555 Timer Breakout Board - Page 1
Forum section: electronics / projects

author: mdszy
author_rank: Unknown
timestamp: January 05, 2014, 01:58:37 am
content: This is a little project I've been messing with for a while. A breakout board for a 555 timer, especially geared towards learning how the chip works. At the basic level, it has a surface mount 555 timer broken out with header pins with silkscreen labels for all the pins. It also has an LED driven by a n-MOSFET connected to the 555's output pin. This way it can be used standalone without any other output so that a seperate LED doesn't have to be connected with a current-limiting resistor, etc. Due to it being controlled by a MOSFET, the LED doesn't affect the output itself.There are a few things I have yet to take care of.1. Actually making it. I have ordered PCBs of an older version, which didn't have the on-board LED at all.2. Figuring out whether it's actually breadboard compatible.3. Writing documentation.I also have to think about voltage limits. The current limiting resistor will protect a 20mA LED up to 5V, but no more than that. I don't want to make the current limiting resistor work up to the voltage rating of the 555, because that will make a needlessly dim LED with voltages lower than the max. If you've got any ideas as to this, let me know!I've attached a PDF of the schematic, and here's a link to the board on gerblook:http://gerblook.org/pcb/NnhHthirkqpSCh6MB3wKsJ. The files are also all on Githubhttp://github.com/mdszy/555_breakout. The board and everything is in the public domain. Do whatever you wish with it.Also, I'm super new to electronics in general and PCB design, any constructive criticism is welcomed!

author: Thor-Arne
author_rank: Unknown
timestamp: January 05, 2014, 06:55:51 am
content: First a general advice, make your schematic with as few as possible global labels and/or named nets, these makes the schematic harder to read. Doesn't matter much on this one, but it will make a more complex one more readable.To make a pcb breadboard compatible you only need to have the grid set to 2.54 mm spacing when laying out the pads for the pins. Also, try the add dimension tool.Try a 1k or even higher resistor for the led, many leds will light up just fine at 5V with a 1k resistor.

author: Jon86
author_rank: Frequent Contributor
timestamp: January 05, 2014, 12:15:48 pm
content: Why have you chosen to drive the LED with a MOSFET? There's no need to do that in this case, as the the 555 can supply an order of magnitude more current than the LED would consume.If you do insist on using a MOSFET, it's good practise to use a gate resistor.

author: mdszy
author_rank: Unknown
timestamp: January 05, 2014, 05:15:11 pm
content: Quote from: Thor-Arne on January 05, 2014, 06:55:51 amAh, alright. I just tried using them because the original schematic was totally awful with the nets just going everywhere. To me the global labels made it easier to read. Thanks!Quote from: Thor-Arne on January 05, 2014, 06:55:51 amOh, nice. I was thinking I'd need to grab the calipers and measure my breadboard, but that works too!Quote from: Thor-Arne on January 05, 2014, 06:55:51 amWon't that make the LED needlessly dim, though?Quote from: Jon86 on January 05, 2014, 12:15:48 pmAh, thanks for the advice. My original design used a jumper to switch between the header output and the LED, and then someone who saw my design suggested that I use a MOSFET instead, since it wouldn't affect the output at all, even the smallest bit. I'll be sure to add a gate resistor, though. Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2014, 05:28:22 pm
content: Quote from: mdszy on January 05, 2014, 05:15:11 pmIf it's not bright enough, reduce the resistor value, a standard 555 can handle up to 200mA.

author: mdszy
author_rank: Unknown
timestamp: January 05, 2014, 05:30:01 pm
content: Quote from: Hero999 on January 05, 2014, 05:28:22 pmWith the MOSFET the LED isn't drawing any current from the 555 itself, I just have the resistor to limit the current through the LED to keep it from burning out.

author: Thor-Arne
author_rank: Unknown
timestamp: January 05, 2014, 05:32:21 pm
content: Quote from: mdszy on January 05, 2014, 05:15:11 pmNot necessarily, compare with the values in other circuits. I.e. the Arduino uses 1k resistors for the LED's.

author: mdszy
author_rank: Unknown
timestamp: January 05, 2014, 05:34:56 pm
content: Quote from: Thor-Arne on January 05, 2014, 05:32:21 pmOh, okay. Thanks!

author: c4757p
author_rank: Super Contributor
timestamp: January 05, 2014, 05:36:07 pm
content: Quote from: mdszy on January 05, 2014, 05:15:11 pmThen you should rearrange things. This can include making a custom part for the 555 with the pins where you want them. That's quite easy in KiCad: pop open the library editor, load the original part, switch to the "projectname-cache" library, edit the part and save under a different name.While you're at it, add non-hidden VCC and GND pins. That hidden-pin crap that KiCad does is a big no-no. I don't know how many times they had to slam their heads on a wallbefore they thought it was a good idea........... even old-style digital schematics might have "omitted" the power pins from the main section, but theyalwaysshowed power distribution on a separate page or at the bottom.The rule of thumb is to make everything painfully obvious. That's why you draw wires: I can follow them and see at a glance everything to which they connect. It's also why you don't use hidden power: I shouldn't have to know the intricacies of the software to know what goes where. Some parts have VCC/GND, some VDD/VSS, some VDD/GND, some an additional VEE..... and if they're hidden I have no clue what has which.Global labels are evil. I have yet to see a use for them, except confusing people, or creating a power net when you're too lazy to use the library editor to make a new power part.Hierarchicallabels are very good, but they should be thought of as more "connectors", used to connect a sub-schematic on a second page to its parent. Normal labels shouldn't be used to make connections unless you really must. They can be used tolabelconnections, to show what that wire does (and as a bonus, the label will carry through to the PCB).One use of labels to make connections, at least in KiCad, that I find acceptable: grouping wires together on a bus. Technically the bus that you draw doesn't do anything; the connections are made by the labels (except for an obscure bus-joining feature that you shouldn't use). For example, you could group together a SPI bus so that you only have one line to draw to route it to everything. But itlookslike the bus makes the connections, so the schematic is still clear.One could argue that using the bus feature for SPI is a perversion of its original purpose (sequentially-numbered nets on a parallel bus). I'd argue that this is 2014 and we don't typically use parallel buses anymore, though if you still want to the feature will still work just fine for it.

author: mdszy
author_rank: Unknown
timestamp: January 05, 2014, 05:41:12 pm
content: Quote from: c4757p on January 05, 2014, 05:36:07 pmThat's great, thanks for the advice!

author: DakLak
author_rank: Contributor
timestamp: January 07, 2014, 05:33:58 am
content: Rather than make a BO PCB dedicated to the great555(designed in1971bylateHans Camenzind- died aged 78 in 2012)why not make a generic 8-pin IC break out board?My company uses generic BO PCBs for prototyping or very low quantity custom production. We even have a BO PCB that can three different sizes/types ofPicAxechips AND an output driver on a 5cm x 5cm PCB.There are many dissertations on the 555 as well.They are still ubiquitous, even sold byweightin Chinese markets!

author: mdszy
author_rank: Unknown
timestamp: January 07, 2014, 04:03:16 pm
content: Quote from: DakLak on January 07, 2014, 05:33:58 amYou're missing the point of my board. It's not just to breakout a 555 timer, it's to assist in learning how they work and what you can do with them.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 07, 2014, 04:52:22 pm
content: Two suggestions:Arrange the breakout pins in the standard schematic configuration, not in the order of the pins on the IC. The Wikipedia pagehttp://en.wikipedia.org/wiki/555_timer_ICshows several illustrations of 555 schematics in the normal schematic layout (inputs on left, output on right, Vcc up top, ground at lower left).Consider using a basic constant current LED driver instead of just a resistor, to eliminate the problems around the LED brightness changing with voltage. Alternatively, make your resistor beefy enough to avoid burning the LED out at max voltage -- it'll be dimmer, but still readable, at more typical voltages.

author: mdszy
author_rank: Unknown
timestamp: January 07, 2014, 05:22:40 pm
content: Quote from: AG6QR on January 07, 2014, 04:52:22 pmI feel like that would make things more confusing, due to when someone goes to actually make their design with a regular 555, the pin layout would be completely different.Quote from: AG6QR on January 07, 2014, 04:52:22 pmI did eventually update the board with a much beefier resistor (1k).

author: jolshefsky
author_rank: Regular Contributor
timestamp: January 07, 2014, 05:44:57 pm
content: Quote from: c4757p on January 05, 2014, 05:36:07 pmI take your point, but I've found (what I think is) a bug in KiCAD where it will treat all gates in a multi-gate device the same. Specifically, a quad op-amp has 5 gates: 4 op-amps and 1 power. But KiCAD treats them interchangeably, so if you number your gates, it will sometimes (often?) pick a power gate when you want an op-amp. There must be a way to fix this ... I'll have to look for a "lock" flag on a placed power gate.Quote from: c4757p on January 05, 2014, 05:36:07 pmI found three uses for them, but you must be diligent and only use them for good.One is when I have a pseudo-power pin, where, say, a switched part of a circuit drives a bunch of other components. Another is when I want to take a well-connected net and make a filled zone on the board: automatically-named nets can change and all of a sudden you refill the zones and everything's suddenly disconnected. A filled net called "GATE_DRIVE" stays the same whereas "N-000008" may be something else at a later time. Third is to break out a debug connector. I make a global label where the signal is, then repeat the label on the test-points sub-schematic.Quote from: c4757p on January 05, 2014, 05:36:07 pmAs a former computer programmer, this sounds like a recipe for obfuscating functionality to someone in the future trying to edit your board. In other words, it seems only acceptable if you are the only person working on the schematic. I could see running a wire only for the SPI data line, for instance, and using a global for the clock, but this "fake lines" thing on the schematic doesn't sound like a debuggable idea.

author: Jon86
author_rank: Frequent Contributor
timestamp: January 07, 2014, 10:15:54 pm
content: Quote from: mdszy on January 07, 2014, 05:22:40 pmThat's not going to be great at lower voltages, like AG6QR said, try a constant current driver. Not difficult to do, and would be much more useful. Also consider adding a power LED with the same type of driver. If you're going to build a board like this, at least try and make it somewhat useful.

author: mdszy
author_rank: Unknown
timestamp: January 08, 2014, 01:18:29 am
content: Quote from: Jon86 on January 07, 2014, 10:15:54 pmWhat? Why would I add a power LED? This is a 555 timer, not a chip that really requires power indication.Quote from: Jon86 on January 07, 2014, 10:15:54 pmSo useful = throwing LED's over every square inch of the thing?If I added Yet Another LED™ it would just lead to confusion, really. "Wait, which LED is power? Or is my OUT LED just constantly on?" It's easier if there's just one LED, that way it's obvious at a glance what the circuit is doing.EDIT: And about the resistor. I decided to lower it to 500 Ohms, taking that idea from the Arduino and the MSP430 Launchpad I have with me right now. They both use 500 Ohm resistors on their LEDs.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 08, 2014, 04:38:15 pm
content: Quote from: mdszy on January 08, 2014, 01:18:29 am(1) Make sure it's a big resistor. i.e. can handle a half watt.The 555 is rated for Vcc = 5V to 18V. If someone feeds your board @ Vcc=18V then its going to drop (18-Vf) = 18 - 2.1 = 15.9 V. The current is I=E/R = 15.9/500 =31.8mA, Power=EI = 15.9*31.8 =500mW. You need a 1210 size resistor.At Vcc = 5V, Vdrop = 5 - 2.1V = 2.9V, I = E/R = 2.9/500 =5.8mA, Power = EI = 2.9*5.8 =16mWSo the max power on that dropping resistor will be 500mW = 1/2W. Even if you assume that the 555 timer is operating at 50% duty cycle max, then it's still 250mW and you need at least a 1206 and then you would be unsafe to exceed the 50% duty cycle.. since you're making a board for others you cannot know what they will do with your board, so you should make it safe. The last thing you want is someone building your board, blinking a LED overnight and smoking the dropping resistor. I say this because your gerber looks like an 0603 size. This is too small given your usable voltage range.(2) Notice how your LED current ranges from 6mA to 32mA. The brightness over that range is quite noticeable. That is why others were suggesting that you use a constant current source to power the led, driven by the 555 output pin. So here is my contribution to your project.. I've drawn the LED circuit for you using a simple constant current sink, that uses just a sot23 and sot223, and (2) 0603 resistors, so it doesn't take up much space on your small board.The BAV99 on the base of Q1 is a simple 1.4V reference based on two diode drops. The emitter of Q1 will be at 0.7V, so the resistor Rset can be chosen as 0.7 / Iled to set the LED current that the transistor will sink when it turns on.Basically, the transistor Q1 is sinking current from the supply, through the LED, but limited by Vbe / Rset.

author: mdszy
author_rank: Unknown
timestamp: January 08, 2014, 04:49:04 pm
content: Quote from: codeboy2k on January 08, 2014, 04:38:15 pmSo should this eliminate the need for the MOSFET driving the LED? Since this is drawing a constant current, it kinda makes it so the MOSFET isn't need anymore?Thank you so much for the contribution, I'll certainly try it out!

author: mdszy
author_rank: Unknown
timestamp: January 08, 2014, 05:04:03 pm
content: Another thing, how did you make the component for the double diode? Is it included, or did you have to make it yourself?

author: codeboy2k
author_rank: Super Contributor
timestamp: January 08, 2014, 05:14:01 pm
content: Yes, it replaces your MOSFET circuit with a BJT sinking a constant current.That double diode is not in the standard Kicad libs. I just made it about 10 mins agoI attached the zip for you. Import the bav99,215.lib into your private EESchema libraryEDIT: fixed the component in the zip file, changed SOT23-3 to SOT23

author: mdszy
author_rank: Unknown
timestamp: January 08, 2014, 05:14:36 pm
content: Quote from: codeboy2k on January 08, 2014, 05:14:01 pmThat's awesome, thank you so much! I really appreciate the contribution!

author: codeboy2k
author_rank: Super Contributor
timestamp: January 08, 2014, 05:26:43 pm
content: No problem, you're welcome.I made a mistake in the LIB file I uploaded (not a mistake, really, just my footprint library has a SOT23-3 but you don't)in the BAV99,215.LIB file, edit it (it's text) and change the SOT23-3 to simply SOT23 for your standard library.I already re-uploaded the file with that change for anyone else.Cheers!

author: codeboy2k
author_rank: Super Contributor
timestamp: January 08, 2014, 05:48:35 pm
content: I just did another caclulation.. on my circuit, you can put a 220 ohm 0603 resistor above the LED, between Vcc and the LED.This will dissipate some power and take it away from the transistor (which right now needs to be a big SOT223 to dissipate up to 500mW in the transistor). If you do this, you can use a small 0603 220-ohm and a smaller transistor in the SOT23 package that only needs to dissipate up to 200mW. For example, the MMBT3904, MMBT2222, BC846, BC848, any one of these in a SOT23 package will handle up to 300mW.If you do this you don't need the bigger PZT2222A in the sot223 package.New schematic attached.

author: Jon86
author_rank: Frequent Contributor
timestamp: January 08, 2014, 06:03:51 pm
content: Quote from: mdszy on January 08, 2014, 01:18:29 amThere's no need to get like that about it, I just suggested adding an LED for power which in my opinion would be quite useful. If it's not oscillating, then it'll be easy to rule out power.You'd have to be really, really stupid to get the two mixed up. Green for power and red for output, simple.At the moment, all I see something that's going to cost far more than the chip itself, just for people that can't read a pin diagram.

author: mdszy
author_rank: Unknown
timestamp: January 08, 2014, 07:26:51 pm
content: Quote from: codeboy2k on January 08, 2014, 05:48:35 pmVery nice! Thanks!Quote from: Jon86 on January 08, 2014, 06:03:51 pmAdding a power LED would take up a lot more space on the board though, too.

author: mdszy
author_rank: Unknown
timestamp: January 09, 2014, 02:18:58 am
content: Quote from: Jon86 on January 08, 2014, 06:03:51 pmI've done some messing around with it and I've combined yours and codeboy's ideas so I'll have both a Power LED and an OUT led but instead of having an external current source, I'll just use a BCR402 LED driver IC.

author: mdszy
author_rank: Unknown
timestamp: January 09, 2014, 03:24:44 am
content: Okay, much much improved board time! Here it is on Gerblook:http://gerblook.org/pcb/TmefhTQ9HLUV4TgbYes45Z. I've also attached a PDF of the new schematic. Let me know what you think! I took ideas from all of you guys, thank you so much! The Github URL is also here:http://github.com/mdszy/555_breakout.Notable changes:Overhauled schematic thanks to tips from many of you.Better ways of managing the LED(s) with help from Jon86 and codeboy2k.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 09, 2014, 12:37:01 pm
content: The new board looks good.Those LED drivers are worthwhile when you have a few LEDS to drive in constant current like you do and also if your space limited or hand-soldering. They take up less space and you have fewer parts to hand-solder. It's a good choice here.I just took a look at Digikey and I see those LED drivers come down in price a lot since I last looked for a singleton LED driver. I haven't looked at them for many years, but I'll look at them again for another time.I really like that gerblook site. That would be awesome here as an embedded forum object. If someone writes an SMF plugin for it, people can easily post their gerbers here.

author: mdszy
author_rank: Unknown
timestamp: January 09, 2014, 01:51:25 pm
content: Quote from: codeboy2k on January 09, 2014, 12:37:01 pmThanks! That's the main reason I chose them, since I'll very likely be hand-soldering these boards.Quote from: codeboy2k on January 09, 2014, 12:37:01 pmI took a look on Digikey as well and was surprised to hear that these chips used to be $3 a piece! Sure am glad they've come down as much as they have.

author: dannyf
author_rank: Super Contributor
timestamp: January 09, 2014, 10:00:20 pm
content: I have to confess that I don't understand this project at all.If you want to learn about the chip, you can use a pdip package + a breadboard, put a led indicator + resistor permanently connected to the output. Plus a few resistors + capacitors, you are ready to go.Or to make a board with pin out so people can simply plug in a few resistors + capacitors to get it working, without a breadboard.Why go through all the trouble to make a breakout board from an smd part + led drivers?

author: SeanB
author_rank: Super Contributor
timestamp: January 09, 2014, 10:01:21 pm
content: Because you can?

author: mdszy
author_rank: Unknown
timestamp: January 09, 2014, 10:23:52 pm
content: Quote from: SeanB on January 09, 2014, 10:01:21 pmThat just about sums it up! I've learned a ton about board and electronics design in general just by building this project and interacting with people, getting input on it, etc. And heck, I might even try to sell these on Tindie, see if people want to buy them.And plus, why would I make it a board that only needs resistors and capacitors and it automagically becomes an astable oscillator? That doesn't teach anything, and that's just one (albeit probably the most common) of the many things you can do with a 555.

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2014, 01:49:49 am
content: QuoteThat might be it: crowd-sourced design.Quote