Oh, a 555 or not? What?

[thomieboy]

Hello everybody,

My sister loves taking care of here nails, and she bought an UV lamp to dry gel nails.
This little device lights up a UV led for 10 seconds. But it needs to light up at least 1 minute.
I thought there would be a 555 timer in monostable mode. But it isn't, I think it isn't the pinout is not correct.
It is a sop8 package.
I've drawn a schematic (I know, it's ugly).

Anyone who knows what this chip is and how I can modify the board so that the UV led lights up for about a minute?

Hope someone can help me with this

Regards Thomas

Here for an extreme high res picture of the board: https://www.dropbox.com/s/z2j2a8hztljc2fc/img170.jpg

[PE1RKI]

maybe an 8 pin pic processor?

[thomieboy]

maybe an 8 pin pic processor?

That is what I tought to, but that would be a whole lot more expensive than just a 555. It seems so illogical fur such a cheap product.

[PE1RKI]

no 555 means also no need for the big capacitor for timing.

[wraper]

Try to play with R2, R3 values to see if it changes timing. Maybe even try to pull down those pins instead or remove them at all. Otherwise I can't see anything could be done with it except making new circuit.

[amyk]

The 10K resistors are almost certainly for I/O pullups, and that SOP-8 is an MCU.

It is not necessarily a PIC, e.g. the HT46R002 has the same power supply pinout.

However, you can certainly replace it with a PIC12Fxxx... and if you're doing so, it might be worth a try to read the ID of the existing MCU with a PIC programmer.

[miguelvp]

Wouldn't it be easier to add an RC circuit to keep the gate of the 50N03 open after the 10 seconds expire?

Not sure how to go about it but I bet someone here can figure out how to keep the gate open after pin 3 goes low on the MCU (green line between the 50N03 and the MCU on your crayonschematic).

Edit: and using a diode so the voltage doesn't go back to pin3

[miguelvp]

Oh and you gotta prevent ringing at the gate so you don't kill your mosfet. But in 10 seconds you should be able to store enough energy to keep that gate open for 6 times of the original duration.

Edit: of course it would be easier to reprogram the MCU if you have the tools.
Pretty much you have to listen to pin 7 for that button press and output on pin3 for the desired time. Pin 6 and pin 5 might have something to do with the unpopulated buzzer.

[miguelvp]

On second thought, why will the 10 seconds be printed on the silkscreen? is there a limit on how long that LED can be on? 

You might not want to modify it after all if keeping the UV LED for longer than 10s will harm it.

[miguelvp]

Scratch all of the above. It might be simpler than you think.

By trying to find the actual device (which I couldn't) I saw that some use a single button interface to select how long the UV light will stay on.
So maybe it's programmed into the MCU already and you just need to read the manual

Just saying, it might not be but if you share what the model is, it might help.