Sensor sockets and user interaction tools (a button and an indicator) are shown below mounted on a test-panel and connected to various parts of the HomeLab-pH module.
Temperature sensor socket is a mini-DIN6 connector that is connected to the JP2 header of the module. The wiring is as required for the DS18B20 smart temperature sensor that comes with the module. The JP2 pin 1 (3.3 V), pin 9 (ground) and pin 7 (data) are connected to the mini-DIN6 pins 2,4 and 6 respectively. Click the image for a larger view.
To connect the button and the indicator a soldering gun and basic skills are needed. The soldering points of the wires are shown below. Mind the position of the indicator ground wire. A red LED indicator is chosen as it would draw more current and thus be brighter compared to a green one.
The module is compatible with almost any type of combined pH-sensor in the market. The sensor only has to be with a BNC connector (most are). The prices vary significantly and reflect the sensor durability, rigidity, response time, etc. The module though would equally well read the voltage coming from a cheap or an expensive pH-sensor.
Also the choice would depend on the application. If only water samples with no harsh constituents are to be measured the cheap probes are OK. Being cheep one can buy a couple of them for replacement until initially some experience in handling of a pH-probe is acquired, if needed.
If the aim is a more specific application, read some pH-probe selection guides which are freely available in the net.
The obvious solution is to detach the measuring block from the RPi header by means of a cable. Some of the solutions described below use just this approach, others do keep the module stacked in place.
The problem except a mechanical has also an electrical aspect as the HomeLab-pH measuring module uses the RPi pin #10 (UART RXD) to both read the on-board button state and set the LED. The electrical conflict is resolved by connecting the module's button line to a different RPi pin and also reassigning a variable in the configuration file main_config.sh to the GPIO number of the selected pin.
GPIO_number_button=15 # default
A solution may also require one or two small pieces of additional hardware.
Solution 1 "With isolator"
The solution is applicable only to a HomeLab-pH module used in tandem with an isolator (the HL-Isol module). It makes use of a jumper-pin header present at the bottom side of the isolator board. The jumper-pin is removed and a short jumper wire is connected as shown on the picture. The other end of the wire is connected to the selected RPi pin (GPIO 27). Then a splitter, an isolator and a measuring module are stacked in consecutive order. In this solution the UART communication is fulfilled through the parallel connector of the Splitter board.
Solution 2 "No isolator"
The solution is applicable only to a non-isolated measuring module. A ribbon cable connects the RPi to the JP2 (male) connector of the module. The cable special connector at the RPi side does not block the UART pins and also connects the selected LED/button pin (GPIO 27). The UART communication is fulfilled via the freed pins #6, #8 and #10 on RPi header.
Solutions 1 and 2 are combined to connect RPi to a splitter by means of the special ribbon cable. A measuring module, with or without isolator, is mounted on a parallel connector of the Splitter board.
Solution 4 "Pi 4"
When using Raspberry Pi 4 you may leave Homelab measuring module stacked on its usual place as the Pi header has other UART pin-sets available. Good alternative pin-sets for UART communication are UART4 (GPIO 8 and 9) or UART5 (GPIO 12 and 13). The change would only require setting OS variables. Find more about RPi UARTs and their configuration.
|Applicability||Additional hardware||Module to RPi|
|Solution 1||Measuring module with isolator||HL-Splitter, short jumper wire||Stacked|
|Solution 2||Measuring module without isolator||Special cable||Remote|
|Solution 3||Measuring module with/without isolator||Special cable and HL-Splitter||Remote|
|Solution 4||Raspberry Pi 4 only||No||Stacked|