FM transmitter (mod4)

[whiz115]

audioguru check this thread... i'm trying to improve a simple FM transmitter by my own (with the help of the forum )

https://www.electro-tech-online.com/threads/antennas-and-standing-waves.94775/

can you tell me why your design needs two LC tanks? what if there wasn't the second one?

 

[audioguru]

My FM transmitter uses a single wire for its antenna. Its second antenna "wire" is its very small capacitance to ground so the antenna is a medium impedance.
The output tuned circuit has its high impedance loaded down by the medium impedance of the antenna so that its tuning is broad. Tuned for a peak at 98MHz it still works pretty well at 88MHz and at 108MHz.

The output tuned circuit might be replaced by a resistor if the transistor is biased properly for it.

 

[whiz115]

what's the output of your FM transmitter in mW and how do you calculate it for the specific circuit with the 30'' wire for antenna?

thanx

 

[audioguru]

My FM transmitter draws about 53mA from a new 9V alkaline battery. Its output power is about 10mW.

You can figure out its power yourself by simulating its output voltage then calculating its power.

 

[mbarazeen]

Emmkay,

i have bought on ebay a very tiny (1cm x 1cm) FM trnasmitter, that can be tuned by PLL just by dip switches( or soldering). i tested them and found working good, the same company offers receivers too to the same size. if you are interested you can search it on ebay and find the details. each transmitter could be tuned to seperatre tiny receivers and couldbe boxed in a small space.

razeen

 

[circuitnoob]

can i use 5pf instead of 4.7pf and a trimmer of 5-40pf??
And if c4 value change for each continent what's for asia??
pls help.....thnx

 

[audioguru]

A 5pf capacitor is almost the same as a 4.7pF so it is fine.
A 5-40pf trimmer is almost the same as 5-35pf so it is also fine.

The value of C4 should be selected to match the amount of de-emphasis (treble frequencies cut) that is used by the FM radios in your area. I don't know how much pre-emphasis is used by FM radio stations in Asia. Ask the engineer at a local FM radio station.

 

[mneary]

Emmkay,

i have bought on ebay a very tiny (1cm x 1cm) FM trnasmitter, that can be tuned by PLL just by dip switches( or soldering). i tested them and found working good, the same company offers receivers too to the same size. if you are interested you can search it on ebay and find the details. each transmitter could be tuned to seperatre tiny receivers and couldbe boxed in a small space.

razeen

Suggestion is appreciated, but can you narrow it down a bit? There are 13,000 FM transmitters on ebay.

 

[circuitnoob]

i cannot find the low drop out regulator mentioned in this circuit.......can i use something else for this?? I saw that in a previous page that a 7805 could be used instead. If i am going to use the 7805 will be there be any changes in the circuit???

 

[circuitnoob]

Other than the 7805 are there any other regulators that can be used??

 

[mvs sarma]

Other than the 7805 are there any other regulators that can be used??

you might get LM1117-5
another LDO regulator.

 

[circuitnoob]

thnx..i might be able to get that....but when replacing that with the LDO in the circuit do i have to make any changes to the capacitors attached to the LDO regulator???

 

[mvs sarma]

thnx..i might be able to get that....but when replacing that with the LDO in the circuit do i have to make any changes to the capacitors attached to the LDO regulator???

i don't feel AG's Mod4 needs any change just because of regulator change, but be sure to follow the pin-out of the new regulator.

 

[audioguru]

A 7805 regulator needs to have a minimum input voltage of 7.5V or 8.0V to work properly. The voltage of a 9V alkaline battery quickly drops to 7.2V then drops more which is too low for a 7805 regulator.

I use a common low dropout regulator with a minimum input of 5.5V.

Every semiconductor manufacturer makes many 5V low dropout regulators. Most need to have a certain minimum value output capacitor.

 

[circuitnoob]

Thanks a lot.....I am half way through the circuit now : )

 

[circuitnoob]

i couldnt even get the LM117 but i could get a KIA 7805 and a C317 (NEC japan). Which one should i use??? Well are there any changes to the circuit because i am using a different regulators??
P.s : Please give me the exact changes of the circuit. (still a newbie )
thnx

 

[mvs sarma]

i couldnt even get the LM117 but i could get a KIA 7805 and a C317 (NEC japan). Which one should i use??? Well are there any changes to the circuit because i am using a different regulators??
P.s : Please give me the exact changes of the circuit. (still a newbie )
thnx

as long as the supply is not reducing, 78L05 would do, BUT, the pinout has to be checked and taken care. you may use a regular source of 9V and NOT the 9V mini battery.
Anyway try to procure few as they are nice to work with.

 

[marcbarker]

What's the Regulator for?

(I mean apart from wasting power burning off voltage)

 

[audioguru]

What's the Regulator for?

It supplies power to the RF oscillator. Its voltage is regulated so the RF frequency does not change as the battery voltage runs down.

It also supplies power to the preamp transistor. Its voltage is regulated so it still is biased properly when the battery is new and is old.

The original FM transmitter circuit that I modified didn't have a voltage regulator.
So its RF frequency changed all over the place and its preamp transistor was saturated with a new battery and was cutoff with an old battery so it did not work for long.

 

[mvs sarma]

What's the Regulator for?

(I mean apart from wasting power burning off voltage)

Nice question. Perhaps, you know the answer.

A circuit is designed at a specific voltage and not around a range of say 9.2V (when the Battery is new) to 7.8 V where after the battery cant deliver any more useful load.

Audioguru has given the reply to this issue long back while reasoning the need for the mod.
Similarly One might raise a question as to why the 3rd transistor is needed. This aspect also has been dealt in detail by Audioguru. I would only suggest those interested to go through the post that is voluminous enough and helps to learn.