transmitter 9v to 3v

i would like to make this transmitter work off of 3 volts instead of 9 volts because i have been told by a couple of people that a 9 volt battery won't work nearly as long as would two AA's.

I lowered R7 to 100 ohm, but all that happened was i heard a clicking noise on the radio when i applied 3 volts. also, would anything need to be done to the audio portion of the transmitter so as to keep its sensitivity?

Thanx very much :lol:

why not use six AA's ..
i suspect that the whole circuit would have to be redesigned to operate lower than 9 volts..

Maybe make all the resistors 1/3 of the 9v value?

why not use six AA's ..

Click to expand...

cuz then it wouldnt be a "bug" it would be a monstrosity :lol:

lol i guess it would .. :lol:

Been doin a bit of clandestine work and came up w/ this circuit here.
Will this give about the same range as my 9v transmitter?

youre right theone, a lot of the resistors are around 1/3 value :lol:

Hi Zach,
If your original transmitter was properly designed, you would be able to simply divide some resistor values by 3.
For example, the base voltage of Q1 must be at least 0.7V. R2 and R3 form an 11:1 voltage divider that provides Q1's base with only 0.82V with a brand new 9V battery and actually less, since Q1's base current will load it down and reduce it.
When the battery's voltage drops to 6V, the base voltage will be only 0.55V or less and Q1 won't work! Will it work with a 9V Ni-Cad rechargeable battery that actually produces only 7.2V? Probably not. Poor design.
If you use resistors with 1/3 the value for R2 and R3, the voltage divider will be the same. It won't work.

In your new transmitter circuit, the voltage for Q1's base is provided by feedack by R2, so it remains fairly stable with supply voltage changes.
Its emitter resistor for the oscillator is 1/3 the value of the 9V transmitter so its Q2 current will be the same at 3V and it will have the same range.

i made the 3v circuit and it works wonderfully. its really weird tho, if i put in that .001uf supply decoupling cap, the transmitter sounds like a tea kettle. think i got a bad cap?

Hi Zack,
Teakettle sound? Ha, ha. Are any of the other parts British?

Maybe the circuit is designed on the hairy edge and therefore goes crazy with a good solid RF supply. Try adding a 10uF to 100uF cap too, for audio frequency bypassing.

haaa haa. just got a chance to fiddle with it a little more and i just found out that the sensitivity is TERRIBLE!! This thing is meant to be able to pick up someone talking across the room, right? cuz all i can get someone putting their mouth up to it and talking. ill try that 10 or 100uf cap. none of my other transmitter circuits needed it tho.

Hi Zach,
The mic preamp, Q1, has negative feedback which cancels its gain. It needs the negative feedback for DC, not AC.
You can easily change it so that it still has DC feedback but no AC feedback. Then it will have lots of gain, almost as much as the other transmitter.

Remove R2, the feedback resistor and replace it with two 47K resistors in series. The DC feedback will be nearly the same as it was before.
Add a 22uF capacitor from the junction of those new resistors to ground. Now it will have no AC negative feedback.

been fiddlin around with the 3v transmitter some more. I don't have room to do what you recommended audioguru (next time).

the range on it isn't very good. I did squeeze a little more out of it though by adjusting C5 from 6pf to 13pf (the 6pf that was already there in parallel w/ a 7pf cap). Is there any way of getting something more out of this thing?

Ill also have to try lowring the value of R5.

Thx :wink:

Hi Zach,
To get more range, lower the value of R5 until its oscillation stops, because C5 can no longer provide feedback into it, or until Q2 melts.
Maybe you need an RF amplifier to follow the oscillator like the illegal 3-transistor transmitter circuits.

oooooh, 3 transistor . . . illegal :lol: . wow, i have wronged the FCC without even knowing it :lol: .

an amplifier transistor . . . ive always wondered this: Wouldn't that make a circuit such as mine lose efficiency? Cuz all the energy that would normally go to the antenna is going into a transistor and then ground. How does that work?

Hi Zach
An amplifier takes a small input signal and makes it bigger at its output. It can be designed to amplify voltage, current or both which makes more power.
You limit the transistor's input current with a series resistor so that it doesn't load-down the source. The series resistor could simply be the collector resistor of a preceding stage.
I'm not certain, but I think the limit is 100mW in the USA. Your transmitter produces only 10mW if you're lucky. An amplifier transistor would take the 10mW as its input and output 100mW or 300mW.
You won't lose efficiency, but you probably will gain, because at RF, you can bias the transistor to be OFF most of the time, drawing no current (very efficient) then it amplifies only at the peak of the input cycle. A tuned circuit or filter at its output smooths-out the resulting harmonics. An amplifier that is operating this way is operating in "class-C".

A guy made a 300mW illegal 3-transistor transmitter. With the FCC (who were called by a complainer) looking over his shoulder with their power meter, he kept clipping his transmitting antenna shorter and shorter until he was operating just barely legally.
I don't know if he had to bribe them for them to say, "Just a warning this time".

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

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:
https://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:

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?