# Schematics (understanding and drawing them) **URL:** \ **Category:** Uncategorized\ **Created:** [May 25, 2020, 3:08pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011 "2020-05-25T15:08:00Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [May 26, 2020, 11:34am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/21 "2020-05-26T11:34:16Z")
40 mA is the absolute maximum rating for an IO pin; it’s not a current limited source so if you ask for more the chip will try to give it to you (\*), and may destroy itself in the process. It’s up to you to limit the current you source or sink through all IO pins (absolute max is 40 mA per pin, 200 mA total on all IO pins). If you don’t, at some point you’ll fry the Arduino instead of the LEDs 😃 And resistors tend to be cheaper than Arduinos, so you might as well wire things up properly. \*) the output voltage will drop as you pull more current from an IO pin, due to the internal impedance, but the graph only goes to 20 mA. Assuming we can extrapolate, you’d be at 50-120 mA depending on what LED you’re using, which is outside the spec. ![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/7/7179867f03226a363b15d5229e224b9c5cb41170.png) ---
**Author:** ![SonOfAGlitch](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonofaglitch/32/2101_2.png) [@SonOfAGlitch](https://lookmumnocomputer.discourse.group/u/SonOfAGlitch)\ **Post date:** [May 26, 2020, 11:56am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/22 "2020-05-26T11:56:48Z")
mmm I see. I’ll try to add the resistors but the LEDs where already not very bright! In any case, the device needs to work for about 2 hours and then I’ll take the Arduino away, coz I don’t need it anymore 🙂 ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [May 26, 2020, 12:56pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/23 "2020-05-26T12:56:56Z")
Do you happen to have any small NPN transistors? If so, you can use the first circuit on this page: [https://learn.sparkfun.com/tutorials/transistors/applications-i-switches](https://learn.sparkfun.com/tutorials/transistors/applications-i-switches) You need both resistors, one to limit the current from base to emitter (and from the IO pin), one to limit the current through the LED (and transistor, but a small transistor can usually manage more current than a small LED). You can tweak the latter until things are bright enough. ---
**Author:** ![SonOfAGlitch](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonofaglitch/32/2101_2.png) [@SonOfAGlitch](https://lookmumnocomputer.discourse.group/u/SonOfAGlitch)\ **Post date:** [May 26, 2020, 1:56pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/24 "2020-05-26T13:56:20Z")
Thanks! The problem is that everything is already soldered and inside a box, so I wouldn’t add too much stuff ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [May 27, 2020, 9:57am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/25 "2020-05-27T09:57:17Z")
To enrich the [Glossary](https://lookmumnocomputer.discourse.group/t/glossary-wiki/528) links a little Potentiometer : ![potentiometer](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/1/19b3fd219007217077aad4ba31f462254f77e6f7.jpeg) ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [May 27, 2020, 10:01am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/26 "2020-05-27T10:01:17Z")
Diode : ![diode](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/f/fc18307e0373176b69d44d4a5921583de80348bd.jpeg) ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [May 27, 2020, 10:05am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/27 "2020-05-27T10:05:00Z")
LDR (light dependant resistor) : Non-polarized component ![ldr symbol](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/b/bed5e15d5ad51f2d472d73d79aa5afb98b040143.jpeg) ---
**Author:** ![CTorp](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/ctorp/32/15023_2.png) [@CTorp](https://lookmumnocomputer.discourse.group/u/CTorp)\ **Post date:** [June 3, 2020, 4:19am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/28 "2020-06-03T04:19:41Z")
![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/3/31028152974d80c52ede07a594c6fd30e7d0100c.jpeg) Working on an adaptation of this circuit, bit confused about that decay pot there. As you can see the standard “US” pot symbol is used otherwise. I see that the symbol is used for a preset/trimmer pot but that wouldn’t make much sense. Is this just a pot with two leads connected? ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [June 3, 2020, 7:04am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/29 "2020-06-03T07:04:27Z")
It’s the symbol for a “variable resistor,” which (for low power requirements (\*)) is realized as a potentiometer with one lead connected to the wiper and the other to one of the ends. It’s common to connect the wiper to the other end while you’re at it: ![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/c/cab04e8f9551df6c17c59259ff8eb8b11587e71c.png) but that’s mostly to avoid dangling leads (and if you use PCB-mounted potentiometers, you want to solder all pins for physical stability). (in this circuit, the resistance controls how quickly the 2.2u capacitor discharges) \*) EDIT: for more on potentiometer ratings, rheostats, etc, see the subthread that starts [here](https://lookmumnocomputer.discourse.group/t/schematics-undestanding-and-drawing-them/1011/13). ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [June 3, 2020, 7:06am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/30 "2020-06-03T07:06:50Z")
yes i think it’s a Trimmer Pot, for the adjust the maximum Decay you want. if I recognize it well it is BD ++ of Thomas Henry ? EDIT : sorry i just compare with the stripboard and like @fredrik said it’s a pot ---
**Author:** ![CTorp](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/ctorp/32/15023_2.png) [@CTorp](https://lookmumnocomputer.discourse.group/u/CTorp)\ **Post date:** [June 3, 2020, 12:27pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/31 "2020-06-03T12:27:08Z")
Thank you @fredrik that makes sense. And @Dud good eye sir yes it’s the bd++. I really liked the sound of yours and the full control set it has compared to other bass drums. I’m probably going to do a kosmo pcb version. ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [June 3, 2020, 3:01pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/32 "2020-06-03T15:01:33Z")
when we have already made a schematic, after having looked at it about 100 times in long and wide to verify everything several times during the build, it is inevitably recorded somewhere in the brain 🧠 😀 ---
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [October 12, 2020, 1:08pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/33 "2020-10-12T13:08:03Z")
I was looking at the North Coast Complex VCO circuit this morning and some (not all) of the pots are shown with four terminals: ![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/e/e884a30c2a781375a29a8a830c73a9233f69fb41.png) From context my guess is this means the metal case of the pot is to be connected to ground, but that’s just a guess. ---
**Author:** ![Jos](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/jos/32/151_2.png) [@Jos](https://lookmumnocomputer.discourse.group/u/Jos)\ **Post date:** [October 12, 2020, 7:34pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/34 "2020-10-12T19:34:05Z")
Your guess sounds perfectly reasonable. On the other hand electronics is a field where lots of stuff is standardized, so this symbol should be too. Maybe you can find this in NEN 5152:2016 or [IEC 60617 ‘Graphical symbols for diagrams’](http://tc3.iec.ch/pdf_files/pdf_preview/info_IEC60617_DB.pdf) ---
**Author:** ![antoine.pasde2](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/antoine.pasde2/32/326_2.png) [@antoine.pasde2](https://lookmumnocomputer.discourse.group/u/antoine.pasde2)\ **Post date:** [October 13, 2020, 12:02am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/35 "2020-10-13T00:02:11Z")
I saw some [4-pin potentiometers](https://www.aliexpress.com/item/32878053251.html) recently and am still wondering what the fourth pin does. ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [October 13, 2020, 3:41am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/36 "2020-10-13T03:41:20Z")
The comments there says “dummy” but potentiometers with a center tap is a thing; search for “loudness” on this page for some discussion: [https://sound-au.com/pots.htm](https://sound-au.com/pots.htm) ---
**Author:** ![Farabide](https://avatars.discourse-cdn.com/v4/letter/f/ecc23a/32.png) [@Farabide](https://lookmumnocomputer.discourse.group/u/Farabide)\ **Post date:** [October 15, 2020, 12:10am UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/37 "2020-10-15T00:10:16Z")
The 4th lug crops up a lot in guitar electronics and usually means the case of the pot and a direct link to the bridge and strings on most electrics or case if it’s metal or heavily shielded. The earth is assumed to be the shielding on the output. If you get a hum that stops when you touch the strings then you need to ground more. ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [November 27, 2020, 6:21pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/38 "2020-11-27T18:21:59Z")
I have a question about the IC bypass cap. Put the cap after or before the wire for +12/-12. Please do you know if one solution is better (why?) or it doesn’t matter at all ? thx ![ic cap 100nf](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/e/e37d82872fdf762f0ea713d3e3966ae72c858913.png) ---
**Author:** ![eric](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@eric](https://lookmumnocomputer.discourse.group/u/eric)\ **Post date:** [November 27, 2020, 6:28pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/39 "2020-11-27T18:28:12Z")
Theoretically, they should be as close as possible to the IC. But I doubt it really matters if they are only one hole away… ---
**Author:** ![Jos](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/jos/32/151_2.png) [@Jos](https://lookmumnocomputer.discourse.group/u/Jos)\ **Post date:** [November 27, 2020, 7:57pm UTC](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011/40 "2020-11-27T19:57:45Z")
The difference would be difficult to notice or measure, I would think because the difference would be very small. [Previous page](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011.md?page=1) [Next page](https://lookmumnocomputer.discourse.group/t/schematics-understanding-and-drawing-them/1011.md?page=3)