Criticize my toothbrush timer project thingy - Page 1
Forum section: electronics / projects

author: Teisybe
author_rank: Newbie
timestamp: October 03, 2016, 08:52:15 pm
content: During this summer I decided to create something out of all the parts that I have. The idea of the project is simple. Once you start brushing your teeth, you press a button, a timer starts, counts down two minutes and starts beeping annoyingly while you try and mess around with the Chinese crap quality tactile switch to turn the damn thing off, thus indicating that you have finished brushing your teeth. My goal for this project was to use only the most basic parts (i settled with simple BJT transistors and a 555 timer chip), and finally create, etch and solder my own pcb. This is my first "real" electronics project and I would like to get some feedback from other people. Please feel free to criticize any part of my project.FeaturesUses a 9 volt battery for powerSingle button for on and off (Thank you Dave for the simple soft latching circuit)It has two LED's to indicate which state it is currently on - red means the device is on, green means the timer is counting downBuzzer is annoyingHere's how it looksBare PCB board without the back plastic bit and a battery mount:GORE WARNING!Soldering on the other side (note to self, buy a normal soldering iron, make traces wider, use a smaller drill bit)The schematic: (Large picture, right click, open image in new tab, for your viewing pleasure)The pcb (took a screenshot of KiCad, traces are wider, but not by much)Things I learned while doing this project/things i will do better next timeMake the traces widerUse a smaller drillGood soldering iron is a mustActually try to choose parts, rather than picking up what is easy to find (regarding A92 and A42 transistors, I could have used much cheaper ones and they would work just as good)Flux is your friendBuy better tactile switchesThe only part while making this project, that made me tear my hair out, was C6 capacitor, without it the circuit either starts beeping instantly or does not work at all and I don't know why. Anybody have any ideas?So EEVblog community, roast/criticize me.

author: suku
author_rank: Contributor
timestamp: October 03, 2016, 08:59:01 pm
content: This kinda reminds me of projects from electronics magazines... ("Back when I was a boy...")BTW, I love the brown resistor with the numeric value printed on it... where did you get that one from? I had on old Soviet analog audio mixing console, the used the exact same ones:D

author: Teisybe
author_rank: Newbie
timestamp: October 03, 2016, 09:06:07 pm
content: Quote from: suku on October 03, 2016, 08:59:01 pmAhh, Suku, you are correct, they are Soviet-era resistors, manufactured in the USSR, still being sold in some electronics shops as new-old stock here where I live (with the same price as regular 4/5 band resistors). I actually have found a few hundred of this type resistors (mostly 750k), have no idea what to do with them.

author: suku
author_rank: Contributor
timestamp: October 03, 2016, 09:31:59 pm
content: Brings a tear to the eye to see that people still using these... very handy when prototyping, much easier to read than 4/5 band resistors. I kinda wish I could get my hands on a some...

author: Teisybe
author_rank: Newbie
timestamp: October 03, 2016, 09:37:39 pm
content: Have a look around ebay (example:http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xsoviet+resistors.TRS0&_nkw=soviet+resistors&_sacat=0). Lots of people are selling them off, though it will cost a bit more.

author: kfnight
author_rank: Regular Contributor
timestamp: October 03, 2016, 10:19:02 pm
content: Takes me back to the 70's.

author: elimenohpee
author_rank: Contributor
timestamp: October 04, 2016, 12:08:48 pm
content: The traces look so tiny! I wonder what their equivalent AWG is. I like your schematic and how you clearly separate each function of the circuit. That is handy for both yourself when you look at this thing 5 years from now, and people who are just looking at it for the first time.

author: Teisybe
author_rank: Newbie
timestamp: October 04, 2016, 04:34:53 pm
content: Quote from: elimenohpee on October 04, 2016, 12:08:48 pmJust looked at KiCad, the traces are 0.25 mm minus some over-etching

author: Buriedcode
author_rank: Super Contributor
timestamp: October 05, 2016, 01:05:09 am
content: In terms of the design, looks good as long as it does what you want! And its not necessarily how 'good' the project is - the list of things you've learned are far more important. In fact that list is pretty much what I was typing until I realized you had already posted them. Although it probably seems old school I think everyone should learn those lessons with a project like this, with less-than-ideal tools/parts.Post your next project and see if there's improvement! Looks a lot like my first few projects... back when I used a simple plug in soldering iron, with a rotten tip, and solder that probably had about as much flux core as a shoelace. But you made it work. Good job sir

author: jitter
author_rank: Frequent Contributor
timestamp: October 05, 2016, 04:00:41 am
content: Quote from: suku on October 03, 2016, 09:31:59 pmBut in a production environment, non-banded resistors are a PITA when it comes to inspecting the values.We use a lot of precision resistors (e.g. from Vishay Dale), and for some reason these still don't use colour coding.The forming machine that cuts them from the tape and then clips and bends the leads ready for insertion doesn't care where the print is. As a result the majority of these resistors end up with the value facing the pcb.Because several different values are used and manually stuffed, errors can easily occur. If I then have to check for the correct values I curse them.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 06, 2016, 03:56:33 am
content: Quote from: jitter on October 05, 2016, 04:00:41 amI have several bags of these and lots of loose ones. Didn't know they were special.Edit: I have both the new blue ones and the old brown russian ones.

author: jitter
author_rank: Frequent Contributor
timestamp: October 06, 2016, 05:16:29 am
content: Measure some, see if they are indeed precision resistors.The ones we use the most are black, a 10k one in the attachment.But thay may come in other appearances as well. The yellow dot in the colour coded one seems to indicate the precision type as there's no tolerance band, only the value, multiplier and, presumably, thermal coefficient.

author: Zero999
author_rank: Super Contributor
timestamp: October 07, 2016, 12:49:47 pm
content: It's possible to integrate the on/off switch and timer. The circuit below turn on when the push button is pressed and deliver power to the load, until either the button is pressed again or the time delay has elapsed. This probably isn't exactly what you need here but could be integrated into an electric toothbrush, which automatically turns off after a certain length of time.https://www.eevblog.com/forum/projects/onoff-switch-with-timer/msg830879/#msg830879