Question

I see that I need to purchase a combination pH-electrode along with your kit. There seems to be a lot of different combination pH-electrodes. I can find them from $20 - $300. Can I use a cheap one?

Answer

Yes, you can.

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.

Question

The HomeLab module's header is blocking spatially the RPi UART pins. I need serial UART communication for other purposes. Is UART communication possible?

Answer

Yes, UART communication is possible by applying small hardware and/or software changes.

Q&A - UART pinsThe question is about pins #8 and #10 (or GPIO 14 and 15) used for TXD and RXD signals respectively.

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.

Example.
GPIO_number_button=15 # default
reassign as:
GPIO_number_button=27

A solution may also require one or two small pieces of additional hardware.

HomeLab Splitter female centre
HL-splitter, female centre
HomeLab Splitter all male
HL-splitter, all male
HomeLab cable 7x10 gray
cable 7x10 gray
HomeLab  cable 7x10 rainbow
cable 7x10 rainbow

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.

Q&A - UART solution 1 Q&A - UART solution 1

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.

Q&A - UART solution 2 Q&A - UART solution 2

Solution 3

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.

Q&A - UART solution 2 Q&A - UART solution 2

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. A good alternative pin-set for UART communication is either 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.

Solutions summary

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