Understanding Electronics

[PotatoBox]

Instead of a inductor coil, could I just use one of these inductors?

[Audioguru]

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.     

[Audioguru]

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.

[PotatoBox]

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.   

Im 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

[Electro Fan]

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.

[PotatoBox]

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.

Is there a way to incorporate a trimmer capacitor so that I don't have to mess with the coil when to change the frequency? 

[Electro Fan]

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

[PotatoBox]

Why wouldn't they just mount it on a perfboard? Making these copper squares and crap seems kinda pointless to me.

[Electro Fan]

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-projects

FWIW, 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

[chris_leyson]

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.

[Zero999]

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

The 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.

[PotatoBox]

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.

Well 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

[Audioguru]

Is there a way to incorporate a trimmer capacitor so that I don't have to mess with the coil when to change the frequency?

My 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 at www.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.

[Audioguru]

Parts labelled on my stripboard layout:

[Electro Fan]

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.

"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.

[nuno]

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

[Electro Fan]

Here is a link with Q&A regarding potential modifications including a trim capacitor with suggested values:  http://anarchy.translocal.jp/radio/micro/howtosimplestTX.html

This link shows the details for a version with a 10pF trim capacitor:  http://anarchy.translocal.jp/radio/micro/simplestTX01.pdf

Looks 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/262542

http://www.digikey.com/product-detail/en/sprague-goodman/GKG20015/SG9029-ND/262541

Or go high end:
http://www.digikey.com/product-detail/en/sprague-goodman/GYA22000/SG3003-ND/65244