Help Reading Schematic  - Page 1
Forum section: electronics / beginners

author: mart1n
author_rank: Contributor
timestamp: January 24, 2017, 07:34:29 am
content: I started learning EE not long ago and recently got an oscilloscope. I wanted to build a simple tone generator to use with my new scope.I chose the simple circuit found herehttp://www.electronic-circuits-for-hobbyists.com/simple-tone-generator-circuit-diagram.phpFor the life of me I can't get it working. I'm new to reading schematics and I'm not sure what I'm doing wrong.Tone Generator Schematic:My breadboard circuit:Thanks for any help!

author: Brumby
author_rank: Unknown
timestamp: January 24, 2017, 07:45:04 am
content: What are the transistors you are using?(Are they the same as listed in the linked page?)

author: Marinated
author_rank: Contributor
timestamp: January 24, 2017, 07:48:11 am
content: Recheck all your connections. There are errors. For example, assuming the transistors are EBC, the base of Q2 is shown connected directly to the collector of Q1, but you have connected it through R2. R3 also seems to be connected to the 3V rail.Edit: Spotting the error in what I said above is left as an exercise for the reader. I was confused by the unconventional schematic.

author: xavier60
author_rank: Super Contributor
timestamp: January 24, 2017, 08:22:34 am
content: I think the capacitor should go to the other speaker terminal. Where did the schematic come from? There should be a resistor in series with the Base of Q2 to prevent excessive Base current.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: January 24, 2017, 08:54:22 am
content: the schematic seems to be wrong

author: xavier60
author_rank: Super Contributor
timestamp: January 24, 2017, 09:21:40 am
content: Just noticed the link,http://www.electronic-circuits-for-hobbyists.com/simple-tone-generator-circuit-diagram.phpI still think the capacitor should go to the Collector of Q2.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 24, 2017, 09:45:27 am
content: The schematic is wrong. C1 should go to the collector of Q2. If you look atanother circuit in the list, the schematic is corrected for that part. Even with that correction it is a very poor circuit.CheersAlex

author: Audioguru
author_rank: Super Contributor
timestamp: January 24, 2017, 02:37:49 pm
content: The schematic is correct and I did not see a "correction" in the list of circuits. Each transistor inverts so the input and output are in phase and the capacitor provides positive feedback for squarewave oscillation.The breadboard circuit shows the middle pin (the base) of the left transistor disconnected.

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2017, 02:49:01 pm
content: Quote from: Audioguru on January 24, 2017, 02:37:49 pmI don't understand why you say the schematic is correct? Looking at the figure in the first post, there appears to be no source of power to drive Q1. One side of C1 is permanently grounded and the only other way Q1 could receive power is through the base of Q2. I can't see a way for this circuit to work.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 24, 2017, 03:25:16 pm
content: Quote from: Audioguru on January 24, 2017, 02:37:49 pm"Simple Siren" has the correct version. However, I suspect that operation of this circuit relies on the internal resistance of the batteries to avoid destruction of the transistors. It's a very "iffy" design. The description in the OP's original link does not match schematic - suggesting that one or both have been lifted from elsewhere!

author: mart1n
author_rank: Contributor
timestamp: January 24, 2017, 03:38:45 pm
content: Thanks to everyone that responded. I'm glad the schematic might not be correct and It's not just me. I'll try moving C1 as suggested.Which way should Q2 be facing? If you take a close look at the photo, the round end is facing us (flat end facing away). So I have 3V connected to the emitter. I wasn't sure witch way to face it because the schematic drawing of the 2n3906 is different than that in the datasheet I have for it.Quote from: Audioguru on January 24, 2017, 02:37:49 pmIn the photo the base of the left (Q1) is connected through R1, sorry if the photo was unclear.Thanks again for the help everyone.

author: Ian.M
author_rank: Super Contributor
timestamp: January 24, 2017, 05:30:41 pm
content: A LTSPICE sim of the corrected circuit (r.end of C1 to Q2 collector) refused to oscillate until I increased R2+R3 to 375K, at which point it oscillated at greater than 25KHz. However the LTSPICE transistor models don't account for B-E junction reverse breakdown and adding simulated avalanche breakdown with a threshold of 8.2V dropped the frequency to a far more reasonable 2.15KHz.Its behaviour also appears to be dependent on the speaker characteristics. I used 32 Ohms, 1.3mH, (similar toCE38MB-32) If the DC resistance of the speaker is under 13 Ohms, it will exceed Q2's collector current rating. (Hint: change the analysis to start saving data at 1u to hide the initial transient.)I suggest using a 1Meg pot. Depending on your transistor characteristics, you may need to increase C1 as well - try 0.047uF.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 24, 2017, 09:19:13 pm
content: Quote from: mart1n on January 24, 2017, 07:34:29 amOn your breadboard, you have the Base of Q2 connected only to R2which is then connected to the junction of the Collector of Q1 andone end of the trimpot R3. This is not how the schematic goes. Theschematic shows the Base of Q2 connected directly to the junctionof the Collector of Q1 and the end of R2 that is not connected to R3.Also, your breadboard shows (I think) a connection (dark blue wire)from one end of R3 to the negative (ground) supply rail -- if you havethat rail connected on your breadboard. This is not reflected in theschematic. The schematic's symbol for the variable trimpot R3 isnot standard.So, your breadboard does not reflect the actual schematic... plus, theschematic itself has the error of the capacitor being connected tothe wrong side of the speaker, and the nonstandard potentiometer symbolmay be confusing to the beginner.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 24, 2017, 09:43:22 pm
content: Corrected breadboard:

author: james_s
author_rank: Super Contributor
timestamp: January 25, 2017, 03:22:38 am
content: Quote from: mart1n on January 24, 2017, 03:38:45 pmWhat do you mean by the schematic of the transistor being different? The 2N3906 looks like any other PNP transistor on a schematic. There is no relation between a schematic and the physical layout of the part, you have to refer to the datasheet to tell which physical pin is which.

author: salbayeng
author_rank: Frequent Contributor
timestamp: January 25, 2017, 03:53:57 am
content: Here's a picture showing the pin arrangement. (I normally bend the base leg towards the round side, that way the triangles of the pins are the same as the way a NPN transistor is usually drawn)If you look down on a 2N3906 , with the curve facing to the left, and the flat facing to the right, the base is in the middle, and the emitter at the bottom.Note that the 2N3904 and 2N3906 are different to most American transistors , typically 2Nxxx has the collector in the middle.European transistors (e.g BC558) are base in the middle, but mirror image, so it is collector down if the curve faces left.Also be careful how the pins are numbered, not all manufacturers number in the same direction.Also your original schematic is wrong, the capacitor should go to the other side of the speaker.

author: salbayeng
author_rank: Frequent Contributor
timestamp: January 25, 2017, 04:04:48 am
content: I concur with the redrawing of the protoboard byalsetalokin4017

author: mart1n
author_rank: Contributor
timestamp: January 26, 2017, 06:00:44 am
content: Quote from: james_s on January 25, 2017, 03:22:38 amI don't know, the schematic for the circuit draws the 2n3906 differently than this datasheet and it confused me.https://www.sparkfun.com/datasheets/Components/2N3906.pdf

author: salbayeng
author_rank: Frequent Contributor
timestamp: January 26, 2017, 07:45:46 am
content: Schematic symbols can be drawn "normal" way or you can rotate them 0 90 180 270 360 degree's and you can flip them around X or Y. But the the emitter base and collector are still the names of the appropriate pins.There rarely is a direct mapping of the schematic symbol to a physical device, except perhaps in some schematics for beginners kits where the designer goes out of his way to do that.The physical layout of the pins of an actaul is determined by a few factors, the most noticeable being the "lead frame"https://en.wikipedia.org/wiki/Lead_framewith most transistors there is a big copper paddle attached to the middle lead , the silicon die is soldered to that, then bond wires attach the other two terminals. Because of the lead frame, the collector or drain terminal is formed by the bulk of the silicon chip, hence the middle wire will be the collector or drain.If you look inside a clear LED you can see the lead frame, the paddle is usually the negative terminal (cathode)About 70 years ago when they first made transistors they used to diffuse in from both sides of the bulk silicon to make a collector on one side and an emitter on the other, the middle pin (= substrate) was then the base, and the transistor symbol was drawn with the base at the bottom. However these transistors had really awful properties, so they eventually started making them by double diffusion from the same side.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 26, 2017, 08:47:32 am
content: Quote from: mart1n on January 26, 2017, 06:00:44 amYou are thinking too much, I think. The SparkFun ST datasheet schematic and the one on your circuit schematic are actually showing the same thing. For PNP/NPN transistors (referred to as "bipolar transistors") the arrow symbol indicates the Emitter (mnemonic: think shooting an arrow as emitting), the simple angled line indicates the Collector (a pipe sucking up stuff from the Base), and the Base is obvious, like a table. PNP has the Emitter arrow pointing toward the Base (poke), and NPN has the Emitter arrow pointing away from the base (no poke).Transistors of the same type/part number can come in different packaging. The packaging we have here is called "TO-92" and with this package, when you are looking at the flat side with numbers on it, leads down, the left-hand lead is always numbered "1", middle "2" and RH lead "3". And for the two transistors you are using, 2n3904 and 2n3906 in TO-92 packages, the leads are 1=E, 2=B, and 3=C left to right as shown on the data sheet.It makes no difference at all how the schematicsymbolis oriented on the actual schematic, as long as you get the Emitter, Base, and Collector leads wired to the right places in your actual build. For PNP transistors, the Collector is generally connected to the ground or lower voltage part of the circuit, while for NPN transistors it is the Emitter that is grounded or at lower potential than the collector. And simple schematics are generally drawn with the Positive supply rail at the top and the Negative or Ground rail at the bottom. So in this case it was convenient and logical to insert the PNP transistor's symbol "upside down" on the schematic as compared to the SparkFun ST data sheet orientation, since the Emitter of this transistor is connected directly to the +3v supply. (Note that the Emitter of the NPN transistor is connected directly to Ground, and hence logically points down on the schematic.)

author: timb
author_rank: Super Contributor
timestamp: January 26, 2017, 09:18:40 am
content: Quote from: alsetalokin4017 on January 26, 2017, 08:47:32 amHere's one I've always been fond of: On an NPN, the arrow is Not Pointing iN. PNPs are transistors that were asked to Point iN, Please.

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2017, 11:31:15 am
content: Or:The arrow indicates the Emitter and its direction indicates (conventional) current flow in the Base-Emitter diode junction in normal operationThat directly gives you Emitter negative for NPN and Emitter Positive for PNP, no analogies, no complex mnemonics, just grade school basic knowledge of simple circuits and diodes.If in doubt about the BJT package pinout check all combos of pins with the diode check range on your DMM. Unless the transistor is more complex than a normal one (e.g. some special purpose power transistors and many Darlingtons) and has extra internal resistors or diodes, its quite easy to find the base and determine whether its NPN or PNP. Distinguishing collector and emitter is more difficult, but as long as you keep the supply under 5V, damage from getting them swapped is unlikely - you'll just see a horribly low gain if they are the wrong way round. Usually you have a good enough idea of the pinout from the datasheet, that positively identifying the base is sufficient to confirm you are reading it correctly.