Understanding Electronics - Page 1
Forum section: electronics / beginners

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 18, 2016, 06:18:37 pm
content: Hi,I'm kinda new with electronic components and I'm trying to understand them better. But the one I don't understand very well is the transistor. Now to my understanding, a transistor can be used as a switch or ampifier. But what I don't understand is how they handle current. Let me explain.Now this picture is ripped from Wikipedia and is probably the simplest schematic you can make of a transistor. Now, what I don't understand is the way the transistor is in the circuit. Why is it put between the ground and light bulb? Shouldn't be placed between the positive and the light bulb? Since current is flowing from the Collector to the Emitter, how does the light bulb get affected in any way if the current is being changed AFTER the bulb?

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 06:35:32 pm
content: HiOk, lets back up to series circuits for a bit:If I put a battery, a light bulb and a toggle switch all in series, current only flows when the toggle switch is closed. All the current going through the switch goes through the light bulb. It also all goes through the battery. It does not matter what order the elements are arranged in.Next I take my circuit that is floating in space and pick a single point to put a sticker on. The sticker says "ground". I can put the sticker on any wire in the circuit, but only on one wire. Putting a sticker on a wire does not change the way the circuit works. Calling a wire "ground" does not change the circuit.So far ok?Bob

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 18, 2016, 07:01:15 pm
content: Quote from: uncle_bob on June 18, 2016, 06:35:32 pmAlright, so the transistor could go before or after the bulb and would still work the same way?

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 07:19:30 pm
content: Quote from: PotatoBox on June 18, 2016, 07:01:15 pmHiYes, that is correct. What matters is that current flows when the switch is closed and none flows when it is open. The order of elements is not significant in this simple case.Bob

author: retrolefty
author_rank: Super Contributor
timestamp: June 18, 2016, 07:38:11 pm
content: Quote from: PotatoBox on June 18, 2016, 07:01:15 pmNo, the bias voltage for the base would then not be high enough in that case to turn on the transistor. This is called a 'low side switch. For 'high side switching' a PNP transistor would be used.

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 07:47:04 pm
content: Quote from: retrolefty on June 18, 2016, 07:38:11 pmHiIn the schematic he shows, the base bias works just fine with the switch "after" the bulb. Given that there is no value shown for the resistor, one could easily come up with one that works regardless of where the bias comes off of (before or after the bulb).Bob

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 18, 2016, 08:02:34 pm
content: Whats a good example of a circuit when you would use a pnp transistor?

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 08:14:51 pm
content: Quote from: PotatoBox on June 18, 2016, 08:02:34 pmHiTransistors are directional. A toggle switch does not care which way it goes in the circuit. A transistor very much cares if the emitter and collector are positive or negative relative to each other. If you want to flip the circuit you have in the original post "upside down" so the transistor emitter is to the positive side of the voltage source, you need a PNP.Bob

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 18, 2016, 08:26:05 pm
content: So lets say we put current through the base of the transistor, and the gain is 100, the current will times by 100 through out the whole circuit right?

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 08:28:42 pm
content: Quote from: PotatoBox on June 18, 2016, 08:26:05 pmHiAs a first approximation, that is right. There will always be nasty details that get in the way under certain circumstances. If you want to be *sure* the transistor is full on (saturated), you will put more current in than the 1/100 that the beta would suggest.Bob

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 18, 2016, 09:55:42 pm
content: So, I'm trying to build this simple circuit here. it is an FM transmitter. But I want to know what each component's role in the circuit is. Now, I believe the C3 cap is to smooth out the voltage, right? and cap C1 is to smooth out the sound waves. I'm really clueless of what the roles of the other components., if some could explain that.

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 10:10:20 pm
content: HiIt's a Pierce oscillator. The capacitor on the base is there to bypass it at RF. It will be quite unstable and hard to work with. Setting it up probably will require a spectrum analyzer. If the objective is a simple wireless mic sort of thing, there are cheap little gizmos that will do that.Bob

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 18, 2016, 11:13:50 pm
content: Quote from: uncle_bob on June 18, 2016, 10:10:20 pmOk well, What are some other good schematics that are simple and fun to build?

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 11:21:17 pm
content: Quote from: PotatoBox on June 18, 2016, 11:13:50 pmHiVHF RF is not a reasonable point to start at. Audio is much more forgiving. The standard "first circuit" is a LED flasher. I made good money back in the 70's doing a variation that changed intensity with sound level. Dave's videos are a very good place to start digging into this stuff.Bob

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 18, 2016, 11:27:37 pm
content: Quote from: uncle_bob on June 18, 2016, 11:21:17 pmBob, the schematic above is an FM transmitter for FM radios.

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 11:32:41 pm
content: Quote from: PotatoBox on June 18, 2016, 11:27:37 pmHiThe VHF radio frequency band starts at 30 MHz and ends at 300 MHz. The FM band is from 88 to 108 MHz in the US. That puts it in the VHF frequency range. The circuit is intended to operate there, thus it is an RF circuit. RF gets you into test gear that is a bit specialized. What kind of bench gear do you have now?Bob

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 18, 2016, 11:35:33 pm
content: Quote from: uncle_bob on June 18, 2016, 11:32:41 pmWell my bench literally consists of a soldering iron, a sponge, a desoldering pump and some containers where I put my salvaged components. Getting an oscillator and other specialised equipment like that is out of the question. If it requires test bench gear like that kind of stuff I'll look at different schematics and such.

author: retrolefty
author_rank: Super Contributor
timestamp: June 18, 2016, 11:57:05 pm
content: Quote from: PotatoBox on June 18, 2016, 11:35:33 pmDon't let the perfectionist around here discourage you. People have build and used such wireless microphone circuits for decades and most didn't own more test equipment then a multimeter. Besides one tends to learn more working on circuits that don't work right off the bat rather then just following a found schematic.But that aside you should go get a multimeter, they don't cost much for a starter model and you can't do much without one as those darn electrons are otherwise invisible.

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 19, 2016, 12:06:45 am
content: Quote from: retrolefty on June 18, 2016, 11:57:05 pmActually yeah I forgot to mention I had a multimeter. A little mastercraft one. And the reason I wanted to start with an fm transmitter is that I see tutorials all over youtube for them and they don't even test them with any of that equipment. Also, I wanted to put a LINE IN on the circuit, how could I do that?

author: uncle_bob
author_rank: Unknown
timestamp: June 19, 2016, 12:28:04 am
content: HiThe gotcha is: what do you do when it does not work? What do you do when if fires up outside the FM band and you can't find the signal? For reasonable performance with a given layout, some of the capacitors will need to be selected.....Line in would go right where the microphone input is. Since all you are doing is changing the bias on the oscillator, it does not matter much what you call the input. Any signal there will drive it a bit nuts. You will get both AM and FM as a result. For that matter, with no buffering on the oscillator, anything that comes near the antenna will modulate it quite a bit as well.Bob

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 19, 2016, 12:34:32 am
content: Quote from: uncle_bob on June 19, 2016, 12:28:04 amIs it because the schematic is flawed? Because there are other ones out there. I found a different one.

author: uncle_bob
author_rank: Unknown
timestamp: June 19, 2016, 01:39:13 am
content: HiAn oscillator like that is purely dependent on the L/C tank circuit for it's stability. When you attach the antenna directly to the tank circuit (as it does), its capacitance forms part of the tank. Any change in the near field of the antenna will change that capacitance. When the capacitance changes, the frequency changes. The answers to the problem are things like buffering and a PLL based approach. The more stuff you add to the circuit, the harder it is to troubleshoot and to get working. That is why you are finding multiple copies of what is basically the same RF part of the circuit.One hint: If you do decide to do that circuit, a 2N3904 is a really poor choice for the RF stage (Q2). It is more oriented to audio work and lower frequency RF. Something like a 2N918 would be a bit better choice.Bob

author: Dave
author_rank: Super Contributor
timestamp: June 19, 2016, 03:39:43 am
content: Bloody hell, he doesn't need a freaking spectrum analyzer to build a freaking RF bug. The old guy that Dave interviewed has been making those for decades, with nothing more than a zener voltage stabilizer and a moving coil instrument.PotatoBox, just build the circuit. Sure, it isn't perfect, but it's more than good enough for you to get a grip on electronics and have some fun with it.Quote from: uncle_bob on June 19, 2016, 01:39:13 am

author: nuno
author_rank: Frequent Contributor
timestamp: June 19, 2016, 04:04:58 am
content: Here's another simple circuit, I had much fun with it in my early teens:http://embeddeddreams.com/site/2008/10/06/fun-and-easy-to-build-buzzer-circuit/Being a VCO (voltage controlled oscillator), you can play with it's "input" to do other fun things; for example, breaking the wire going vertical into P1 and inserting a pushbutton there, and connecting an electrolytic cap from P1's top wire to GND, it makes a "piiuuuuu" sound when you press and release the button.You can use other small signal NPN/PNP transistor pairs such as 2N3904/2N3906.

author: Electro Fan
author_rank: Super Contributor
timestamp: June 19, 2016, 11:07:34 am
content: A spectrum analyzer would make this project easier but it isn't necessarily required. If the OP builds the transmitter and all goes well the signal should land somewhere in the FM band. If the OP transmits a known song (by playing say a recording of the Beatles on a smartphone and connecting the smartphone's earphone output to the line-in of his transmiitter) he can put his transmitter very near a FM receiver radio - and then slowly tune up and down the dial on the FM receiver radio until he hears the Beatles song. The challenge will be if the transmitter frequency happens to be a little high or low and falls outside the FM listening band, in which case he will have to do some trial and error to adjust the transmit frequency. This might be possible by adjusting the inductor coil. It's a good project for getting introduced to circuits, home made board layout, and soldering in general, and radio in particular. The OP will probably have a smile when he first hears his transmitted recording being received on the receiver radio.

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 19, 2016, 02:46:09 pm
content: Instead of a inductor coil, could I just use one of these inductors?

author: Audioguru
author_rank: Super Contributor
timestamp: June 19, 2016, 04:36:26 pm
content: The "Minature FM Transmitter" was posted on another website and did not work:1) If the gain of the preamp transistor is high and/or if the battery is new then the preamp transistor is saturated and does not work.2) If the gain of the preamp transistor is low and/or if the battery voltage is low then the preamp transistor is cutoff and does not work.3) The radio frequency changes when the battery voltage runs down because the voltage is not regulated.4) The radio frequency changes when something moves near or away from the antenna because there is no RF amplifier to separate the oscillator from the antenna.5) It sounds muffled when heard on a normal FM radio because it is missing pre-emphasis (treble frequencies boost) like all FM radio stations have and the de-emphasis in all FM radios cuts the high frequencies down to normal.So I fixed it by biasing the preamp transistor properly, using a low dropout voltage regulator and adding pre-emphasis. The RF oscillator also uses the voltage regulator and I added an RF transistor. It works well and sounds very good. It probably will not work when built on a solderless breadboard because of too much stray capacitance between the strips of contacts and the many jumper wires. I built mine on a soldered compact stripboard layout.

author: Audioguru
author_rank: Super Contributor
timestamp: June 19, 2016, 04:46:01 pm
content: My very cheap Chinese solar garden lights have this very cheap inductor that probably does not work at radio frequencies. An FM radio or transmitter can use a simple coil of insulated wire with air as its core.

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 19, 2016, 05:04:58 pm
content: Quote from: Audioguru on June 19, 2016, 04:36:26 pmIm having trouble finding an lm2931 chip. I see lots of variants of them as some of them are IC's some of them have 4 legs etc etc

author: Electro Fan
author_rank: Super Contributor
timestamp: June 19, 2016, 05:22:32 pm
content: All the issues that Audioguru noted can be challenges and they can also be learning opportunities. Part of learning is to understand not only the components individually and how they get placed within circuits but also how they interact with each other. Having said that electronics can start out simple, reveal some important fundamentals, and still be marvelous. The point is take it in digestable increments. Some people because of their prior experience or knowledge can take much bigger bites than others. Electronics is like an endless? onion with layers - it just keeps going but it's also like a beautiful flower - it's elegant and smells much better than an onion figuratively and literally (especially if you like the smell of solder - but try to work in a well ventilated area). Ok, enough philosophy.Here is a radio project that might be a good starting point:Sometimes seeing the circuit and the physical layout can be helpful. The coil will lend itself to adjustments. If you want to modify the design you could add sockets to provide pressure fit attachment points for the ends of the coil. That way you can easily remove the coil, change the length and winding dimensions and then reinsert the coil into the circuit to see the impact on the transmitter's frequency. The size of the board could be made a tad larger than what is shown to make it slightly easier to place all the components on the board. Remember to keep the transmitter very close to the receiving radio - with this particular design you might have to nearly touch the transmitter to the receiver antenna.More info including schematic:http://makezine.com/projects/super-simple-fm-transmitter/This design was originally popularized by Japanese multimedia artist Tetsuo Kogawa. The circuit itself is a slight variation on Kogawa’s simplest FM transmitter design, and the method of building it is sometimes referred to as “Manhattan style.”It uses a piece of copper-clad circuit board but, rather than etching the circuit traces through the copper layer, a large piece of continuously-plated board is used to make all the circuit’s ground connections, and small sections of plated board are glued to the surface to form nodes or “pads” that are insulated from ground. Besides being a convenient way to assemble circuits using minimal tools, this building method encourages you to think about circuits in an interesting way — as groups of connections that are either grounded or “floating above” ground.

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 19, 2016, 05:51:32 pm
content: Quote from: Electro Fan on June 19, 2016, 05:22:32 pmIs there a way to incorporate a trimmer capacitor so that I don't have to mess with the coil when to change the frequency?

author: Electro Fan
author_rank: Super Contributor
timestamp: June 19, 2016, 05:58:13 pm
content: I'll leave that to more experienced EEVers but I think it's safe to say that there is a very important set of relationships between inductance and capacitance (and resistance) in electronics in general and radios/antennas in particular - so I'd encourage you to explore your idea. Maybe not for this project but on another project you might enjoy looking into resistance vs impedance and the topic of reactance. The flower just keeps getting more beautiful.- in the meantime you could build the design in the video and I think it will have a high probability of working; you can always modify it and/or build a second version - it's a process

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 19, 2016, 09:06:43 pm
content: Why wouldn't they just mount it on a perfboard? Making these copper squares and crap seems kinda pointless to me.

author: Electro Fan
author_rank: Super Contributor
timestamp: June 19, 2016, 09:17:58 pm
content: Give it a try. Should be easy to assemble pretty quick. If it works great, if not you can reuse all the components.Or you could take a look at this or other similar articles:http://electronics.stackexchange.com/questions/105688/using-copper-clad-for-rf-projectsFWIW, I have been trying to learn electronics at the hands-on level since I first discovered EEVblog. It's been a pretty slow and methodical (often tedious) process of studying plus trial and error but with feedback from other EEVers and other learning vectors (the Internet is of course a magnificent resource) the journey has yielded steady if not rapid progress, and even though it's been kind of slow it has yielded what I consider to be significant progress (at least compared to where I started). But even though Ohm's Law, resistors, LEDs, DMMs, batteries and DC power supplies, transistors, capacitors, inductors, antennas, spectrum analyzers, signal generators, oscilloscopes, AC vs DC, impedance and reactance including phase angles and more have become increasingly understandable the concept of ground remains consistently among the most commonly used and yet the most elusive. I don't know what I'm missing but I know I'm missing something, probably a lot, and probably some things that are very key. Good luck with "ground" and everything related (which I have a hunch is most of electricity).(just to be square, that's me - but I've found it helps to be a little humble and be able to laugh at yourself when trying to learn electricity)PS, I think understanding ground is also important to personal and test equipment safety - two other great reasons beyond DUT performance and safety to figure out ground and it's various implications sooner than later

author: chris_leyson
author_rank: Super Contributor
timestamp: June 19, 2016, 10:11:17 pm
content: You could build it on perfboard but it probably wouldn't work, hence the construction over a ground plane, what you are trying to build is a high frequency RF oscillator after all. Cutting up a few squares of FR4 PCB isn't that big a deal and to honest trying to build it on perfboard is "kinda pointless". Building or prototyping stuff dead bug style over a ground plane is a good technique for VHF and even UHF, I've even seen engineers use this prototyping method at much lower frequencies with good results.With perfboard you've got stray capacitance between the strips and each strip has unwanted inductance at the sort of frequencies you want to work at. How do you wire the ground in ? It just becomes a mess of stray inductances and capacitances.I remember building FM bugs as a kid a long time ago, it was great fun, go ahead and build it dead bug style it'll work, you can always use perfboard at lower frequncies.

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2016, 10:30:09 pm
content: Quote from: uncle_bob on June 18, 2016, 07:47:04 pmThe correct answer is: with the NPN connected in series with the negative side of the bulb, it's a common emitter amplifier, which has both current and voltage gain. When the NPN transistor is connected to the positive side of the bulb, it becomes an emitter follower which has no voltage gain but lots of current gain.A BJT only starts to turn on when the voltage on the base is around 0.6V greater than the voltage on the emitter. If the transistor is connected on the negative side, then it can be made to fully turn on (saturate) if there's sufficient base current. If the transistor is connected in series with the positive side of the bulb, then the voltage on the bulb will always be at lease 0.6V less than the voltage on the base and it's not possible to saturate the transistor, unless the base is connected to more than 0.6V above the positive rail.

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 19, 2016, 10:57:49 pm
content: Quote from: chris_leyson on June 19, 2016, 10:11:17 pmWell audioguru's schematic design isn't mounted on a copper plate and I see other fm transmitter tutorials on YouTube that use just a normal perfboard

author: Audioguru
author_rank: Super Contributor
timestamp: June 20, 2016, 01:54:27 am
content: Quote from: PotatoBox on June 19, 2016, 05:51:32 pmMy project uses trimmer capacitor C6 to adjust the radio frequency and uses trimmer capacitor C13 to adjust the circuit for maximum output which is affected by the antenna and its surroundings.The "Super Simple ..." is a nightmare with no schematic, no parts list and none of its parts labelled. It will have the same problems I noted with the other simple circuit.I used stripboard, not a perf board because the strips of copper are cut to length and form half of a printed circuit board. The parts and a few short jumper wires form the other half.The parts must be fairly close together since a wire has inductance that you do not want in many parts of the circuit.There are thousands of 5V low dropout voltage regulator ICs in 3 pins packages available. Look for them atwww.digikey.com. An ordinary 5V regulator fails when a 9V battery voltage drops a little as it runs down. A low dropout 5V regulator works perfectly until the 9V has dropped to 5.5V.

author: Audioguru
author_rank: Super Contributor
timestamp: June 20, 2016, 01:58:40 am
content: Parts labelled on my stripboard layout:

author: Electro Fan
author_rank: Super Contributor
timestamp: June 20, 2016, 05:05:36 am
content: Quote from: Audioguru on June 20, 2016, 01:54:27 am"Nightmare" might be a bit harsh. The video shows all the parts and tools needed in the first minute (a couple screen captures would provide printable lists). The schematic with the parts values is shown at the makezine link above.

author: nuno
author_rank: Frequent Contributor
timestamp: June 20, 2016, 09:50:14 am
content: As a kid I have built the simple 1 transistor FM bug (replace the cap in parallel with the coil with a variable capacitor "trimmer", use non-metalic tool to adjust frequency), with microphones from piezo elements extracted from defunct hand-held electronic games, on both screw and solder terminal strips and they worked.These pages have a bunch of projects using the simple 1 transistor FM transmitter stage, but they are in (Brazilian) Portuguese. Nevertheless you can still see the schematics and PCBs (almost at the end of the 1st link there's a diagram of a terminal strip assembly).http://blog.novaeletronica.com.br/circuitos-de-transmissores-de-fm/http://tutoriaiscaseiro.blogspot.pt/2014/09/como-aumentar-o-alcance-do-transmissor.html

author: Electro Fan
author_rank: Super Contributor
timestamp: June 20, 2016, 12:00:48 pm
content: Here is a link with Q&A regarding potential modifications including a trim capacitor with suggested values:http://anarchy.translocal.jp/radio/micro/howtosimplestTX.htmlThis link shows the details for a version with a 10pF trim capacitor:http://anarchy.translocal.jp/radio/micro/simplestTX01.pdfLooks like you could add one of these to the original design for about 81 cents:http://www.digikey.com/product-detail/en/sprague-goodman/GKG30015/SG9030-ND/262542http://www.digikey.com/product-detail/en/sprague-goodman/GKG20015/SG9029-ND/262541Or go high end:http://www.digikey.com/product-detail/en/sprague-goodman/GYA22000/SG3003-ND/65244