Advice regarding my Thermostat project

Whatever suits you, as long as they are suitable for mains in your country.

This is the sort of idea I was mentioning with the schematic layout:


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I was thinking only about 5V side.
I don’t see the reason you need “quick blow” there. Even quick blow fuse is very slow compared with how fast electronic elements can be damaged. The fuse at 5V side will not protect electronic against their damage but will only protect against giving off too much heat in case of damage in electronic and even very slow PTC should do the job. PTCs are slow, slow and once more slow as to blow they need to heat their entire volume up to about 80°C and not only a very thin wire as in standard fuses.

ah ok I see so input on the left and output on the right, just so I understand more, is U1 at the top for clarity, would it be an issue being say under u2 below the k2 and J2 screw terminal?

ah OK, I understand, well that definitely advice I will consider. This was the tutorial I was basing , my design from here so I of course went with their consideration to use a quick blow fuse.

Yes. It is also the star performer in the circuit: the sensor.

No, but up top, there are fewer crossed wires (it is not mixed up with the relay circuitry), so easier to read.

My bedtime now; I’ll catch up tomorrow.

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jmk gave you ‘sort of idea’, as he said. Details are not important. Idea is important.
Supply block can be even drown as separate part with no connection with wires with the rest of schematic (just VCC and GND symbols will do the job).
Some time ago I have shown one of my schematics:

supply is at bottom, relays are in top center.

If the voltage and current is low then read about conditions necessary for self-cleaning of relay contacts. I would start from measuring that voltage and current switched by my circuit. May be you can or may be you can’t use first off the shelf relay. But you should check it if you want your device to work reliably for many years.
May be reed relay be better choice.

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@jmk thank you for advice thus far.

@Piotr thanks for the link, I have a few things to read up on now and try and develop my schematic knowledge.
first I’ve heard of a reed relay, I know of reed switch, but a quick search shows me they are the same thing , but packaged as one component with the magnetised coil instead of how I have used them in the past as door contacts with a magnet.
how strange I was just reading a thread on another forum regarding build up of oxide layers and wetting current required for dry contacts. Thank you for the interesting component tip and direction regarding self cleaning relay contacts.

I have a lot of reading ahead.

I didn’t had in mind ‘self cleaning relay contacts’ but ‘conditions (switched voltage/current) needed to make relay contacts be self cleaned’ or using relay with gold plated contacts if conditions are not met.

As @retiredfeline pointed out, you should use a wall wart and take the high voltage off the board. It is not just a safety concern, but a liability consideration. If someone gets shocked and killed you could be sued personally. If your house burns down and they trace it to your device, your insurance may well not pay out.


There is another issue to consider:
Distances between potentials.
This is, surprise surprice!, something one could study a year about. There are many norms to relate to, and even the level above sea has to be taken into account, no joke!
Here comes the wallwart again…
3mm between N and L, 6mm between mains and low voltage could under certain circumstances maybe be values to apply here. This is is just to show the type of range that might apply in your case. These values are not prooven to be correct!
Don‘t forget, mains voltage is dangerous, and you are the one to blame if anything goes wrong. Example: lightning inducts a voltage surge to mains, while your neighbors son touches the 5V display. The chance is low, but the damage is high. That is why you MUST know what you are doing, even if „usually“ nothing bad ever happens.

How times change over the course of one man’s life !
Having at home sockets like this (you see contacts inside):
being 4 years + 4 moth old I managed to connect the two pins of the plug with my finger when they were already in contact with 220V AC (I liked to turn the Christmas tree on and off by plugging it in and unplugging it from the socket).
We also had unusual Christmas tree lights - wires directly soldered to 16-18 colored 14V bulbs and connected to 220V. These bulbs were the first thing I soldered.


I design power supplies for a living (many AC/DC) and even experienced EEs should not be designing boards to handle mains voltage, unless that is within their expertise.

For my personal projects I will use an external AC/DC power supply unless there is a compelling reason to do otherwise.

Layout is not safe.

Your design does not follow safety requirements for a board that will handle mains power, 220 VAC at that !

I am not in a position to tell you in detail what you have to do to make a “safe” PCB, however I know your high voltage layout is not acceptable.

I would scrap the high voltage in and purchase a CE / UL approved 5V adapter.

I also have a bunch of left over (right under??) laptop power adapters. Those provide 18V - 20V. In any case if you shop, it is much cheaper to buy a power adapter than to make your own AC/DC power supply.

This is especially true if you consider possible resulting bills from a hospital or repairs after a fire.

OK, potentially lethal design, help from internet… The thread will be closed soon.


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