Well done Dan! It’s a great feeling when it finally works.

I have the parts coming in this week for this project. With the bitter cold I have retreated indoors for some building projects.

Have a suggestion based on another transmitter project. Rather than a range of inductors and trimmer cap I used a ferrite core antenna coil from my junk box. With the core fully inserted the coil is about 260 uh with it removed it is about 15 uh. I just leave the trimmer cap out and peak the transmitter with the inductor. Works over a nice wide range of frequencies and antenna lengths. I am using it at 1228 kh right now. It does help that I have about a dozen of these coils on hand that are NOS.

I have now built 2 of these and a 3rd on the protoboard for experimenting. I eliminated all the caps on the LM386 except C2 and I used a .001 because I have a lot of them. Seems to work fine. I found some small variable inductors that look a lot like IF cans in a transistor radio. They mount to the circuit board and have a range of 90 uh to about 400 uh. I can peak the antenna with just the inductor. I went with all PCB mount components on this one. It really does not even need a case. Makes a very compact unit.

Put it in a clear box to show off it's simplicity.
Nice work!

Great idea. Thanks!

Still got my transmitter on the breadboard. Sounds pretty good on my 1950 Zenith, not so good on my 70's Zenith previously pictured. I picked up the 70's one because I knew I'd be doing a bit of testing and wanted to keep the miles off the G500. The 70's Zenith with no external antenna picks up considerably more stations including one over 100mi away right at 1050. So I'll be grabbing another osc and have 2 L2's using jumpers on the final board, because why not. Also on both radios (without my xmitter powered) there is an audible alternating frequency right around 1050, I get a similar sound at 600 but much louder.



Also realized most my ceramic cap values were incorrect on my previous bread board with the .1 being .01 and the .01's being .001. Currently I don't have a 200v .01, using a .001. I'm guessing a 50v .01 wouldn't cut it for C5?

Grabbed a handful of these air caps for not much more than the one I got from digikey. They make it super easy to tune the antenna even though my LED doesn't really change. I couldn't find one without a set voltage (unregulated?) so against previous advice I got a 12v one. Perhaps someone could link a meter that would work with this setup.

Where would a guy put a power switch in this set up? I'd guess on the 12v side but would that leave the 120v side of the transformer "hot" all the time?

Dan

Dan; if the LED does not peak the inductor is not in the right range for the frequency or length of antenna wire. How long is your wire? You should see a brightness peak when the inductor is the correct size. If you have a choice use an inductor that requires the least amount of capacitance to peak the antenna.

If you want to be certain there is no power you can use a DPST or DPDT switch and disconnect both hot and neutral. I would not put it on the 12v side. Lowes has the DPDT for about $5.

Not sure about your 12v question. If you are concerned about the voltage just add in extra LEDs. They will drop 1.5 to 2.0 volts each depending on their specs.

Thanks for the info Rick. I like you double pole suggestion and will use it. I hadn't thought of it, would basically make the power cord an extension of an outlet with nothing plugged in when switched off. I was focused on single pole I guess.

I have tried a few inductors combinations of the max and min on the schematic including 330 and 390 for L2 (1mhz osc) and 150 to 680 for L1. None of which required any notable change in the air cap. The antenna is 4ft-ish. Also the air cap is 5-100pf, so good range. After checking the LED data sheet it has a built in resistor and appears to be at its maximum brightness at 10mA (12v). I have another LED on the way after deciding I don't want to mess with a meter, simpler is better. The data sheet makes no mention of a resistor and the specs make it appear as if it will vary with current. I feel confident it was peaked, the signal faded smoothly in either direction on the cap from peaked and given its position it's requiring just over 50% of the caps range which would be expected with the antenna length (from what I've read here). Also am turning another station (just over 100mi away) into a barely noticeable hum.


Tom, I was able to get this sounding very good on both my radios yesterday. Even with another station at the same frequency and expect it to sound even better when I get it on a another frequency, on a board and in a case, which I'm finally working on. Thanks!

Dan

Update. I have been using these voltage regulators because I can use a cheap 12v wall wart and still get a solid stable and adjustabe voltage. Since I seldom throw away a wall wart I have plenty in my stash.

https://www.amazon.com/gp/product/B01GJ0SC2C/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

Because they are adjustable you can set the voltage to the 386 exactly. I think I came to the same conclusion as Tom; about 8.6v seems to be a sweet spot. Much above or below that starts to affect modulation and/or distortion. I have noticed no noise or rf from this module and have even mounted it inside the case with a transmitter.

Cheers.

I agree 8.6V seems good, since the oscillator (pin5) sees only half that voltage.
I have a couple DC-DC but just use a 7810 with 2 diodes.
Also, my little `386 with player works great on a 9V battery. Without an antenna (but with a small cap in it's place) I can set it next to, or within 3' of a radio.

Guys! Finally got my first transmitter done! I'm amazed by the performance of the circuit and beyond pleased with the final result. Even through my phone with no audio processing it sounds better than any of the stations around here and sounds even better on the computer. learned A LOT, so figured I'd share for others starting where I was.

Running it on 6 AA batteries (9v). The LED is running on the 9V. Has a 220uh and 330uh L2 and works with a huge range of antenna sizes. I could have used just a 270uh with the 5-100pf tuning cap but antenna length was limited to about 6ft max at 1228 MHz and 2ft minimum at 1000 MHz while still being able to peak the antenna. A 40in antenna was perfect for a 270uh L2 peaking at about 33% and 66% of the capacitance range for 1228 MHz and 1000 MHz. Grounding the audio jacks not only to the circuit but to earth too made the biggest difference in audio quality for me. Though for my location battery power helped a lot too. Added a DC input jack because I had one and the switch had the extra connectors, so OFF on batteries is also DC inputs ON. So it can't turn off if plugged in, no LED on DC either.











Dan

Looking great Dan. Nice finished look in the case.

So is your LED peaking now with the inductor / capacitor / antenna combination you are using?

Looks good Dan, very tidy. I was wondering why you needed a separate supply for the LED. How is it wired?

Colin

Thanks guys.

Rick, the replacement LED I got exploded when I hooked it up to a 9v. The top half blew off with an incredible noise and force, I'll link it. So I just used the 12v ones I had, I'm happy with the brightness but never did get it to vary at all (at 10v). Not sure what happened with the replacement, you could see into it, it didn't have any built in resistor.

https://www.digikey.com/product-detail/en/dialight/5219270F/350-2682-ND/2429372

Colin, I decided to go that way with the LED since I couldn't get my specific one to change brightness in the circuit along with deciding to run off batteries. Some of it can down to battery holder size and available configurations. Some of it was about maximizing battery life. I was going to forgo it all together, but like the DC jack the switch had the extra connectors for it so decided to hook it up as a pilot light isolated from the Xmitter, in case I were to turn something down and forget it was on. Here's how it's wired if you can decipher it, it's a 3PDT switch. The squiggly lines are wires, all but the one prong is used.



Dan

Thanks Dan. That explains it.

I took Mike's tip and found a clear plastic box and put together my "travel" transmitter. Perfect for taking to a show to demonstrate radios working. Went with the 9v battery external so I can use any supply with a connector. iPhone feeds the 3.5 mm stereo jack and 3 meter antenna connects with a banana plug or just wire it to binding post. BTW; I really like the board I am using. It is called the "Permaboard" and is sold by Adafruit. High quality epoxy / glass board with all holes plated through and then plated in gold. Makes soldering a breeze.

Nice work! What's your source for the case?

Electronics Goldmine

Upgraded my antenna setup. Sounds the same as a wire as far as I can tell, much more appealing and manageable though. Doesn't make a difference if its extended or not for my purposes.
I thought I had good sound quality before, I have since found the one corner in my house that gives crystal clear sound, when I turn the gain down on the Tx down the radio is dead silent.
I'm not using my external ground that was hooked to the audio jacks. That was actually adding a very steady hum that was covering up some interference caused by location, I didn't notice until I turned the volume up a bit. Where I set my phone seems to be equally important as antenna location. Yep, that is from a ZTO, one beyond salvation unfortunately. Only other thing I'm going to do to this one is add some RFI shielding tape to the inside of the case, see what kind of difference it makes in noisier locations. I calculate the battery life for the Tx part to be well over 400 Hrs, possibly up to 500. Though when I apply the same calculations to the LED I get 45 Hrs max.

That is due to the audio jacks being grounded to the circuit ground. Had a similar issue and extra hum with my two tube transmitter. I solved it by using a 1/4" insulated jack and an Edcor WSM 15K/15K balanced/unbalanced to balanced/unbalanced transformer to isolate the circuit ground from the input ground.