Merge pull request #69 from leigh-hackspace/RaspberryPI-32Channel-ADC

First publish for 32 ADC

content/blog/2024/raspberrypi-32channel-adc/index.md

@@ -0,0 +1,257 @@
+---
+title: "Creating a 32 Channel ADC for Raspberry PI"
+subtitle: "My first entry into create a Raspberry PI HAT"
+date: 2024-07-03T00:00:00Z
+toc: false
+tags:
+  - hardware
+  - raspberry pi
+draft: false
+author: Andrew K
+author_email: leighhack@shamrock.org.uk
+listing_image: images/finished_hat.jpg
+---
+Over the last 12 months I had been looking at building a hydroponics set up using the Raspberry PI. I wanted a cost effective and repairable solution for handling soil/moisture measurements. 
+
+There of course many HATs already available which solve many problems, but after quite a bit of research found that the HATs available severly restricted the capability of the PI and increased costs should I ever want to scale up.
+
+## Starting simple
+
+I initially started very simply by looking at some of the solutions available and found several options, the simplest was this module [Gravity: I2C ADS1115 16-Bit ADC Module
+](https://thepihut.com/products/gravity-i2c-ads1115-16-bit-adc-module-arduino-raspberry-pi-compatible)
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/adc_i2c.jpg" width="400" height="400" float="inline-block">}}
+{{< /bgallery >}}
+
+It is not too expensive, although buying multiple soon adds up. It can service 4 analogue inputs and you can put up to 4 ( two I2C bus on the Raspberry PI and two per bus ).
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/moisture_sensor.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+I did also purchase this sensor [Gravity: Analog Waterproof Capacitive Soil Moisture Sensor](https://thepihut.com/products/gravity-analog-waterproof-capacitive-soil-moisture-sensor) 
+
+This works very well and I can get measurements from the sensor, some work is required for calibration but that is expected.
+
+## How to increase the number of sensors
+
+After some more research I also found several modules using the MCP3008 IC. This is an 8 channel, 10 bit, SPI package. 
+{{< bgallery width="60" >}}
+{{< fimage src="images/mcp3008-ic.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+There are several Raspberry PI hats that use this package, the [MCP3008 Specifications](https://www.microchip.com/en-us/product/mcp3008#document-table) are straight forward along with penty of example code to drive it. It does
+have advantages over other interfaces, however it is restricted to 10bit resolution, but for the purposes of soil moisture measurements this is not an issue.
+
+## Building a prototype
+
+The MCP3008 is very simple to get going and to test my idea of getting more than one integrated I build a prototype on a breadboard
+{{< bgallery width="60" >}}
+{{< fimage src="images/adc_prototype.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+The SPI interface is daisy chained from one device to another, and separate GPIO pins on the Raspberry PI are used for selecting which IC should provide data.
+
+The amazing people at [Adafruit](https://learn.adafruit.com/mcp3008-spi-adc/python-circuitpython) have a simple demo to get it going and I will post the demo code I used below.
+
+```
+import time
+import busio
+import digitalio
+import board
+import adafruit_mcp3xxx.mcp3008 as MCP
+from adafruit_mcp3xxx.analog_in import AnalogIn
+
+spi = busio.SPI(clock=board.SCK, MISO=board.MISO, MOSI=board.MOSI)
+cs = digitalio.DigitalInOut(board.D5)
+
+mcp = MCP.MCP3008(spi,cs)
+#
+channel = AnalogIn(mcp,MCP.P0)
+#
+while True:
+    print ("Raw ADC Value: ", channel.value)
+    print ("ADV Voltage: " +str(channel.value)+"V")
+    time.sleep(1)
+```
+
+The simple thing to note is the CS definition which allows you to put the chip select on different GPIOs for different packages. Each input for each package selected is done with the line
+In the above there is 1 second delay between readings and for our purposes is good to see instant changes. 
+
+```
+channel = AnalogIn(mcp,MCP.P0)
+```
+
+By changing this from MCP.P0 to MCP.P1 you get channel one, up to P7 to the last channel , so 0 to 7.
+
+I then tested this with the moisture sensor I already had and it provided a simple interface which worked well.
+
+## Research on Raspberry PI HATs
+
+Now I have a prototype working, whats next ?
+
+To make this something more useable in the real world I did needed to make this into a Raspberry PI Hat. At first glance this seemed simple enough, taking the connection schematic and then producing a layout to match the correct form factor.
+
+### Raspberry PI Hat IDs 
+
+The Raspberry PI Hat specification suggests (is optional) to have an ID for your Hat. I had never done this before, so I thought this would be a good opportunity to see what this is about. 
+
+The simplest guide for doing this I found by [MadeByMikal](https://www.madebymikal.com/raspberry-pi-hat-identity-eeproms-a-simple-guide/). It notes the gotchas and although does use a slightly more expensive EEPROM it works well.
+
+After you have read the guide, the file you are looking at is eeprom_settings.txt. These are the following values I used
+
+```
+product "32 Channel ADC"
+product_id 0x0001
+product_ver 0x0001
+vendor "Leigh Hack Space - https://leighhack.org"
+```
+
+The one element I use which is not covered is the custom data and I took some text and added it as hex.
+
+The text I added was
+
+```
+This is a 32 channel ADC using the library found here
+
+https://learn.adafruit.com/mcp3008-spi-adc/python-circuitpython
+
+It uses serial SPI0 and GPIO lines 5, 6, 26 and 16 for control
+```
+
+I then used a text to [hex converter](https://www.browserling.com/tools/text-to-hex) to generate the hex information needed, and it became
+
+```
+custom_data
+54 68 69 73 20 69 73 20 61 20 33 32 20 63 68 61 6e 6e 65 6c 20 41 44 43 20 75 73 69 6e 67 20 74 68 65 20 
+6c 69 62 72 61 72 79 20 66 6f 75 6e 64 20 68 65 72 65 0a 0a 68 74 74 70 73 3a 2f 2f 6c 65 61 72 6e 2e 61 
+64 61 66 72 75 69 74 2e 63 6f 6d 2f 6d 63 70 33 30 30 38 2d 73 70 69 2d 61 64 63 2f 70 79 74 68 6f 6e 2d 
+63 69 72 63 75 69 74 70 79 74 68 6f 6e 0a 0a 49 74 20 75 73 65 73 20 73 65 72 69 61 6c 20 53 50 49 30 20 
+61 6e 64 20 47 50 49 4f 20 6c 69 6e 65 73 20 35 2c 20 36 2c 20 32 36 20 61 6e 64 20 31 36 20 66 6f 72 20 
+63 6f 6e 74 72 6f 6c 
+end
+```
+
+I added this to the breadboard
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/adc_prototype_sensor.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+Now when I went into the hat location on the file system I get
+
+```
+/proc/device-tree/hat $ more *
+::::::::::::::
+custom_0
+::::::::::::::
+This is a 32 channel ADC using the library found here
+
+https://learn.adafruit.com/mcp3008-spi-adc/python-circuitpython
+
+It uses serial SPI0 and GPIO lines 5, 6, 26 and 16 for control
+::::::::::::::
+name
+::::::::::::::
+hat
+::::::::::::::
+product
+::::::::::::::
+32 Channel ADC
+::::::::::::::
+product_id
+::::::::::::::
+0x0001
+::::::::::::::
+product_ver
+::::::::::::::
+0x0001
+::::::::::::::
+uuid
+::::::::::::::
+f9768a88-e105-4ee1-adc1-36fa5602df09
+::::::::::::::
+vendor
+::::::::::::::
+Leigh Hack Space - https://leighhack.org
+
+```
+
+### Raspberry PI Hat designs
+
+In order to design a PCB for the Hat I chose to use [KiCAD](https://www.kicad.org/). I have used it before and although takes a little practise is good for someone (like me) who just wants something simple and files can be exported to various PCB manufacturers without much pain.
+
+There are various Raspberry PI Hat designs available however many appear to be either corrupt, old or just 'good enough', so when you use the KiCAD checking tool it flagged lots of errors. The dimensions of the various Hat sizes can be found [here](https://github.com/raspberrypi/hats) so it was straight forward to create something which matched what I needed.
+
+#### Design considerations
+
+As part of the ethos of useability, upgradability, repairability and of course cost I chose to make all the IC locations to be sockets rather than solder in components. This then allows anyone else to decide if they want the ID EEPROM, and use all 32 channels, so adding 8 channels at a time if needed.
+
+The only part that would be fixed are 2 x 3.9k resistors, which are only required if the EEPROM is being used. They are designed as 1/6W, but perhaps they should have been 1/4W to make it more general.
+
+The PCB design I ended up with is 
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/PCBLayoutKiCAD.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+The KiCAD rendering suggested it should be ok once created with all the parts
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/pcb_render_kicad.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+At this point I got the PCB made from [PCBWay](https://www.pcbway.com/) and decided although I could just get 5 made, based on the shipping cost and the differential I had some panelized to be more efficient and also they can be bought from the HackSpace either as bare boards or finished products (tbd).
+
+## Building the first board
+
+I bought all the parts needed along with stackable headers for the Hat, so when the boards can back it was time to soldering it all up.
+
+A single bare board looks like this. As you can see, and from the PCB design, I added text to the boards to show which IC is controlled by which GPIO on the Raspberry PI, just to help with deployment.
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/pcb_bare.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+### How to go about soldering it all up
+
+A simple gotcha was to solder the GPIO header FIRST. I went through a board figuring that one out, thankfully it was just IC sockets and was more painfully to remove than just throw away the board.
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/pcb_rasp_connector.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+Next was trying to figure out how to make all the connectors straight. Single line headers are somewhat fiddly to get straight, common techniques are solder one into place, while holding then do the rest. As I had several blanks it was easier to make a PCB sandwich so holding them in place with small bull dog clips.
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/pcb_sandwich.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+You can see in the picture all the headers are straight so I used this technique for adding in the IC holders and completed the PCB soldering quite quickly. 
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/pcb_conn_complete.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+All the connectors, sockets and resistors in place with an EEPROM ready for testing. It provided the same results as the breadboard prototype so I knew it was working for the EEPROM at least, and testing each location I found the moisture sensor worked as needed.
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/pcb_first.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+The design worked and I was happy with it, however I also thought it was somewhat hard to determine the direction of connectors (they are marked on the board), so I went one step further and bought some coloured connectors.
+
+As you can see this is much more appealing and make is significantly easier to see the connection orientation and what the jumpers are for. I used red for positive, black for negative and yellow for signal. I matched the orientation to the sensor I had.
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/finished_hat.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+When the Hat is on the Raspberry PI it looks nice and compact ready to be used.
+
+{{< bgallery width="60" >}}
+{{< fimage src="images/pcb_on_pi.jpg" width="400" height="300" float="inline-block">}}
+{{< /bgallery >}}
+
+This little project helped me learn quite a bit about the Raspberry PI and how relatively straight forward it is to build something for it. I am working on a couple of new HATs for the next part of my hydroponics set up so will post what they involve when they are done.