How Does Project 122 Snap Circuit AM Transmitter Work?

[topito2]

TL;DR Summary

AM transmitter

Hi, guys! Could anyone provide an explanation about how the following AM transmitter works? It is similar to the AM transmitter for project 122 of the Snap Circuit kit by Elenco. Does it work as a switching modulator?

 

[tech99]

What does signal to be transmitted mean? Is it the modulating audio or the amplitude modulated carrier?

 

[berkeman]

Summary:: AM transmitter

It is similar to the AM transmitter for project 122 of the Snap Circuit kit by Elenco.

Could you provide a link to help us find the explanation?

 

[topito2]

Could you provide a link to help us find the explanation?

You can see the system working in the following video. The audio is in Spanish but it shows the system working.

 

[topito2]

Could you provide a link to help us find the explanation?

Snap Circuits Manual
https://www.elenco.com/wp-content/uploads/2017/10/753098-2.pdf

Projects 122 and 213

This is the kit
https://www.google.com.pa/url?sa=t&...p/B0000683A4&usg=AOvVaw1XqTGc7s8KYSWapKVfLp3r

 

[topito2]

What does signal to be transmitted mean? Is it the modulating audio or the amplitude modulated carrier?

Audio signal

 

[berkeman]

Snap Circuits Manual
https://www.elenco.com/wp-content/uploads/2017/10/753098-2.pdf

Projects 122 and 213

This is the kit
https://www.google.com.pa/url?sa=t&...p/B0000683A4&usg=AOvVaw1XqTGc7s8KYSWapKVfLp3r

Sorry, that doesn't really help. I need a full schematic of the whole circuit, preferably with an explanation of how it works. Is that included anywhere in the documentation?

 

[Baluncore]

Summary:: AM transmitter

Does it work as a switching modulator?

Only the positive peaks of the audio turn the transistor on and sink current.
That current flows through the tuned circuit and causes bursts of oscillation.
The tuned circuit is being “plucked” like a string.
Bigger positive peaks sink more current than small peaks.
The amplitude of the oscillation is determined by amplitude (and frequency) of the audio.
In a sense it is a class C transmitter.

 

[berkeman]

In a sense it is a class C transmitter.

Yoiks, so illegal to build and transmit at any reasonable power level then? Hopefully the range of that AM transmitter kit is less than 10 meters...

 

[Baluncore]

Yoiks, so illegal to build and transmit at any reasonable power level then?

It is a very clever implementation of a low power AM transmitter. It reminds me of tuned-output spark transmitters.

I don't think class C prohibits it's use. The harmonics generated are of the audio, not the RF. The rise-time of the RF pulse is determined by dv/dt of the audio, so "key clicks" should not spread across the band to be a problem.

 

[berkeman]

It is a very clever implementation of a low power AM transmitter. It reminds me of tuned-output spark transmitters.

I don't think class C prohibits it's use. The harmonics generated are of the audio, not the RF. The rise-time of the RF pulse is determined by dv/dt of the audio, so "key clicks" should not spread across the band to be a problem.

Interesting. I may have to simulate it to see what is going on. It's hard to believe that snapping a 2nd order LC resonator on and off won't result in out-of-band harmonics, but you might be right. The bandpass filters that I'm more familiar with in the WiFi band are much steeper than 2nd order.

 

[Baluncore]

It's hard to believe that snapping a 2nd order LC resonator on and off won't result in out-of-band harmonics, but you might be right.

I do not believe it is being "snapped" on and off.
Another interesting point, worthy of simulation, is the purpose of the diode.

 

[Tom.G]

Another interesting point, worthy of simulation, is the purpose of the diode.

I'll go out on a limb here:

Probably to give the edges to shock-excite the tuned circuit.
When the diode is Off it would also serve to isolate the Collector capacitance of the driver from the LC cicuit.

 

[Baluncore]

 

[tech99]

It looks as if the transistor forms an oscillator at the frequency determined by L and C. Not sure how exactly. It is base modulated by the audio.

 

[sophiecentaur]

I don't think class C prohibits it's use. The harmonics generated are of the audio, not the RF.

Would a simple tank circuit really eliminate harmonics? Perhaps you mean that the actual power level would be low and that the (very short) antenna response would also be low.

The graph in that post suggests it would be OK.

 

[tech99]

Another possibility comes to mind for the operation of this circuit. I have noticed that audio amplifier ICs, such as LM386, create large amounts of noise which extends far above the audio range. It is possible to hear this noise on a nearby receiver. If the IC is amplifying audio at the time, the signal is heard as amplitude modulated noise. It is so severe that if you use the IC with a ferrite rod receiver for the European Long Wave band, the ferrite rod will pick up the modulated noise signal and it will cause "howl round" or audio oscillation of the receiver. It looks to me that the audio IC supplies a noise signal to the transistor, extending up to the broadcast band, and also supplies modulating audio to the base of the transistor. The LC circuit will create a noise spectrum having the shape of the LC response.

 

[DaveE]

It's hard to believe that snapping a 2nd order LC resonator on and off won't result in out-of-band harmonics

Not if your "snapping" is band limited in the audio range.

The bandpass filters that I'm more familiar with in the WiFi band are much steeper than 2nd order.

Yes, this is the point the RF emission spectrum is essentially determined by the resonant tank properties, spread out a bit by the audio AM modulation. So, good Rx or Tx circuits use narrow filters of some sort.

Another interesting point, worthy of simulation, is the purpose of the diode.

Yes, if the diode switches in response to the tank oscillation, then you'll get RF Harmonics. If the switching is fast but driven by the audio, then the HF noise will be spread out. If you look for it, I'd bet you see both effects.

 

[DaveE]

Another possibility comes to mind for the operation of this circuit. I have noticed that audio amplifier ICs, such as LM386, create large amounts of noise which extends far above the audio range. It is possible to hear this noise on a nearby receiver. If the IC is amplifying audio at the time, the signal is heard as amplitude modulated noise. It is so severe that if you use the IC with a ferrite rod receiver for the European Long Wave band, the ferrite rod will pick up the modulated noise signal and it will cause "howl round" or audio oscillation of the receiver. It looks to me that the audio IC supplies a noise signal to the transistor, extending up to the broadcast band, and also supplies modulating audio to the base of the transistor. The LC circuit will create a noise spectrum having the shape of the LC response.

That's an interesting observation. I wouldn't expect that from a typical class AB amp. Perhaps from cross-over distortion? Or PSRR effects? Did you ever find out why?

 

[topito2]

Sorry, that doesn't really help. I need a full schematic of the whole circuit, preferably with an explanation of how it works. Is that included anywhere in the documentation?

Well, that's exactly the information I'm looking for. I'm asking if someone can explain to me how this system works since it is not explained in the documentation

 

[sophiecentaur]

Well, that's exactly the information I'm looking for. I'm asking if someone can explain to me how this system works since it is not explained in the documentation.

If you want a real noddy description. The output power of a RF oscillator will depend on the 'power supply' volts. The transistor acts as both RF oscillator and AF amplifier. The peak amplitude of the RF will be proportional to the AF signal.

The problem with a circuit like this is that, instead of working like the block diagram of a 'typical' Amplitude Modulator circuit, they have been too damn smart and have condensed it down to a minimum number of components with the single transistor doing everything. Really not the best way to start on simple electronics.

 

[topito2]

If you want a real noddy description. The output power of a RF oscillator will depend on the 'power supply' volts. The transistor acts as both RF oscillator and AF amplifier. The peak amplitude of the RF will be proportional to the AF signal.

The problem with a circuit like this is that, instead of working like the block diagram of a 'typical' Amplitude Modulator circuit, they have been too damn smart and have condensed it down to a minimum number of components with the single transistor doing everything. Really not the best way to start on simple electronics.

OK, so it seems two phenomena happen in the system: we have RF oscillations and AF amplication happening at the same time, which could not be explain by conventional block diagrams of AM systems. So if I use this system in a technology class for middle school students to have them to explore AM modulators and AM transmission it might better if I just use this system for them to have a practical approach to AM transmission, but not to dive in into the specifics. Would you agree?

 

[sophiecentaur]

OK, so it seems two phenomena happen in the system: we have RF oscillations and AF amplication happening at the same time, which could not be explain by conventional block diagrams of AM systems. So if I use this system in a technology class for middle school students to have them to explore AM modulators and AM transmission it might better if I just use this system for them to have a practical approach to AM transmission, but not to dive in into the specifics. Would you agree?

It's ages since I did any simple construction but there will be dozens of simple AM project circuits on the www in which the two functions are achieved with two separate transistors. If it's taken PF thirty posts to discuss what this 'smart Alec' circuit actually does then it is clearly beyond the scope of young, first-time minds. The total amount of lacking knowledge in an average science class is quite staggering so it has to be made really easy for them.

 

[topito2]

It's ages since I did any simple construction but there will be dozens of simple AM project circuits on the www in which the two functions are achieved with two separate transistors. If it's taken PF thirty posts to discuss what this 'smart Alec' circuit actually does then it is clearly beyond the scope of young, first-time minds. The total amount of lacking knowledge in an average science class is quite staggering so it has to be made really easy for them.

Thank you so much for the helpful advice.

 

[topito2]

Only the positive peaks of the audio turn the transistor on and sink current.
That current flows through the tuned circuit and causes bursts of oscillation.
The tuned circuit is being “plucked” like a string.
Bigger positive peaks sink more current than small peaks.
The amplitude of the oscillation is determined by amplitude (and frequency) of the audio.
In a sense it is a class C transmitter.

Thank you so much for your answer.

 

[topito2]

Another possibility comes to mind for the operation of this circuit. I have noticed that audio amplifier ICs, such as LM386, create large amounts of noise which extends far above the audio range. It is possible to hear this noise on a nearby receiver. If the IC is amplifying audio at the time, the signal is heard as amplitude modulated noise. It is so severe that if you use the IC with a ferrite rod receiver for the European Long Wave band, the ferrite rod will pick up the modulated noise signal and it will cause "howl round" or audio oscillation of the receiver. It looks to me that the audio IC supplies a noise signal to the transistor, extending up to the broadcast band, and also supplies modulating audio to the base of the transistor. The LC circuit will create a noise spectrum having the shape of the LC response.

Thank you so much for sharing. You're very kind.

 

[DaveE]

If you feel like they have to build something, perhaps more of a "black box" approach like this one?

https://www.sciencebuddies.org/scie...own-low-power-am-radio-transmitter#background

I'm not sure it makes sense to try to really explain how radios work if they don't already know about transistor amplifiers, spectrum analyzers (or Fourier transforms, the frequency domain), etc.

Most practicing EEs work 80% in the "black box" domain anyway. We are really detail oriented system engineers; choosing ICs and using them they way the designers say to. Of course with a lot of background knowledge about how to connect things. Then sometimes you do real circuit design for the tricky bits. Honestly, even a 2N2222A is a "black box" to me.

 

[topito2]

If you feel like they have to build something, perhaps more of a "black box" approach like this one?

https://www.sciencebuddies.org/scie...own-low-power-am-radio-transmitter#background

I'm not sure it makes sense to try to really explain how radios work if they don't already know about transistor amplifiers, spectrum analyzers (or Fourier transforms, the frequency domain), etc.

Most practicing EEs work 80% in the "black box" domain anyway. We are really detail oriented system engineers; choosing ICs and using them they way the designers say to. Of course with a lot of background knowledge about how to connect things. Then sometimes you do real circuit design for the tricky bits. Honestly, even a 2N2222A is a "black box" to me.

I will take your advice. Thank you so much.

 

[topito2]

I just want to say thank you to all the participants who have given me very valuable information. Thank you so much for sharing your knowledge with me. Blessings!

 

[Baluncore]

Thank you so much for your answer.

That is what we are here for.

In these times of the www it is necessary to give a quick reward to maintain interest in a subject.

For success, a fishing lure must be designed for a dual purpose. The lure must first be marketed to catch the fisherman, then the lure must quickly catch a fish.

Likewise, this attractive minimum circuit can be quickly built, then like magic it has generated an AM signal.

The fact that topito2 has requested more information demonstrates the efficacy of the circuit at selecting budding engineers.

 

[sophiecentaur]

The fact that topito2 has requested more information demonstrates the efficacy of the circuit at selecting budding engineers.

Hmm, not having built it, I have no idea just what the output would actually look like on a scope. I suspect it may look pretty scrappy although an AM receiver could probably give some recognisable audio out.
I really think this is a case for Back to the Drawing Board and asking what is the purpose of the lesson that @topito2 is planning. Will there be number of groups building and testing this or will it be a demo? I wouldn't dream of giving a lesson to teenagers, based on that circuit - although I could contemplate building it at home (if I still did that sort of thing). The circuit has only one thing going for it and that is it uses very few components - none of which do their job perfectly.
I don't know that sort of syllabus the kids are being taught to but school students need quick success with experiments and rapidly lose interest when problems arise or results are confusing.
To demonstrate AM, why not use an RF oscillator circuit with its Power supply derived from a lab AF tone source.
I suggest that the OP is looking for a simple presentation which will not tax his/her electronics skill too much. I don't think anyone on this group would think of that circuit as an entry level exercise into AM. It would be unfair, IMO, to encourage the OP in this direction, even if the topic is fun to discuss.

 

[topito2]

Another interesting point, worthy of simulation, is the purpose of the diode.

Just to share some information I found that gave me some light about the purpose of the diode:
https://www.edn.com/increase-piezoelectric-transducer-acoustic-output-with-a-simple-circuit/

This is for an acoustic transducer, but might be the reason the Elenco engineers included the diode.
And I found a printed Snap Circuits manual that mentions that the music IC provides the modulated audio signal in the system (I don't post the exact text here since I don't want to violate any copyrights law).

 

[Baluncore]

Just to share some information I found that gave me some light about the purpose of the diode:

When a red LED is used for that diode, it sheds even more light on how a significantly greater voltage compliance can be available for ringing the LC tank. With an LED the 3V power supply also seems more appropriate.

 

[StandardsGuy]

Summary:: AM transmitter

Hi, guys! Could anyone provide an explanation about how the following AM transmitter works? It is similar to the AM transmitter for project 122 of the Snap Circuit kit by Elenco. Does it work as a switching modulator?
View attachment 282911

The diode serves no useful purpose and looks to be in backwards, but no polarity of voltage is shown. There is little current limiting in the circuit, so the transistor may fail from excess current, except that the transistor has no bias. The inductor is a very poor antenna. How does it work? Poorly if at all.

 

[Averagesupernova]

The diode serves no useful purpose and looks to be in backwards,

Turn it around and this transmitter will have an output cleaner than any other in existence.

 

[Baluncore]

The diode serves no useful purpose and looks to be in backwards, but no polarity of voltage is shown.

Both the diode and the transistor will be forward biassed and will operate with the +3V supply. The LED, (as shown in the video), illuminates when there is sufficient audio input. The LED is forward biassed, so it gives a couple of volts compliance for the oscillation of both phases. Build it and take a look, or simulate it and you will see.

There is little current limiting in the circuit, so the transistor may fail from excess current, except that the transistor has no bias.

The transistor bias current is limited by the audio source provided in the kit. The LED and the +3V supply in the kit limit the collector current. It is the half wave bias that generates the AM signal.

The inductor is a very poor antenna.

Yes, but it is more than sufficient to demonstrate the AM modulation on an AM receiver where only 10 uV of antenna input is needed.

How does it work? Poorly if at all.

If it worked any better it would need more than a "very low" EIRP class license.