Or, Broadcast to an Old AM Radio Using Bluetooth

I was recently given a Hallicrafters S-38B radio receiver that had been rescued from a flood but was otherwise was doomed to be thrown out. I immediately started thinking about how I could modify the radio so that I could play music from my phone or computer through the tube amplifier. But, having worked with 110 V AC-powered tube amps in the past, I knew that I really didn’t want to mess with mains power directly. So I thought about finding a way to broadcast from my phone to the radio using a radio transmitter.

Many of you have probably used an FM transmitter to send audio from a phone to an FM radio in a car. FM works very well for this purpose even if it isn’t perfect. But back when my Hallicrafters radio was built (this model was produced from 1947 to 1953), AM was still the common mode for radio. So I searched around for a personal AM transmitter and found they weren’t readily available.

Next stop—Instructables. I was able to find a couple of projects for building an AM transmitter. I tried one using a 555 timer but was not able to get it to work the way I expected it to. So I bought a 1 Mhz crystal oscillator and worked on building one of the projects based on that. (This project is a remix of bmlbytes' transmitter.)

In this Instructable I will show you how I paired the AM transmitter project with a basic Bluetooth speaker and how I am able to play audio on my phone wirelessly to the radio. I can also plug my computer’s audio into the transmitter using an ⅛” audio jack on the Bluetooth speaker if necessary.

Step 1: Needed Parts

  • Some type of Bluetooth amplifier or speaker that you can use to get the signal into the transmitter. I used a Vivitar Bluetooth speaker that I bought but rarely used.
  • 1 Mhz full-can crystal oscillator. I could not find one locally despite having a couple of big electronics suppliers in town. They can be found at eBay, Amazon or a few other online retailers.
  • Transformer. I am not entirely sure what will and will not work for this purpose, but you are basically using it to inject some DC voltage into the audio signal from the Bluetooth module. An audio transformer would probably be best, but I tried an AC (115 VAC to 12 VAC) transformer and it worked. That was chosen simply because I had it sitting in my junk pile. Since it worked, I never tried anything else. I am not an electrical engineer and never really understood all the properties of a transformer (as you can see in my mystery question at the end).
  • Some hookup wire.
  • Something to build the project on and to enclose it.

Step 2: Preparing the Bluetooth Module

This part of the project can be done by buying a cheap Bluetooth amplifier instead of hacking up a Bluetooth speaker. But, as I already had the speaker that I don’t really use that much anymore, this was a good choice.

Cut the speaker wires off the output of the Bluetooth module and solder on a couple of wires that will go into the transformer. For testing purposes (until you figure out what wires need to go into which side of the transformer), leave yourself enough wire to feed into the transformer. Later, when you know how it all works, you can solder the transformer straight to the Bluetooth module.

For now, that is all we will do to the Bluetooth module. At some point you have to decide how you are going to power everything. So if your Bluetooth speaker was powered by a battery previously, you should leave it in place for the time being. If it was powered by a power cord you should be ready to go with that part for now.

Step 3: ​Build the AM Transmitter

There are a couple of other Instructables that teach how to build the type of transmitter I am using. I include links to them here because I am building the exact same transmitter. If my instructions are not clear, then maybe you will benefit from reading someone else's way of saying it. Here is a basic transmitter that will get you going. Here is one that takes more time to explain the details of how it all works, but is essentially the exact same transmitter as the previous link. And, if a video explanation helps, here is the same transmitter on YouTube.

You need a 1 Mhz crystal oscillator. The kind we are using for this is called a full can crystal oscillator. This type of crystal has 4 legs on it. The type of crystal with 2 legs (that you can find on a computer motherboard) is just the crystal and does not have the circuitry to make it oscillate.

You can actually use any oscillator that will put your signal in the AM band, but the 1 Mhz ones are fairly easy to find on eBay.

Using the diagram above, here is how you will build the transmitter and hook the Bluetooth module to it.

Bluetooth to Transformer

The speaker wires from the Bluetooth module get plugged into the low side of the transformer. In my case the low side of the transformer has a smaller plug which made it easy to figure out. But if your ransformer is not marked in any way, it will not hurt anything to plug it in and test both sides.

Oscillator Connections for the Transmitter

  • Pin 1 (bottom left corner) is not connected to anything. This is the 90 degree corner (the other 3 are rounded). There will probably also be a dot or some other type of marking on that corner.
  • Pin 7 (bottom right corner) gets the ground lead from the transmitter power source. In my case it is the ground from a 5 V AC to DC power supply. But you could also use 4-AA batteries (or a 9 V battery, but that may not be the best choice as you will see later).
  • Pin 8 (top right corner) goes to the antenna. To get started I just hooked a 10" (25 cm) piece of wire to this pin. You can add a longer wire later if necessary.
  • Pin 14 (top left corner) connects to one of the wires on the high side of the transformer—the other lead on the transformer connects to the positive 5 V of the power supply.

Step 4: ​Power

For testing purposes, you can power the Bluetooth module and the transmitter separately. In my case, my Bluetooth speaker had a small battery connected to it. Fully charged it was 4 V. Therefore, I assume that it is a standard 3.7 V battery. As far as the crystal transmitter is concerned it seems that anything from about 3 V to 9 V will work. However, I have read that much more than 6 volts is not good for the crystal. The datatsheet for my specific crystal oscillator says it operates at 5 V plus or minus 10%.

I am using a 5 V AC adapter (that I pulled out of a junk pile and tested with a volt meter) which can be used to power the transmitter crystal as well as the Bluetooth module. I cut off the plug end and marked the positive side with tape. (You can determine the positive and ground leads by using a volt meter). The positive wire from the power supply will go into one of the wires on the high side of the transformer. The ground wire goes into pin 7 on the crystal.

Since my Bluetooth module is charged by using a 5 V micro-USB adapter, I knew that I could safely feed 5 V into the amp. However, I was not sure if I needed to feed the 5 V into the USB charging connector for it to work, or if I could just remove the battery and send 5 V right into that spot. Fortunately for me, I was able to feed 5 V into the battery connector. This meant I could use one 5 V AC to DC power supply and feed both the transmitter and Bluetooth module.

Depending on your Bluetooth module, you may have to have 2 separate voltage sources. However, if your Bluetooth module is either powered by, or charged by, a USB phone charger, then you can probably do the same thing I have done. My guess is the vast majority of Bluetooth speakers that are cheap enough for you to be willing to hack up are powered this way. However, a Bluetooth amp that is designed for larger speakers probably have much higher power requirements.

Step 5: Test With the Radio

Power up your new Bluetooth/AM transmitter and connect to it with your phone, tablet or computer. Tune your AM receiver to 1000 kHz (or whatever your crystal will broadcast on) and see if you hear your broadcast. You may need to move your radio or its antenna around to see if you can pick up the signal. Move your transmitter closer to your radio. To get a better signal try lengthening the antenna wire.

If you are actually using this with a vintage radio like mine, you may be pleasantly surprised to find that many of these radios will allow you to connect an external antenna. When I first started working on this I was not getting a good signal out of my transmitter. Because I don’t really care about whether this is wireless or not (my goal is to use the radio without having to modify it), I attached the transmitter's antenna to the AM antenna lug of my receiver. Needless to say, you get very good reception that way.

If everything works as expected it is time to unify everything into the final project.

Step 6: Final Wiring

First I breadboarded everything to make sure it worked as expected. At this point I was still using the battery on the Bluetooth amp and only using the 5 V wall plug to run through the transformer to the crystal.

Then I pulled it all apart and rebuilt it into a more permanent setup, which you can see are exactly the same steps as the previous pages except now I hook up power to both the transmitter and Bluetooth module from the same power source.

Again, the positive of the 5 V power supply goes into one of the wires of the high side of the transformer. The other wire on that side of the transformer goes to pin 14 of the crystal. The ground of the power supply goes to pin 7 of the crystal.

The antenna is connected to pin 8 of the crystal package. The antenna should be as long as necessary for the receiving radio to pick up the broadcasted signal. Be aware that, in the United States, broadcasting your signal more than 200 feet (61 meters) is in violation of the FCC regulations for a low power unlicensed AM station. However, unless you work hard at building a good antenna and some power amplification into your unit, you are not likely to have any problems with the power limitations.

For the Bluetooth side I spliced into the 5 V positive coming out of the power supply and ran that into the Bluetooth amp at the solder pad of the positive battery connector. I then ran ground from pin 7 of the crystal over to the Bluetooth battery ground connection.

What would have been the two wires of the speaker output from the Bluetooth amp goes into the low side of the transformer.

Ready to Go!

Congratulations! The Bluetooth to AM Transmitter to AM Receiver project is done. Now you just need to house it in some clever way. Keep reading to find out about how I will house mine, a major potential risk to this project, and a mystery I have uncovered.

Step 7: Cleaning Up

I know some people build beautiful enclosures for their projects. I was never one of those. Most of the things I build get to a working state and live the rest of their lives without some fancy enclosure. I might find a plastic box to put this in and hide it behind a book on my bookshelf. But the reality is that I will probably have it sitting right next to the radio so I can show it off to anyone who seems mildly interested.

If, like me, you are making this to be used with a very specific radio, you could consider housing your project inside the radio itself. Pop the back off that old radio and hot glue everything inside. There is probably plenty of empty space to work with.

The video on this page is a timelapse video of me putting the Bluetooth/AM transmitter onto the final perf board. It was during this step that I found my mystery I talk about in a couple of pages.

Turn the page to read about the potential risks of working with old tube-type radios.

Step 8: Warning

If you are acquiring an old tube radio to build this project, be aware that these radios can kill you! They operate on line level voltage. Your radio may or may not have a step-down transformer, but it will still be AC powered (not our kinder, gentler DC power). If something is poorly grounded (my particular radio is known to be prone to this), then you could receive a deadly shock by touching something on the radio that was not originally harmful. Before turning an old radio on, you should at least replace any insulating grommets that have broken down through the years and strongly consider having the radio checked by a qualified electrician.

It is also recommended that you use an isolating transformer between your radio and mains power. This is a good recommendation even after having the radio checked out.

Step 9: A Mystery

At least this is a mystery to me.

There was one thing that I tried in this build that didn’t work as I expected that I thought might be of interest to others. When I connected the positive lead from the 5 V power supply to the transformer’s high voltage side (primary winding), I measured the voltage coming out with ground and appropriately got 5 volts. Therefore, I expected that I could take the voltage coming out of the other wire on the high voltage side and run it to my Bluetooth module to get power.

But that was not the case. Something happens to the voltage signal after it goes through the transformer winding. Even though my volt meter said I was getting 5 V, it would not power the Bluetooth module.

I ended up having to split the positive 5 V line and running it to the transformer separately from the positive 5 V going into the Bluetooth amp.

If anyone knows what is actually happening, I would love for you to leave a comment and let me know. I suspect with an oscilloscope I could see that the 5 V is pulsing in some odd way. The volt meter shows 5 volts but it can't respond quickly enough to see that the output from the transformer alters the rate of the signal.

The specs on my transformer are:

  • Input: 115VAC/60Hz
  • Output: 12VAC 0.5A

I appreciate those who voted for this in the Make Noise Challenge in January 2017. I got a nice T-Shirt out of it!

<p>I believe the 5v problem you were seeing is from the fact that the transformer is a load and so is the Bluetooth module. As soon as the Bluetooth module tries to 'power up' the circuit is now loaded, and the voltage will droop. You wouldn't see that with a volt meter, because the circuit was unloaded at the time.</p><p>Great way to update an antique radio without actually changing or modifying the radio.</p>
<p>Thanks for your input nanosec12. I will have to check into the idea that my power supply was getting overloaded. Thanks for the comment.</p>
Great job!
<p>I attribute the build to being able to get a Fresnel lens out of your neighbor's junk pile last week. :-)</p>
<p>Awesome project! :)</p>
<p>Thank you Swansong. It was a fun build.</p>

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More by dpeach:LED Trunk Light Upgrade How to Add Bluetooth to a Vintage Radio 
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