Audioguru's FM Transmitters

[audioguru]

Thank you Audioguru! I made 2 quantity of your MOD4. One with regulator, which cost a lot here in Pakistan about 75 rupees, or equal to 90US Pennies.... it works perfect with mic.

90 US pennies is cheap for Canadians. It is almost the same as one litre of gasoline.

Another one I made without regulator, because it's too expensive, but I can't get audio from mic.

I designed the biasing of Q1 the mic preamp for a regulated 4.75V to 5.25V supply. If the voltage is lower then Q1 is cutoff and if the voltage is higher then Q1 is saturated. it is also very important for Q2 the FM oscillator to have a regulated supply so that the frequency does not change as the battery voltage runs down.

Can you please recommend any suggestion about my second unregulated transmitter! I'm operating it on 3 volt DC, 2 AA Cells......

My transmitter was designed for a 9V battery and a 5V low-dropout regulator. You can re-design it for 3V (that drops to 2V) if you want.

 

[k7420]

I can also buy a litter of gas here in PK about 68 Rupees, lol.... But I don't know what things can be changed to re-design it for 3 volt AA cells.....

And can i power up my first regulated transmitter to a 12 volt car battery? it's has a 3 volt deference compared to a 9 volt battery...

 

[k7420]

I'm sorry, I forgot to let you know that I couldn't find "R2-160K" I put 150K resistor and "R3-30K" I put a 33K resistor.... Do you think it's the problem which is causing Mic stage not to work?

 

[k7420]

Okay, I just removed Regulator from my first perfect transmitter and installed its connection to my second transmitter, and supplied the 6 volt DC power, it's working magically, and sound quality is good too, and has a good sensitivity, I can hear it on other room clearly.......

I understand that your circuit need 5 volt regulated supply, but I will be using this transmitter for a 2.4 volt rechargeable AA Cells. Would you please let me know what regulator do I need to regulate it at 1.8 volt dc, and what parts needs to be changed in transmitter? Thank you....

 

[k7420]

This is my second MOD4 transmitter I'm trying to operate at 2.4 volt DC....



Looks pretty decent...



Isn't it!!!
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Okay This one is my First transmitter it works perfect with regulator... at 6 volt DC....


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My Work place... lol really small one....

 

[audioguru]

I don't know what things can be changed to re-design it for 3 volt AA cells.....

Changing the voltage also changes the currents. Then nearly every resistor will need to be re-calculated and its value changed.

can i power up my first regulated transmitter to a 12 volt car battery?

Like you said before, then maybe Q3 will get too hot. Also, maybe the 5V regulator will get too hot but it can be replaced with an ordinary 7805.

I forgot to let you know that I couldn't find "R2-160K" I put 150K resistor and "R3-30K" I put a 33K resistor.... Do you think it's the problem which is causing Mic stage not to work?

Yes.
I designed my FM transmitter for The West where ordinary 160k and 30k 5% resistors are available everywhere. Yuor transistor is saturated with its collector voltage as low as it can go and it will amplify loud sounds with extreme distortion.

I just removed Regulator from my first perfect transmitter and installed its connection to my second transmitter, and supplied the 6 volt DC power, it's working magically, and sound quality is good too, and has a good sensitivity, I can hear it on other room clearly.......

Of course, that is the way it was designed.

I will be using this transmitter for a 2.4 volt rechargeable AA Cells. Would you please let me know what regulator do I need to regulate it at 1.8 volt dc, and what parts needs to be changed in transmitter?

Then you need a low-dropout 1.8V regulator but I don't know one.
Nearly all resistors will need to be re-calculated and replaced with the new values. I will not do it.
The range will be low and the audio will probably be distorted most of the time with such a low supply voltage.

Your circuit board is neat and tidy.

 

[k7420]

Thanks, I'm gotta play with it at 6 volt DC battery....

 

[audioguru]

I used a low-dropout 5V regulator that still regulates perfectly when the 9V battery drops to 5.5V.
A 7805 or 78L05 regulator fails to regulate when the 9V battery drops to 7.5V but the battery is still fairly new and is still powerful.

A 6V alkaline battery drops to 3.2V or less during its life. Its voltage drops to 4.8V quickly then slowly drops the rest.
A 6V battery made with five Ni-MH cells is about 7.5V when fully charged and is 4.5V when it needs to be recharged.

 

[mvs sarma]

Can we try out a fixed low voltage rail that could be used for biasing. perhaps you remember this method as used in pocket radios working with a single AA dry cell. it might work well over a wide range of 9V battery.

 

[k7420]

Audioguru, looks like it's a complete copy of your FM transmitter, but it's designed to work at 3 volt DC.

I'm eager to know should I put a coil and a trimmer at the RF output stage, and remove the 330R resistor?

**broken link removed**

 

[audioguru]

Audioguru, looks like it's a complete copy of your FM transmitter, but it's designed to work at 3 volt DC.

It is completely different:
1) Its output power is low so its range is only as far as you can talk.
2) Its oscillator does not have a voltage regulator so the radio frequency changes as the battery voltage runs down.
3) It does not have pre-emphasis (treble boost which is the opposite of the de-emphasis [treble cut] in all FM radios) so it will sound muffled. In fact, the 1nF capacitor at the base of the oscillator transistor cuts high audio frequencies instead of boosting them.

I'm eager to know should I put a coil and a trimmer at the RF output stage, and remove the 330R resistor?

The 150k base resistor results in a low current which makes low output power. Reduce its value to 15k or 16k and add the coil and capacitor.

 

[k7420]

Thank you, tomorrow I'm going to try to mix up things by looking at both schematic, and will change the base resistor to a lower one....

 

[short_circuit_]

Is it possible to receive the signal from audio guru's fm transmitter using the radio in our cell phone? or do we need any super hetrodyne receiver?

 

[audioguru]

My FM transmitter has the same 88MHz to 108MHz radio frequency as FM broadcast band stations. If your radio can receive an ordinary FM radio station then it will also receive my FM transmitter.
A super-heterodyne radio has good sensitivity and selectivity but a crystal radio or super-regen radio will also work but poorly.

 

[mdanh2002]

Hi, is it possible to replace the LM2931A in the circuit with LM7805?

 

[audioguru]

Hi, is it possible to replace the LM2931A in the circuit with LM7805?

A 9V battery's voltage quickly drops to 7.2V as it is used then slowly drops to 6V and less.
A 7805 needs a minimum input of 7.0V so a 9V battery will quickly cause the circuit to fail.
An LM2931A or any other low dropout 5V regulator works perfectly when its input drops to 5.5V so a 9V battery will last a long time and the circuit will keep working properly.

All semiconductor manufacturers make low dropout 5V regulators.

 

[mdanh2002]

A 9V battery's voltage quickly drops to 7.2V as it is used then slowly drops to 6V and less.
A 7805 needs a minimum input of 7.0V so a 9V battery will quickly cause the circuit to fail.
An LM2931A or any other low dropout 5V regulator works perfectly when its input drops to 5.5V so a 9V battery will last a long time and the circuit will keep working properly.

All semiconductor manufacturers make low dropout 5V regulators.

Thanks. Can I replace it with LM2940CT-5.0 instead (see here **broken link removed** It's a low dropout 5V regulator.

Also, which resistors should be power resistors? I saw some power resistors in your veroboard picture but this is not mentioned in the schematics.

 

[audioguru]

Can I replace it with LM2940CT-5.0 instead

It is a huge monster but it will work. There are hundreds of small ones the size of the one I used.
Will E-Bay provide a real one or a counterfeit one?

Also, which resistors should be power resistors? I saw some power resistors in your veroboard picture but this is not mentioned in the schematics.

Nothing in my circuit uses a high power so every resistor is 1/4W. The 220 ohm resistor is extra long because it is 50 years old.

 

[mdanh2002]

Will E-Bay provide a real one or a counterfeit one?

My experience with ebay so far has been quite positive. As long as I choose a reliable seller with more than 500 positive feedback (more than 99%) and don't order expensive stuff, it should be fine. Most of my electronics orders from ebay arrive in 1-2 weeks time. Never had a lost package.

 

[Samrajih]

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What is the range of the transmitter and how can i manipulate it for higher range?