How Does Project 122 Snap Circuit AM Transmitter Work?

[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.