Understanding Electronics

[PotatoBox]

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?

[uncle_bob]

Hi

Ok, 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

[PotatoBox]

Hi

Ok, 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

Alright, so the transistor could go before or after the bulb and would still work the same way?

[uncle_bob]

Hi

Ok, 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

Alright, so the transistor could go before or after the bulb and would still work the same way?

Hi

Yes, 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

[retrolefty]

Hi

Ok, 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

Alright, so the transistor could go before or after the bulb and would still work the same way?

 No, 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.

[uncle_bob]

Hi

Ok, 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

Alright, so the transistor could go before or after the bulb and would still work the same way?

 No, 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.

Hi

In 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

[PotatoBox]

Whats a good example of a circuit when you would use a pnp transistor?

[uncle_bob]

Whats a good example of a circuit when you would use a pnp transistor?

Hi

Transistors 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

[PotatoBox]

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?

[uncle_bob]

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?

Hi

As 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

[PotatoBox]

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.

[uncle_bob]

Hi

It'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

[PotatoBox]

Hi

It'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

Ok well, What are some other good schematics that are simple and fun to build?

[uncle_bob]

Hi

It'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

Ok well, What are some other good schematics that are simple and fun to build?

Hi

VHF 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

[PotatoBox]

Hi

It'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

Ok well, What are some other good schematics that are simple and fun to build?

Hi

VHF 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

Bob, the schematic above is an FM transmitter for FM radios.

[uncle_bob]

Hi

It'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

Ok well, What are some other good schematics that are simple and fun to build?

Hi

VHF 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

Bob, the schematic above is an FM transmitter for FM radios.

Hi

The 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

[PotatoBox]

Hi

It'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

Ok well, What are some other good schematics that are simple and fun to build?

Hi

VHF 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

Bob, the schematic above is an FM transmitter for FM radios.

Hi

The 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

Well 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.   

[retrolefty]

Hi

It'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

Ok well, What are some other good schematics that are simple and fun to build?

Hi

VHF 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

Bob, the schematic above is an FM transmitter for FM radios.

Hi

The 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

Well 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.

 Don'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.

[PotatoBox]

Hi

It'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

Ok well, What are some other good schematics that are simple and fun to build?

Hi

VHF 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

Bob, the schematic above is an FM transmitter for FM radios.

Hi

The 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

Well 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.

 Don'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.

Actually 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?

[uncle_bob]

Hi

The 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

[PotatoBox]

Hi

The 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

Is it because the schematic is flawed? Because there are other ones out there. I found a different one.

[uncle_bob]

Hi

An 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

[Dave]

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.

... a PLL based approach.

[nuno]

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.

[Electro Fan]

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.