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914World.com _ 914World Garage _ SOT: AM iPod Transmitter

Posted by: McMark Jan 12 2006, 10:19 PM

I'm rebuilding a 57 bug with the original AM radio. I'm interested in the feasibility of an AM transmitter for an iPod. I found this circuit and description, but it's over my head. Could this be made small enough to be portable?

BTW, I'm familiar with FM transmitters and cassette adapters and radios hidden in the glovebox, etc. I'm not interested in anything else. Just AM transmitters.

QUOTE
In this circuit, a 74HC14 hex Schmitt trigger inverter is used as a square wave oscillator to drive a small signal transistor in a class C amplifier configuration. The oscillator frequency can be either fixed by a crystal or made adjustable (VFO) with a capacitor/resistor combination. A 100pF capacitor is used in place of the crystal for VFO operation. Amplitude modulation is accomplished with a second transistor that controls the DC voltage to the output stage. The modulator stage is biased so that half the supply voltage or 6 volts is applied to the output stage with no modulation. The output stage is tuned and matched to the antenna with a standard variable 30-365 pF capacitor. Approximately 20 milliamps of current will flow in the antenna lead (at frequencies near the top of the band) when the output stage is optimally tuned to the oscillator frequency. A small 'grain of wheat' lamp is used to indicate antenna current and optimum settings. The 140 uH inductor was made using a 2 inch length of 7/8 inch (OD) PVC pipe wound with 120 turns of #28 copper wire. Best performance is obtained near the high end of the broadcast band (1.6 MHz) since the antenna length is only a very small fraction of a wavelength. Input power to the amplifier is less than 100 milliwatts and antenna length is 3 meters or less which complies with FCC rules. Output power is somewhere in the 40 microwatt range and the signal can be heard approximately 80 feet. Radiated power output can be approximated by working out the antenna radiation resistance and multiplying by the antenna current squared. The radiation resistance for a dipole antenna less than 1/4 wavelength is

R = 80*[(pi)^2]*[(Length/wavelength)^2]*(a factor depending on the form of the current distribution) The factor depending on the current distribution turns out to be [(average current along the rod)/(feed current)]^2 for short rods, which is 1/4 for a linearly-tapered current distribution falling to zero at the ends. Even if the rods are capped with plates, this factor cannot be larger than 1. Substituting values for a 9.8 foot dipole at a frequency of 1.6 MHz we get R= 790*.000354*.25 = .07 Ohms. And the resistance will be only half as much for a monopole or 0.035 Ohms. Radiated power at 20 milliamps works out to about I^2 * R = 14 microwatts.

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Posted by: jkeyzer Jan 12 2006, 10:31 PM

Looks doable. The hardest part is winding the coils for the inductors unless you can find them premade.

Overall the circuit will be pretty small except for the 140uH inductor and the antenna, which you could probably make into a coil.

The fidelity will probably be pretty poor and you may have to retune the radio to follow the transmitter since open loop circuits like that tend to drift. If you picked one frequency to operate at and used a crystal it would be better. The frequency also tends to be "pushed" by nearby metal objects etc. You will also have to fight ignition noise, I don't see much filtering on that circuit and it is probably intended to operate with batteries.

Here's a kit that is equally simple, but you won't have to wind your own inductors.
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=AM1C

There may be AM transmitter IC's around. I would think you could make the thing the size of an altoids box with a more modern circuit design.

This kit uses a better approach:

http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=AM25

Posted by: 914GT Jan 12 2006, 10:52 PM

Just AC couple the audio from the iPod into the volume control of the AM radio. You might need to switch out the antenna if outside signals interfere with the sound quality.

Posted by: McMark Jan 12 2006, 10:53 PM

Hmmmm.... but is the volume control pre or post amp?

Posted by: 914GT Jan 12 2006, 10:55 PM

QUOTE (McMark @ Jan 12 2006, 09:53 PM)
Hmmmm.... but is the volume control pre or post amp?

Usually it's in the preamp section.

Posted by: McMark Jan 12 2006, 11:08 PM

Now, which ones to tap into on the Pot? The three white ones connect up front, and the green cloth covered one connects on the back.

Posted by: McMark Jan 12 2006, 11:08 PM

Use your imagination, or this picture.

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Posted by: McMark Jan 12 2006, 11:22 PM

I just confirmed that the stereo does, in fact, work. So on to step 2. I'm off to Target to get a cheap headphone cord that I can cut and jack into the pot. You better have an answer for me by the time I get back.

Posted by: 914GT Jan 12 2006, 11:23 PM

Well, it's not exactly easy figuring out the wiring from the photo but I do see one end of the ganged pots connected through a ceramic disk cap. That might be a good place to try injecting an audio signal. One way to check it is to power the radio up to the speakers (leave the antenna unplugged) then touch your finger or a test lead to the terminals on the pots. Have the volume control up a ways, maybe halfway. If you hear buzzing or humming when you touch it, and can control the volume level of the noise, you've found a spot you can couple your external audio into. Assuming the radio is all transistors and 12V you won't get shocked. You can inject the external audio in through a capacitor (something like a 0.1 uF - 0.5 uF should be reasonable value). This will block any DC level from getting back into the iPod. If necessary an external resistor divider or trimmer pot could be used to set the level. You'll probably need to couple the iPod's left and right channels through a couple resistors anyway so you'll have a mono signal.

Posted by: McMark Jan 12 2006, 11:27 PM

Will the signal and ground wires go to the pot? Or will the ground go to "chassis" ground (case) and the signal go to the pot?

Posted by: McMark Jan 13 2006, 12:18 AM

Well I tried all the connection combos I could think of. But not sound.

Wait, what's this? I'm a moron? Yes, yes, that's it. A wire has come loose. I've found the wire and will be adding an audio in jack to this 1957 Bendix AM radio for iPod use. Thanks for the idea! I might even add a small 1W amp to boost the signal just a bit.

Now to deal with the scratchy pot.

Posted by: jkeyzer Jan 13 2006, 01:37 AM

That was fast. I'm impressed! Nice alternative solution!

Posted by: McMark Jan 13 2006, 02:06 AM

New Question: I just found out the radio is 6 volt supposedly. Since it obviously works on 12 volt, what's going on? Electronics make me feel stupid.

Is this one of those situations where it will work in the short term, but will probably die quickly?

Upon reading more I'm starting to wonder if this radio has been converted. The bulb didn't burn out as I've been led to believe a 6v bulb will when connected to 12v.

Posted by: 914GT Jan 13 2006, 08:50 AM

Mark,
Usually something designed for a particular voltage will not last long if you double the voltage on it, especially if it's 'solid state' (haven't used that term in awhile). If the radio has 6V stamped on it and it's running on 12, it must have been modified and that would mean having a dropping resistor or regulator put in somewhere on the DC input. Dropping resistors are a poor way for a car radio since the load changes with music and volume level. If the bulbs in the radio are looking normal brightness it must be OK.

On your earlier question - the shield or ground would connect to the chassis (I'm assuming this is a negative ground setup on the battery).

Depending on how sophisticated you want to get with this, it should be possible to separate the preamp section from the AM tuner at the volume control. Then you could bring these out to a switch and have it selectable from AM radio to iPod input.

Have fun!

Posted by: KenH Jan 13 2006, 11:36 AM

Google this "am modulation IC" lots of circuits are listed.

Ken

Posted by: Eddie Williams Jan 13 2006, 11:52 AM

Wow an iPod in a 50 year old Bug with the original 6volt AM radio.... Who'd of thunk it?
I love this site!!!