Science Buddies: "Ask an Expert"

Hi,

I am in the process of building the Low-Power AM Radio Transmitter (https://www.sciencebuddies.org/mentorin ... p024.shtml) and was wondering how I can attach a microphone or other device to the unit so that the direct sound source would be my voice. I understand simply connecting a microphone to the headphone jack will not work - is there a way to achieve live voice-broadcasting (as opposed to for example an iPod as my sound source)? I would appreciate any knowledge you can share on this topic!

Many thanks in advance,
Erica

ericaz - I would first build the circuit as described and establish that it does work with an audio source like a portable radio or MP-3 player. The audio transformer in the circuit is used to modulate the power to the crystal oscillator to achieve AM or Amplitude Modulation of the transmitter. Although the article says the transformer "amplifies" the input, that is not correct. Rather it changes the impedence of the input. The transformer is taking the low impedence input of 8 ohms, and transforming it to 1K ohms. This significantly increases the voltage of the signal on the secondary (but at much lower signal current). There is no amplification of the audio power by the transformer. Actually, all transformers will introduce a power loss. Sorry for the side-track. In order to get your voice to modulate the oscillator, you will have to introduce an audio signal level to the input of the transformer similar to that provided by the radio or MP-3 player. That means a true amplifier stage to connect your microphone to. You might find something like an LM386 IC amplifier (Radio Shack listed for $1.49) and build an amplifier stage. Do a search on this part number and the various manufacturers data sheets might have a typical circuit. Good luck.

Rick Marz

The previous comments about amplification and transformers and impedance is a bit technical.

From a practical standpoint, the modulator aspect of this particular oscillator/transmitter circuit is driven by the voltage and any impeadance/power/current aspects of the audio signal aren't very important for this circuit. The transformer in the original circuit transforms a low voltage audio AC signal on the primary winding to a higher voltage audio AC signal on the secondary winding which is used to feed the modulator.

In a sense, the transformer is increasing/amplifying the AC voltage of the audio signal.

There are inexpensive "crystal" aka "piezo" microphones that produce enough voltage to be used directly without the use of an audio amplifier or transformer with this transmitter.

Rick and Craig-

Thank you for your responses. I have since gotten the AM Transmitter to work with my iPod, and to broadcast my voice (albeit through a very stupid process of hooking it up to my laptop and condenser mic). I was looking into microphone amplifiers, and I was wondering if I were to connect to a factory-made amplifier + mic, would I have to reverse the transformer? I read elsewhere:

The two leads are the low impedance side of the transformer, (the 8 ohm side). The three leads are the high impedance side (the 1000 ohm side). The middle of the three leads is called the center tap, and we won't be using it in this circuit.

To get the best range, we put the low impedance side of the transformer in series with the oscillator. This means that the signal source must be capable of driving heavy loads, like an 8 ohm speaker.

I guess my question is, would a standard store-bought amplifier drive the signal great enough to warrant a reverse of the transformer?

I had looked into crystal microphones, but I don't have much time now, and don't think I can get one shipped to me in time.

Thanks again,
Erica

Craig- Are all contact microphones piezo mics, and will work the same way? Do you know if stores like RadioShack carry piezo/crystal microphones?

ericaz - If you have found a 'store-bought' amplifier and mic that is capable of driving a low impedence speaker or load (4-16 ohms) that should work fine. Do not reverse the transformer, as you want the output impedence of the amplifier to match the input impedence of the transformer as closely as possible. The transformer, in it's present configuration, is amplifying the voltage of the audio signal from the primary side to the secondary side. Because you are using a 6 volt battery supply, the modulation voltage at the secondary of the transformer output should probably be a few volts for relatively full modulation. For example, if the peak-to-peak audio voltage on the secondary was 4 volts, the modulated voltage supplied to the oscillator would be 4-8 volts (6 volts, +/- 2 volts). I think the use of a crystal microphone (with an impedence of tens of megohms) would not work on the low impedence transformer you have here without some form of amplification and impedence matching.

Rick Marz

ericaz wrote:Are all contact microphones piezo mics, and will work the same way?

Most contact microphones (aka pickups) are piezo; however, they are usually designed for acoustic instruments that are fairly loud on their own. Other than using a large funnel similar to what a "Victrola" needle phonograph used to amplify sound, using one of these pickups for voice isn't likely to work well.

ericaz wrote:Do you know if stores like RadioShack carry piezo/crystal microphones?

I looked at RadioShack's online store and they don't carry one.

rmarz wrote:I think the use of a crystal microphone (with an impedence of tens of megohms) would not work

Mosts crystal microphones I've dealt with over the years were in the 3 to 10K range. Example: http://www.mouser.com/catalog/specsheets/KT-400025.pdf is 9 K ohms. For this circuit, 9 K ohms is a bit high.

I went looking for a pre-assembled inexpensive audio amplifier and I didn't find one quickly. Given where you are time wise, you should consider asking yourself WHY you need to use a microphone as an audio source for this project? It isn't likely to be inexpensive or quick or both to do so.

I am having the same issue. i am working on sound effects for an installation about spacemen going to other dimentions, and wanted to incorporate audio feedback with the static qualities of AM radio. i have a month to get the sound together at least, and was hoping for some dumbed-down explanation of what i would need to get to connect a live mic to an am transmitter. hooking a mic to a small guitar amp and then hooking the amp to the audio plug on the transmitter, this idea came to me but i've no clue as the the ohm output of the amp. what am i to do?