Hacking home weather station transmitter

Recently I've found this piece of electronic on the dumpster, it was looking interesting - compact case with battery holder, LCD display, temperature and humidity sensor. It has also radio transmitter, but I'm not interested in it since I don't have the receiver station. I decided to bring it back to life.

It seems that was previously immersed in water - whole PCB was covered in white stains, in addition radio transmitter was covered in wax, that made removing of its components much harder. At first multimeter hasn't shown shortcut of battery electrodes, but the device was drawing a lot of current, roughly 1A. I decided to clean it first using trisodium phosphate solution, then rework all of the joints with a lot of flux and clean it again. This fixed the problem of shortcut and the device started to work.

Unfortunately, while temperature readings were correlated with the environment, they were much too high. The temperature sensor is just a termistor, for those who want to see signals on its pins, below is a screen, as visible - current doesn't flow through this element all the time, only, when a measurement is made. This will probably save some battery life. Nice.

It's interesting that we may deduce how the measurement is done by the IC. It probably has a small capacitor loaded to Vcc, during the measurement, the capacitor is being connected to the sensor and discharged by it, the chip measures time to discharge the capacitor to 1V.

I don't know, why the measurements were incorrect, maybe the IC or the thermostat was damaged by the humidity, maybe [this is cheap Chinese consumer electronic] manufacturer run out of the correct thermistors and replaced them with new ones with different curve, or maybe those were cheaper than originals. I don't know. I decided to try to hack the curve of a thermostat by using serial and parallel resistors.

The problem is that I don't have thermal chamber, so I can't easily draw thermistor curve before and after the fix. I could improvise, but for such gadget I don't care. I've removed thermistor from the PCB, and soldered on its place electrodes, then I placed the thermistor on a breadboard, and tried different values of resistors. The room temperature was between 21-25°C - I could modify it by opening a window - maybe not the best way, but it works. My whole room had become a temperature chamber :)

Finally, I've added two resistors, as visible below. It works for room temperature, but for outdoor use it's still not good. Anyway, I will leave it as it is, at least for the moment.

I've also removed all components related to radio transmitter - no reason to waste battery. Electrolytic capacitors were replaced too. Worth to note is that they were rated for 105°C - were they genuine? I don't know :)

Making your own PCB cleaner mixture

Commercial PCB washing solutions are relatively expensive, much cheaper and still powerful one can be prepared at home. It's based on trisodium phosphate, highly alkaline substance able to reduce oxides and react with grass forming soaps. Soaps in chemical meaning, not something we buy at store.

A sample PCB smashed into two pieces - on the left original state, on the right washed up.

DIY water based PCB solvent: before and after

Ingredients

The procedure is simple - a PCB is just placed into the solution and shaken from time to time to speed up the cleaning. Heating is not needed.

I'm using a cheap ultrasonic washing machine - I still don't know if it's working or if it's just a placebo and a bit of a noise, but it seems that at least it mix the water. I've placed the mixture into a glass and then placed it into the water in the washing machine. This is less efficient, but device is easier to clean after use.

DIY water based PCB solvent: ingredients

Note: trisodium phosphate is highly alkaline - it's visible on below below on below universal indicator. Use protective glasses and gloves.

In case of skin contact, wash the solution wash it with a lot of water. In case of a skin contact with the powdered substance whip it by using a rug and then wash it.

DIY water based PCB solvent: universal indicator

Trisodium phosphate was widely used till 70's, it's an inexpensive and effective cleaning agent, but is not environmental friendly.

Home-made mixture for chemical PCB tinning (alcaline bath)

Today I will present a method for PCB tinning that uses alkaline bath. What does alkaline bath mean? It says that the solution that we're using for tinning has this pH, in opposition acid bathes also exists - one of the example I presented in my previous post about PCB tinning at home.

This method is less expensive that the acid one, the results of both are comparable, but because of how a relatively dangerous alkaline bath is, I don't plan to use it. If you're searching a method to tin your PCBs, I would strongly suggest either using Lichtenberg's alloy (described here) or chemically tin them by using acid bath.

Ingredients:

Instead of pure metallic tin, a solder can be used, to increase its reactive surface, it can be melted on a small scrap of a laminate.

Note: NaOH is highly aggressive substance, it can easily permanently damage eyes or skin. Dissolution of an alcaline is strongly exothermic which means that water heats up, it can even start boiling and splashing dissolved NaOH!

Note: protective glasses and appropriate gloves should be used.

Note: don't use any containers, spoons or other small tools that you are also using for cooking, have a separate set for tinning.

PCB and tin (or mentioned above laminate) is placed in a high beaker. 100ml of water is added, then extremely slowly 1/2 teaspoon of NaOH and SnCl2 mixture is added. Mixture is heated (not boiled!) in heated bath similar to presented in this post. Tinning takes 10-20 minutes.

I haven't use polishing paste, so the surface isn't smooth and glossy, but technically it doesn't matter.

Solder wets surface very well, it's visible on below image.

To summarize, despite good results and low proce, I think that it's too dangerous to tin a PCB this way.

PCB tinning using Lichtenberg's alloy

The Lichtenberg's alloy is an interesting way of home PCB tinning - it's inexpensive and there is no need to polish cooper or use specialized tools and chemicals. The negative side is that the alloy is hard to obtain, fortunately it's not expensive. For some countries, it might not be possible at all to buy it.

The alloy contains 50% of bismuth, 30% of lead and 20% of tin, but what makes it interesting is that its melting point is 95-100°C. It means that it melts in boiling water - that's the idea if this technique.

Below you may see drops of The Lichtenberg's alloy.

We will need a couple of grams of the alloy, pan with lid, a tablespoon of citric acid, silicon spoon and old tweezers or similar tool.

  • Fill pan with tap water to 1/2 - 1/3 of volume, put inside the alloy and your PCB, cover the pan with lid to speed up heating.
  • When the water boils, use tweezers to immobilize the PCB - squeeze it to the pan. Dispatch alloy on whole copper surface by using a spoon. Put the lid again.
  • After a couple of minutes, immobilize the PCB again and by using a spoon, remove alloy excess from the PCB.
  • Clean the PCB in water, dry it by using a rag.

Citric acid is used here in two ways: as a flux and to decrease boiling temperature.

Note: don't use pans or spoons that you are also using for cooking, have a separate set for tinning. Please keep in mind that Lichtenberg's alloy contains lead, the method involves citric acid, a product of those (lead citrate) is (at least) unhealthy if digested.

Note: several similar alloys exists (e.g. Lipowitz's alloy), probably they could be also used, but always check theirs recipe. Especially make sure you don't use something that contains cadmium!

Note: don't spoil on your body any boiling water during above procedure.

Below is an example of a PCB tinned by using this method, it's a USB Li-Ion battery charger

If you liked this idea, you may also check the article about inexpensive chemical PCB tinning.

Home-made mixture for chemical PCB tinning (acid bath)

While it's possible to buy ready to use tinning formulas, making your own is much less expensive. Various recipes exist for tinning copper, today I will present a simple one that uses ingredients available at online chemical stores. It's one of acid bath methods.

Ingredients:

Simply put PCB into the solution and wait a couple of seconds, the more time the PCB is in the solution, the thicker a tin layer is. It's not needed to heat up the solution - room temperature is sufficient.

The substances are irritating, if you want to apply this method at home, please use gloves and store ingredients and mixture away from children.

The first PCB that I tinned this way is presented below. Solder wets the surface easily! Next time I will use polishing paste before tinning - this should help to obtain smooth and shiny surface.

Here is the device before testing it - it's a driver for a dosimeter based on DKP-50. Not the tidiest construction, but still looks nice.

In the next post I will present PCB tinning by using Lichtenberg alloy. You may also check related previous post about galvanic copper plating.