well, i got the range that i had w/ my 9v transmitter w/ the 100 ohm resistor, but the thing is, at first, the sound is real nice and clear, but after a while, it starts clicking, and thats all it will do. i can receive the clicking a far distance off. Also, i am getting a signal around 96 and 107 MHZ (remember thats only a guess) no matter how far i am

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Hi Zach,
I just realised that the oscillator's base bias resistor of your transmitters (R6 = 10K in the 9V one, and R4 = 4.7K in the 3V one) have a value that is much too low. This will cause the oscillator to be saturated most of the time creating all kinds of phantom signals and reducing their range.
Now that you have a 100 ohm emitter resistor on your 3V transmitter, change its R4 to 27K. It should work much better.
Again, 100uF across the battery might stop those clicks.

 

Hi Zach,
I discovered an RF engineer's ramblings about FM transmitter kits, including Harry's. See "Where do I begin", about half-way down. He talks about range improvement from a legal 3rd transistor and detailed measurements of frequency shift due to temperature change (including warm-up when it is first turned-on), simply holding the circuit without a shielding metal box, loosely folding its antenna and due to the battery running down.

There are other links including discussion of an FM stereo transmitter IC that's obsolete now, that worked very poorly (many kits used it).

The site is a few years old so some important links don't work, like "How far will my signal go". The site is here:
http://members.tripod.com/~transmitters/

 

Hi audioguru. Words cannot express how grateful I am for ur giving me all of the help that you have with all of my projects, thank you.

Thanx for that website, that was pretty cool.

I couldn’t find a 27k resistor, so I got a 22k and slapped it in there. No change

I found out that the clicking was not from my transmitter, but from somewhere else. The reason I was hearing it when I had my transmitter on, and not hearing it when it was off was because the two signals must have been mixing or something.

I’m going to look into just how much longer those two AA’s will last compared w/ the 9v and see if the longer battery life is worth the headache. :evil:

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Wow!! what a difference!! AA=2850mah, 9v=625mah, among other figures where the AA lasts much longer.

Well, i do want to go w/ AA, so what is my next move?

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Hi Zach,
I'm glad to help you. I've been there and done that a long time ago and it is fun to refresh.
It is good that you see the big difference in battery life. Reconsider using only two AA battery cells. Their voltage will drop to 2V and your circuits won't work. Three AA cells will start at a little more than 4.5V and end at 3V.

You can use your un-needed 9V batteries on my Stethoscope project here:
http://www.electronics-lab.com/proje...019/index.html

I've just finished another low battery voltage project and are making them for family and friends:
http://www.electronics-lab.com/proje...003/index.html

Along with another low battery voltage project that I posted:
http://www.electronics-lab.com/proje...018/index.html

 

those are cool projects audioguru :lol:

i made the mistake of making this circuit on veroboard and then hacksawing it off, so now i cant really experiment w/ it (i was sure it would work the first time). but now i know the first rule of electronics (4 me neway) it almost never works the first time (again, thats 4 me :lol: ). ive never had a circuit work the first time. It might be a while b4 i build it again.

oh yeah, is that 27k instead of 22k alright?

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Thanks Zach,
Most of my designs work fine every time. Yours will too after a little more experience. I always use Veroboard for my prototypes and sometimes must change a resistor value or add one to make it work even better. Luckily I have many 1/10W resistors from answering machines and clock radios that were killed by lightning. Those tiny resistors can be sqeezed in and 2 put in series to make a 1/4W value that I've run out of.
On my last project I made the circuit first, then took it to the store to match a box to it. Mistake! It didn't quite fit into a nice box. So I re-layouted the Veroboard for my next assembly.

22K or 27K, it won't make much difference (only 1.3 times). They are both nearly 5 times higher than the original 4.7K and the variations (current gain, etc.) between samples of the same transistor number are much higher than only 1.3 times anyway.

 

hello again :lol:

Duz overdriving an oscillator limit the range?

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Hi Zach,
A simple oscillator has its drive uncontrolled. An oscillator oscillates continuousy because it has gain (amplification) and positive feedback. It takes some of its output back to its input, amplifies it, gives that larger signal back to its input, amplifies it even more, etc. It can't amplify more when its transistor "hits the supply rails" when it is saturated (turned-on hard) for a half-cycle and cutoff (not conducting) for the other half cycle. Its output is the largest when the saturation and cutoff timing are symmetrical. Bias for the transistor's base (with 4.7K it is saturated most of the time, with 27K it is more symmetrical) determines its output symmetry.
However, your oscillator has a tuned LC network as its load, so its collector voltage can actually exceed the supply voltage when the transistor is cutoff. Just like a child on a swing. Give a little push (the transistor conducts a small amount) and the swing swings like a pendulum (the tuned LC network also swings) until it is back at the position that you pushed it. Give it another little push and it swings higher. The tuned LC network does too. If the transistor is an RF one with a high gain at FM frequencies, it could be operated in "class-C" where it is cutoff most of the time but has a very large output. Your transistor is a "general-purpose" audio transistor that has hardly any gain at FM frequencies so it can't take advantage of operating in class-C.

 

really now. what type of transistor do you suggest i get then?

Thanx for the explanation, that helped :lol:

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Hi Zach,
I'm not an RF engineer, but I could probably find a good RF transistor. It wouldn't make any difference in your oscillator. An oscillator would need to be designed for it to take advantage of its good RF performance.

It would be a lot easier just to add an RF amplifier made from a "general purpose" transistor.

 

alrighty than :lol:

i just finished assembling the circuit on cardboard so it is really easy to take parts in n out of it. ill let you know how it goes :lol:

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Hi Zach,
What do you use for soldering and desoldering tools? Mine makes moving parts easy.

I have and recommend a Weller temperature-controlled soldering iron. Mine is the simple inexpensive "mechanical" controlled one, and I used the same at work. I found mine in the trash at my very 1st job about 40 years ago and it still works fine. When I found it, its mechanical switch was stuck so I replaced it. It has a good cord (teflon that doesn't burn) but I had to supply my own 24V transformer. I use a 700 degree tip that seems to last forever. I never have to "tin" the tip. With it I always use a damp sponge to clean the tip frequently. The sponge-holder is Weller too.

I also use a good solder-sucker. Mine has a sturdy but lightweight aluminum barrel and a conductive teflon small nozzle. My thumb quickly latches it and when I push its button the solder is gone. It is also nearly 40 years old. I clean it out and lube it every couple of years but have never changed its "O" rings. I don't like using the cheap and huge plastic (static problem?) solder suckers nor that Dry-Wick braided stuff.

Are you going to make a high-power transmitter? Keep looking over your shoulder for the FCC and stay away from a little above 108MHZ or the FAA will visit too!

 

I have a really nice xytronics temperature controlled soldering station (reeealy nice :lol: ). a friend of mine gave it to me because he didn't need it, along with a bunch of other nice stuff. ill have to look into getting a solder sucker, i always just use the braid stuff, works fine.

oh yeah im gonna make a high power transmitter one of these days. gotta walk before i run though in making this little bug. i thought i had it a little while ago w/ the 9v transmitter which works fine, but then you told me about AA's lasting MUCH longer than a 9v batery, so i am getting closer to finally having a bug that i will be satisfied w/.

FAA, federal aeronautics agency?

btw audioguru, do you have a website?

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