This time, my focus is on a cheap TDS meter.
TDS meters are small hand-held devices used to indicate the Total Dissolved Solids in a solution, usually water. Since dissolved ionized solids (such as salts and minerals), increase the conductivity of a solution, a TDS meter measures the conductivity of the solution and estimates the TDS from that measurement.
TDS meters are easy to order over the internet and seem to be a simple electronics solution for testing the overall quality of the water, as well as its potential health effects. I recently bought a cheap TDS meter for some quick hacks!
The TDS Meter looks a little like a highlighter though it’s slightly bulkier. There’s a cap at one end that protects the electrodes.
The other end is for two replaceable LR44 coin batteries (https://docs.rs-online.com/e11c/0900766b81650940.pdf). There’s a small LCD screen which sadly isn’t backlit, so, not always that easy to read.
The TDS Meter is easy to use. Since it comes with coin batteries already installed, you can use it straight out of the box. All you have to do is put some water you want to test into a cup (or the protective cap).
To take the water quality test, just press the ON/OFF button (the reading should be zero), and put the meter into the water. You only have to wait a few moments (30 to 45 seconds) to get a stable readout. Fortunately, there’s a HOLD button and so when you take it out of the water, it can gently hold the readout. The auto-off function turns off the meter after 10 minutes of idle state to conserve its batteries!
Note, the TEMP button can be used independently to measure the temperature in °C and °F (press the button over again to toggle the scale).
As far as I understand, the TDS meter measures the number of ions in the water. Anything under 200 mg/L (or ppm) is good and 100 mg/L is considered excellent. As the TDS level goes below this level, as the minerals are removed, the water has a lower pH and becomes more acidic or corrosive. This causes a noticeable difference in the taste of the water. TDS levels over 400 mg/L are considered non-potable, so don’t drink that stuff (https://www.aquasana.com/info/tds-meter-what-is-it-and-do-you-need -it-pd.html).
The user guide is a little vague. It indicates that this is a dual-range 0-9990ppm TDS Meter. That measures 0 to 999ppm with a resolution of 1ppm, but from 1000 to 9990ppm the resolution is 10ppm. Note that a value of 40 ppm means that from one million particles there’re 40 dissolved ions and the rest (= 999 960) are water molecules. At this point, I need to emphasize that a higher TDS reading doesn’t mean unsafe water. It means the water isn’t pure and has been fortified with something!
This is a closeup of the exposed electrodes.
Out of curiosity, I checked the wiring scheme of the electrodes, which looks like this. Through my oscilloscope, I found that the electrodes inject bipolar pulses into the water being tested.
As pointed before, TDS can be calculated by measuring the conductivity (reciprocal of resistance) of a water sample. However, the measurement of conductivity is not quite as straight forward because it’s caused by the dissolved salt molecules in water.
So, if DC current is injected by a TDS tester, it will cause these molecules to break down (ionize) and migrate to the electrodes, the conductivity value measured will be keep on varying, thus making the reading useless.
This limitation, however, can be overcome by passing an alternating current instead of direct current. If the frequency of the alternating signal applied is appropriate (>5kHz), the molecules no longer break down, thereby rendering a sufficiently accurate readings.
Another important factor that affects the conductivity is the temperature of the water sample. The conductivity of water sample increases with temperature since the mobility of molecules or ions increases. As observed, some cheap TDS meters do not have integrated temperature sensors, but a temperature compensation algorithm
is included in the sample code, default to 25°C.
As guessed, my TDS Meter has an onboard chip thermistor – see below.
A few more snaps showing the inside electronics!
After looking around on the PCB for a while I did find two key components – an unknown uC and a serial EEPROM. The marking on the 24-pin microcontroller chip has been rubbed out by someone, but the serial EEPROM chip has its label – 24C02N. The 12-pin passive LCD seems to be a custom display.
Looking for a similar design idea?
See the reference design of the RENESAS TDS Water Quality Tester https://www.rs-online.com/designspark/reference-design-of-renesas-tds-water-quality-tester
A Little Secret!
I planned to start an economic remote water quality monitoring project for a farmer and bought the cheap TDS meter in hopes that it could be used with an external microcontroller.
Hack inspiration https://dankar.github.io/hydrowatch/2014/11/20/interfacing-a-cheap-tds-meter-with-micro-controller/
But I failed, because my TDS meter is not as easy to hack as it seems!
So, I picked an Analog TDS Sensor Module as the frontend (water quality sensor) of the project.
The next thing to do would be to transfer the data wirelessly to a remote location. But it’s not over yet. I will post as soon as it’s completed.
Keep the questions coming! Also, if anything isn’t clear, let me know. I’ll amend this write up with anything I may have missed. Always be creative and innovative 👍