1.5 Volt Tracking Transmitter

[Bogidar]

So now I've got a 2N3904, a 2N3906, a 2N2904, a mystery transistor that is marked SPS 2003 M 724 and it has been very convincingly argued that I replace the BC547 with a 2N2222. Can anyone suggest which combination would work best. This is my first circuit. I can feel I'm getting close.

 

[audioguru]

A 2N3906 is a PNP transistor and can be used to replace the BC557 PNP transistor. Its pins are the opposite so look at the photos in their datasheets.
A 2N3904 NPN transistor is used in hundreds of FM transmitter circuits on the internet.

 

[forumlicker007]

Bodigar, you're not just getting close..you've done it according to me
Replace 2n3904 for BC547 and 3906 for 557. Like uncle scrooge told I've also seen thousands of FM X'mitters using 2n3904. Even my prototypes also used this very reliable transistor quite a number of times. So start building it and let us know. I've homeworked pinouts for you.
I've a collection of old metal can 2n2222 that're breadboarded by my grandpa. Now I'm using modern 2n4401 for the same ratings.

 

[audioguru]

Notice that the pins on the 2N3904 and 2n3906 are the opposite (like a mirror image) of the BC547 and BC557 transistors.

 

[duffy]

And when you discover it doesn't work worth a damn, swap out the crappy 2N3904 for a 2N2222.

 

[audioguru]

And when you discover it doesn't work worth a damn, swap out the crappy 2N3904 for a 2N2222.

Of course it won't work worth a damn. It is a simple crappy circuit.
The transistor's part number has little to do with improving it.

 

[duffy]

Simple crappy circuit, yes, but I've been friends with those transistors for thirty years and know their personalities. Forrest Mims introduced us. I employed them both in this circuit on a few occasions, wrapped wire around a pencil to make the coil, twisted two bits for wire together for the "gimmick" tunable capacitor (ever do that?).

 

[audioguru]

Hi Duffy,
Haven't you seen my FM transmitter? It works very well and sounds great.

 

[duffy]

Yes, very nice, I like the RF amp, I like the voltage regulator (pulling double-duty as an RFC).

I question your use of the term "Miniature", however. All that plus a honkin' 9V battery? Not going to impress 007 when he shows up at Q division.

 

[audioguru]

My FM transmitter has a range of across the street to a cheap scanning radio from The Dollar Store.
Its range is down the street to my cheap Sony Walkman radio.
Its range is over 2km across a river valley to my very sensitive home stereo and car radio.

It draws 53mA from the 9V battery that drops to about 8V in one hour.Three lithium coin cells are smaller and might power it for a few minutes.

The name "miniature" was on the original project that I fixed. It didn't work.

 

[forumlicker007]

I think bodigar is now directionally moving behind in the dark to catch someone..

 

[duffy]

Well then post vids please, Bogidar - freaky green nightvision with a voiceover doing play by play, thanks.

"Now she's getting out of the car... I think I can see...
yeah, that's her, and she's standing...
no, she's meeting up with him...
oh my God, is that a BLACK GUY???"

By the way, my experience with these things is that they are always bad juju. Always. If your relationship wasn't bad before, they will become cursed afterwards.

 

[forumlicker007]

No doubt, a worthy gadget for alphadog.

 

[Bogidar]

No doubt something like this could end up being "bad juju". Sorry to disapoint but there's no night-vision footage of my lady and here in Canada pot plants aren't so rare or precious that anyone would bother tracking them. ;^)

 

[andrebc]

My FM transmitter has a range of across the street to a cheap scanning radio from The Dollar Store.
Its range is down the street to my cheap Sony Walkman radio.
Its range is over 2km across a river valley to my very sensitive home stereo and car radio.

It draws 53mA from the 9V battery that drops to about 8V in one hour.Three lithium coin cells are smaller and might power it for a few minutes.

The name "miniature" was on the original project that I fixed. It didn't work.

Audioguro:Can I ask you,? What does your C2 capacitor doing, to me for certain frequencies it will short the tranistor and makes VBE=0 instead of 0.7V, but what's the value of that capacitor and what frequencies you had to block( of course this has to do with the value of your C2). Thanks

 

[audioguru]

Audioguro:Can I ask you,? What does your C2 capacitor doing, to me for certain frequencies it will short the tranistor and makes VBE=0 instead of 0.7V, but what's the value of that capacitor and what frequencies you had to block( of course this has to do with the value of your C2).

C2 is only 100pF. It is a capacitor so it does not short the base to the emitter at DC, it stops the preamp transistor from amplifying the RF from the output of the transmitter. 100pF has a reactance of 80k ohms at 20kHz so it has no effect on the signal from the microphone. At 100MHz its reactance is only 16 ohms.

 

[andrebc]

would not be the same idea if you use C2 a coupling capacitor before the base of the transistor, that way you certainly will block any DC from coming in.

 

[Roff]

would not be the same idea if you use C2 a coupling capacitor before the base of the transistor, that way you certainly will block any DC from coming in.

C1 blocks DC from the microphone. As Audioguru said, C2 kills the gain of Q1 at high frequencies, to prevent the RF output signal from screwing up the mic preamp.

 

[andrebc]

C2 is only 100pF. It is a capacitor so it does not short the base to the emitter at DC, it stops the preamp transistor from amplifying the RF from the output of the transmitter. 100pF has a reactance of 80k ohms at 20kHz so it has no effect on the signal from the microphone. At 100MHz its reactance is only 16 ohms.

Thanks for the reply, I got this. why there is no coupling capacitor before C2, and what does that LM IC doing to your transmitter with the capacitors in parallel?

 

[audioguru]

Why there is no coupling capacitor before C2?

C2 reduces the gain of the preamp transistor at radio frequencies. C1 is the coupling capacitor that couples the signal from the mic to the preamp transistor.

What does that LM IC doing to your transmitter with the capacitors in parallel?[/quote]
The LM2931A-5.0 is a low dropout 5V voltage regulator. Unlike an ordinary 7805 5V regulator that has a minimum input voltage of 7V, the LM2931 has a minimum input voltage of only 5.2V so the 9V battery will power it for a long time as it runs down. The 1000pF capacitors are for RF bypassing and the 100uF capacitors are for audio bypassing. The LM2931 must have a 100uF capacitor on its output for stability. The regulator keeps the oscillator from changing frequency as the battery voltage runs down and also keeps the preamp transistor biased correctly.