Multi-turn potentiometer modification for cheap power supplies

As covered in an earlier post, I have four MC Power/MC Voice NG1620-BL power supplies. These serve me quite well, although adjusting the current limit at the low end of the scale is quite fiddly. Some time ago (a year or so) I replaced the potentiometers on one unit with multi-turn wire wound potentiometers. This simple modification completely fixed all problems with setting the output. The first time around I only ordered two potentiometers as they are quite expensive. Now I have replacement pots also for the rest of them.

Figure 1. Chinese multi-turn wire wound potentiometers.

I ordered the potentiometers off eBay. They are nominally 6.8k wire wound 10 turn potentiometers (WXD3-13-2W). Made in China, perhaps of brand Xinghuo. Based on how they look, I suspect they are recycled from old hardware. Anyway, these are the same ones I used for the first unit and I have been happy with them.

Figure 2. De-solder these.

Figure 3. Pots came off easily.

Figure 4. Close up on the old pots.

The first thing of course was to remove the original pots. The knobs just pull off, and the pots themselves are secured to the case with hex nuts. I don't have a long enough socket, so I just used pliers to loosen the nuts. An important thing to check is which way the pots are wired. Both pots in this power supply are connected in the way, in which the counter-clockwise end gives the minimal resistance. For the new pots this means that I need to connect to the terminal closest to the shaft in addition to the wiper, which is the terminal furthest away from the shaft.

Figure 5. The new pots are slightly too long to fit between the board and the front panel.

Figure 6. Board mounts unscrewed to make space to solder new pots in.

Figure 7. New holes drilled to mount the board slightly further back. New pots fit nicely.

The new pots use the same thread, so I just re-used the washers and hex nuts from the original ones. However, the new pots are slightly too long to fit nicely between the front panel and the main board. However, there is more than enough space in the enclose, so I just drilled new mounting holes for the board and moved it slightly back.

Figure 8. 0.8mm welding wire bent into U-shapes.

Figure 9. This is how the U-shaped wires attach to the pot. The knob goes over this with a drop of hot glue.

After closing everything up I still needed to put the knobs on. The shafts are different, so the old ones don't go straight on. I cut short lengths of 0.8 mm welding wire and bent them into U-shapes. This grabs the slot on the end of the shaft and slides in the knob quite tightly. A small drop of hot glue secures them in place. This sounds like a kludge, but it has held up perfectly in the one unit I modified earlier.

Figure 10. Voltage is now easy to adjust within 10 millivolts. Current is easy to adjust within 1 milliampere.

Figure 11. I have four of these power supplies. Two on the left are still unmodified, while the two on the right now have this modification.

Weather display update

My wall mounted Ferrograph Aurora 63 is now displaying weather data.

Figure 1. The sign displaying weather icons.

Figure 2. The sign displaying temperatures.

The weather data is obtained through the Finnish Meteorological Institute's open data service. The format is a bit on the convoluted side, when compared with for instance what is provided by openweathermap.org. However, openweathermap.org data seemed quite a bit off for my area, so FMI data is much more reliable.

Currently only temperature and a general weather symbol is shown (current symbols are clear, partly cloudy, overcast, light rain, moderate rain and heavy rain). These displays alternate every couple of seconds. I'll probably add precipitation intensity and wind data as well. Also I'll need to draw more weather symbols.

See the display in action on YouTube.

FPV with Hubsan H107L - fail!

I got a Hubsan H107L quadcopter (or a clone thereof) from a friend. I've been flying it quite extensively lately, and thought it would be cool to try some FPV on the thing.

I tested the carrying capability of the thing by attaching some self adhesive wheel balance weights on its bottom. Turns out it can only lift about 15 grams, so any video gear needs to be really light.

I ordered the smallest cheapest camera I could find on eBay as well as the smallest video transmitter. Although the copter is controlled over 2.4GHz and the small video transmitters are also 2.4GHz I decided to give it a try anyway.

Figure 1. The small camera straight off from eBay.

According to the data sheet, the video transmitter board operates from 3.3V to 5V, so operation from the single LiPo cell of the quadcopter is possible. The camera however was specified for operation from 9 to 12 V. Opening the camera revealed a single low dropout linear regulator providing 3.3V to everything important (only the bias voltage for the small electret microphone was fed from the supply voltage). The dropout voltage of the regulator was around 0.5V, so power could still be supplied from a single cell as long as it wasn't too empty (also the camera remained operational long after the regulator dropped out of regulation). As a side note, the specified 9V is probably too high for the camera. As it draws about 200mA of current, the regulator needs to dissipate about 1 watt of power. That is quite a lot for a SOT-23-6 package.

By coincidence the video transmitter is exactly the same size and shape as the camera board, so both of them fit inside the camera housing. It was then just a matter of connecting the boards to each other and wiring the power. The whole set-up weighs just 10 grams.

Figure 2. Camera and transmitter boards connected. Yellow loose wire is the antenna, while red and black are the power leads.

Figure 3. Camera and transmitter boards fitted in the camera housing. A small hole was drilled for the antenna wire.

Unfortunately, with the high power video transmitter so close to the quadcopter receiver, the control range is reduced to just 2 meters. Switching the video transmitter channel doesn't affect the range, so the high power is probably just saturating the receiver. Complete failure in that respect. You can't win them all. Perhaps I'll find some use for this neat little camera later.

Ferrograph Aurora 63 LED sign wall mount

So my wife OK'd the wall installation of my Ferrograph Aurora 63. The idea was to have it over the entryway door showing the weather forecast. This makes it easier to choose appropriate clothes when dressing the kids. The Bluetooth modification of my Ferrograph Aurora 63, covered earlier in this blog, was the first step in this project.

Figure 1. The display showing a message on my work bench.

The wall mount I built is just a 1100x200 mm wooden shelf attached to the wall with three brackets. The display hangs from the shelf with 4 M3 screws. The heads of the screws are captive in the mounting slot on the top of the display. I first attached the shelf to the wall and then slid the display onto the hanging M3 screws. After everything was fixed, I drilled a hole through the wall for the power cable.

Figure 2. The screws can move in the mounting slot of the display so that the display can be slid from side to side.

Of course not everything went quite to plan. The screw holes for two of the brackets originally hit some concrete inside the wall, and while I could have drilled the holes with my hammer drill, I opted to move the brackets instead. This of course left some slightly ugly extra holes in both the shelf as well as the wall. I'll probably fix these later, as I still have to take the display down to paint it white. Also the shelf is a quarter of a degree crooked, which is visible as it is so close to the ceiling.

Figure 3. Close up on the power cable junction (I'll install a junction box for this) and the extra holes in the shelf and the wall.

Figure 4. This is how it looks like.

Anyway, I declare this a success. Now to write some code to have it display useful stuff.