jeinzi

Thank you very much :) If you already were aware of the exception for resistors with gold/silver in the fourth band that MrOtingocni mentioned, where have you encountered that before?

Close; as it turns out, it is a snubber resistor :)

Thank you very much for your detailled answer, this resolves my question and I can sleep peacefully again :) The device is also fixed; as I wrote in response to BigDanishGuy, the problem was caused by a connected pressure switch. The circuit board seems to have no issue at all.

I solved it, thanks for your help :)

The relay winding is powered directly by the mains and current can only be interrupted by the main power switch and one of the two pressure switches. As there was electrolyte in the connecting gas line when I got the device, I’m suspecting the switch is damaged and doesn’t have enough hysteresis anymore, causing the rapid switching.

• The heatsink is for a triac doing phase angle control. The second pressure switch is supposed to be set to a lower pressure and if it closes, it inhibits the phase angle control, stopping gas production. The problematic pressure switch seems to be just a redunandancy that removes power from the entire circuit using the relay.
• As I found out, “manually affecting the circuit driving the relay” is just operating the main power switch, and that works without any issues.
• There is no discrete transistor in the circuit :)

My solution was to move the set points of the two pressure switches further apart (there are screws on them for that), so the switch inhibiting the triac triggers first again.

Thanks for your tips :) I already finished drawing the schematic, this resistor was the last part I hadn’t clearly identified. Now I just need to understand it :D

I don’t know how serious your project is and how much reliability and predictability you need; but regarding

I would need a 2-300ohm resistor in series to protect the microcontroller

I will just say: I have yet to kill an AVR.

I haven’t used any gate drivers yet, but I remember an introductory video about MOSFET control that helped me a lot, and at about 7:40 he shows his go-to setup.

And again, I don’t know your requirements, but my default logic level MOSFET is the SI2302. It’s available from a bunch of different manufacturers, has a threshold voltage less than 1V and can conduct several amps continuously.

I have no clue about wireless earbuds in particular, so I won’t be of help with question 1.

But I could give you some general remote help. I don’t know how successful that will be with probably limited tools and documention, but I’m willing to try :D

Is this a thing? Have you encountered resistors with 5 color bands without a multiplier?

With all other resistors on the board, my measurements match the color code. This one is the only exception :/

As stated I measured; the question is how and why the measurement is so far off from the encoded resistance, and why those values are so strange no matter how I read them.

Broken solder joints is a very good idea, thank you. I already found one earlier, but I haven’t resoldered them all yet.

The relay is not performing as intended. The circuit board is controlling an electrolysis reactor which works fine until apparently the pressure switch decides the H2/O2 pressure is high enough and it tries to turn the reactor off. At that point the relay goes brrr, switching on and off very fast, producing a lot of sparks, and it stops when the fuse blows. Deactivating the pressure switch “fixes” the problem, but that is dangerous in itself.

The horizontal bar is commonly used to indicate ground, meaning the part of your circuit you define to be at 0V. So they are all connected, yes. I don’t know anything about a capacitor symbol with a dot though, never seen it until now.

I can highly recommend surgical scalpels to work on PCBs. It also sounds like the shorting of the two pads only has to happen once to flash the firmware, so maybe you could use one of those cheap needle probes to temporarily get contact with the via under the ripped of pad.

Image

They are only a solution if you use a gate voltage higher than 5V, in which case NPN would probably also work. Try to use maybe 6V or 7V to control the NPN base and see if that improves your situation. But as that voltage is impractical to get from the Arduino outputs, PNP/PMOS would be another solution.

Edit: If I understand your issue correctly, then your choice of transistor is not the problem. The problem is that you are forcing them to be less conductive than they could. If a p and an n doped region touch, they form a diode. In an NPN transistor, you therefore have a diode between base and emitter, which is also indicated by the arrow in the transistor symbol. A silicon diode drops around 0.7V, and because its steep I/V curve, that’s more or less independent of the current passing through it. When you apply 5V to the base (ignoring the small drop over the base resistor), you’re forcing the emitter to sit at around 4.3V. But that would not be possible if the transistor was fully conductive, because that then the emitter would be shorted to the collector sitting at 5V. The transistor has no choice but to increase its resistance between collector and emitter in such a way that it also drops 0.7V there, ultimately limiting current through your laser. The same would happen with a FET, just that the voltage drop between source and drain would be determined by its threshold voltage, not the 0.7V diode drop of the NPN.

The Arduino Nano uses 5V logic, so PNP/PMOS should work.

I don’t know much about lasers. Also, I don’t entirely understand your schematic. What is connected to J5 and J6?

That being said, my first thought is: Why you are using an NPN transistor for high-side switching? I would try switching it out for a PNP. If that still does not work, then a p-channel MOSFET. There are more than enough FETs btw that don’t need any external driver for use with an Arduino, or even with 3.3V systems.