Oh, a 555 or not? What? - Page 1
Forum section: electronics / repair

author: thomieboy
author_rank: Contributor
timestamp: July 25, 2014, 07:35:08 pm
content: 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 thisRegards ThomasHere for an extreme high res picture of the board:https://www.dropbox.com/s/z2j2a8hztljc2fc/img170.jpg

author: PE1RKI
author_rank: Regular Contributor
timestamp: July 25, 2014, 07:59:56 pm
content: maybe an 8 pin pic processor?

author: thomieboy
author_rank: Contributor
timestamp: July 25, 2014, 08:04:37 pm
content: Quote from: PE1RKI on July 25, 2014, 07:59:56 pmThat 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.

author: PE1RKI
author_rank: Regular Contributor
timestamp: July 25, 2014, 08:31:19 pm
content: no 555 means also no need for the big capacitor for timing.

author: wraper
author_rank: Unknown
timestamp: July 25, 2014, 09:00:41 pm
content: 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.

author: amyk
author_rank: Super Contributor
timestamp: July 26, 2014, 02:31:32 pm
content: 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. theHT46R002has 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.

author: miguelvp
author_rank: Super Contributor
timestamp: July 26, 2014, 07:26:42 pm
content: 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

author: miguelvp
author_rank: Super Contributor
timestamp: July 26, 2014, 07:38:04 pm
content: 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.

author: miguelvp
author_rank: Super Contributor
timestamp: July 26, 2014, 07:44:00 pm
content: 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.

author: miguelvp
author_rank: Super Contributor
timestamp: July 26, 2014, 07:55:21 pm
content: 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 manualJust saying, it might not be but if you share what the model is, it might help.