3v transmitter questions

yup, im still working on transmitters :lol: . i perfected my 9v transmitter, but then found out the battery life is crap compared to a 3v transmitter . . . so, now im onto the good 3v transmitter . . . and ive got my questions.

I am doing nothing different from the schematic accept for using 2n3904's and using a 8pf cap instead of 5.6pf cap for C5. One thing that must be known is that i have the circuit assembled on a piece of cardboard with long leads.

1. It transmits beautifuly. The thing is, when i use the .022uf decoupling cap, the thing starts sounding like a fly over the radio. I tried using a 100uf cap to take this away, but it only added to the problem.

2. Are there any simple improvements that i could make on this circuit to improve range, etc.?
Thanx much :lol:

The long leads are probably causing it to oscillate at strange frequencies. The easiest way to increase range is to raise the voltage. Matching to a good antenna is the 2nd best way.

aw CRAP!!! i forgot to post a picture of the circuit :evil: :evil:

ok, now that ive got the picture posted, here's my questions:

1. Why is the decoupling cap making the thing sound like a fly!? i thought it was supposed to suppress that!

2. Does having C6 there inhibit the range @ all? i don't mind taking it out even if it makes the frequency a bit unstable.

3. How long would you suppose this thing would last on two AA batt's? Should i use a voltage regulator or anything like that?

Thanx very much. when i get these questions answered i am going to make my very first PCB :lol:

looks like you got problem with feedback.
try adding small capacitor like 2n2 from base to emiter of Q1
and of course try to make everything nice and short...

panic mode said:

looks like you got problem with feedback.
try adding small capacitor like 2n2 from base to emiter of Q1
and of course try to make everything nice and short...

Click to expand...

how would that help??? has this got something to do with miller capacitance?? just asking???

i tried that panic, and it didn't do a thing

i'd be very greatful if somoene would answer my other 2 questions:
1. How long will this circuit last on two AA batteries? should i use a voltage regulator or anything?

2. Does that 27pf cap in front of the aerial impede the range?

EDIT: i posted this right before you did panic :lol:
panic, i forgot to mention that this does result in harmonics ALL over the place. All over the dial, even if i walk a good distance away from the transmitter.

this is low pas filter which acts as short circuit for high freq signals.
it's supposed to remove any unwanted signal before it is amplified by Q1.
feedback signals casue unwanted oscillation which can cause effects as described above. to high gain also causes clipping of the signal which
results in harmonics all over the place...

I would break the circuit in 2 parts. First power only the last RF part to make sure that the feedback problem is not with the RF part. Check that you can receive a clean carrier. If that is OK, the problem is most likely in the first AF part or feedback between the 2 stages. It is often useful to put some decoupling in the power rail between the RF and AF stage. Small resistor, say 100Ohm with a 0.1uF cap to GND (-ve) rail on the AF side.

Hi Zach,
Answers:
1) The length of time the AA battery cells will last is depending on the current that is drawn from them and how low you allow the voltage to drop before replacing them. If you use a voltage regulator to keep the range and mis-tuning constant, it will have reduced range because the regulated voltage will always be minimum.

2) You can get more range by reducing the value of R5 which will allow more current for Q2. Don't use less than about 100 ohms. Of course the battery's life will be reduced.

3) Use C6 which doesn't do any harm. It may help the transmitter by reducing the pickup at its antenna of strong lower frequency signals.

4) The "harmonics" that you have all over the FM band I think are caused by receiver overload, which mixes your transmitter's signal with all other signals. If the transmitter is operating at 100MHz, the first harmonic (called the 2nd harmonic) is 200MHz, the 3rd harmonic is 300MHz, etc.
"Harmonics" could also be caused by your long wires around Q2's circuit picking-up all other signals and modulating Q2 with them. RF circuits must have extremely short wires and a solid ground (a ground plane on the other side of the circuit board is preferred).

5) To eliminate the "fly" noise, the resistor to feed filtered power to Q1 is a good idea but the filter cap should be big enough to filter audio frequencies. Use about 100uF in parallel with a 0.01uF ceramic cap to filter RF frequencies. Of course you are using extremely short wires to your mic or a shielded cable to the mic, aren't you?

hi audio, thanx.

You can get more range by reducing the value of R5 which will allow more current for Q2. Don't use less than about 100 ohms. Of course the battery's life will be reduced.

Click to expand...

strange thing. when i upped the resistance from 100 to 470 ohms, i actually got beter range dont ask me why :roll: .

i don't think that you understand that the "fly" and the "harmonics" is one in the same. They only occur when i put any type of decoupling cap into the circuit. I get a fly sound all over the radio, even if i walk 30 feet away.

i am testing how long this circuit will last on two AAs right now. its been on for an hour so far. this thing is amazingly stable, its only drifted about 1MHZ, maybe not even that! Of course this is without any kind of decoupling capacitors. I'm thinking about just leaving my circuit without the decoupling caps, depending on how it performs when the voltage gets low. If i don't need them, that will be great cuz my bug will be even smaller.

yeah, the mic has very short leads.

Depending on the test that i am running on it right now, i might try your suggestion theone.

Thanx everyone!! :lol:

Hi Zach,
Ha, I can see you sitting there counting the hours of battery life. Watch what you say, it will be transmitted. According to Energizer's datasheet for their e2 AA alkaline cells, if your transmitter draws 10mA (just guessing) it will keep going for about 1 week! Its frequency will keep drifting and its range will get less until you decide it is too low.

I estimate that a single AA NiCd would last 500 hours. You don't need C6 unless the antenna wire is long but I suspect your antenna is capacitive anyway (short). C6 provides some isolation so that the transmitter is not detuned by people walking by.

Q1 could be an oscillator, like Q2 because of the feedback thru C2 and C3 (compare to C5). Try connecting C3 to the positive rail. Do you have C7 in the circuit? What is the value of the resistor between R3 and R4?

Hi Russ,
How can you get 500 hours from a 500mA/hr Ni-Cad? I think the transmitter draws much more than 1mA, and besides needs 3V, not just 1.2V.

WOW!! A WHOLE WEEK!! i dont think that the frequency will drift any. ive had the transmitter on for 7 hours so far and it hasn't drifted one kilhohertz, even when i turned it on, it drifted a tiiiiiny bit, and this radio is an old clock radio, so i doubt there's any phase locked loop in it.

the cap in front of the antenna does a lot of good. i can practiclly touch it without it getting messed up.

and audio, u hit it right on the head. i think this circuit draws exactly 10ma. im not sure because my multimeter is broken. the only amperage reading that i can get is when i turn it to the 10A setting, and when i do, i get .01

now i just gotta get a better receiver :lol:

EDIT: how hard would it be to put a third transistor on there to make a three transistor transmitter?

Hi Zach,
Your clock radio probably has "AFC" as they say, "to prevent the radio stations from drifting".
Sure your transmitter's frequency will change as the battery runs down. The capacitance of Q2 changes when the voltage across it changes, that's how it does FM. If you want its frequency to be much more stable and the battery to last a lot longer, use Energizer's AA lithium cells. Ha-ha, I got free samples.

Haven't we posted many 3-transistor "almost illegal" transmitters? On most, adding a 3rd transistor is simple.

audioguru said:

Haven't we posted many 3-transistor "almost illegal" transmitters? On most, adding a 3rd transistor is simple.

Click to expand...

But will drastically reduce your battery life!.

But will drastically reduce your battery life!.

Click to expand...

audio, i thought u sed that since the third transistor is a C-class amplifier, it would barely do anything to the battery life (or am i making things up?).

well, i would like to know how to put a third transistor on there. I tried taking the third transistor part from a 9v circuit and slashing the resistor values 1/3, but it didn't work. how would i put this third stage on mine?

if wut u said audio is true, and the transmitter will run for about a week, i wouldn't mind if the battery life gets slashed in half :lol:

thanx :lol:

The guys I bought my Stereo Transmitter from have a 3 transistor DIY design on their site that they claim has very good performance.

**broken link removed**

zachtheterrible said:

But will drastically reduce your battery life!.

Click to expand...

audio, i thought u sed that since the third transistor is a C-class amplifier, it would barely do anything to the battery life (or am i making things up?).

Click to expand...

Sorry, but magic isn't working this week :lol:

If you increase your output power, you increase your power consumption accordingly - it doesn't matter much what class of amplifier it is, the same rules apply.

well, i would like to know how to put a third transistor on there. I tried taking the third transistor part from a 9v circuit and slashing the resistor values 1/3, but it didn't work. how would i put this third stage on mine?

if wut u said audio is true, and the transmitter will run for about a week, i wouldn't mind if the battery life gets slashed in half :lol:

Click to expand...

If an extra transistor would only double your output power it's not worth adding!, you should be looking at least ten times the power output (and only 1/10th the battery life) and giving about double the range.

Hi Zach,
For class-C, the transistor is turned-off most of the time then conducts heavily (using lots of battery current) for a short pulse then its tuned load swings much higher than the supply voltage. It uses lots of current because the pulse happens 100 million times per second. Class-C is more efficient and has more output than the class-A circuit in The One's DIY article. That circuit uses diodes as voltage-variable capacitors for true FM, unlike your transmitter that makes AM and FM at the same time.

It takes a lot of power to double the range because your antenna isn't directional (like NASA's very directional dishes) so your signal spreads over a complete half-circle (including upward if the antenna is not exactly vertical).