seems like there are more than a few problems with some of the newer versions.  I think I will switch over to the 1.8" display I used in my SNA Jr. instead.  Slightly larger, but I think it will fit in the same size case. I have only found one set of drivers for it, and that might eliminate some of the problems that have been reported with the 1.44" display.
Have a couple on order and when they get here I will rewrite the sketch to reflect the changes.