Fm transmitter with tones changed by light

[mstechca]

After looking at the circuit at the following URL:
**broken link removed**

I thought there must be a ridiculously easy formula that tells me the exact frequency of the tone I am sending. I know that the L and C connected in parallel determine the radio frequency I send the tone to, but I still can't quite figure out how to determine the frequency of the tone. I figured out that omitting R2 gave me better results.

Somehow, I want it where I include a LDR (light-dark-resistor or photocell), and the tone is high when the room is dark, and the tone is low when the room is bright. The reason why I want that is because then I can pick it up in the receiver's LED, and therefore I get myself a remote light detector.

What I was thinking is somehow combining the output of two 1 transistor oscillators together with two resistors, one of them being the LDR.

If anyone can point me to a 1 transistor oscillator that WORKS, and contains the fewest parts (even if it has the worst amplification), I will be happy. Also, I need an equation to determine the frequency of the oscillator shown to me.

 

[Nigel Goodwin]

After looking at the circuit at the following URL:
**broken link removed**

I thought there must be a ridiculously easy formula that tells me the exact frequency of the tone I am sending. I know that the L and C connected in parallel determine the radio frequency I send the tone to, but I still can't quite figure out how to determine the frequency of the tone. I figured out that omitting R2 gave me better results.

Omitting R2 will prevent it oscillating (at least in the way it's supposed to work!

The formula to calculate the frequency is simply F=1/(2*PI*SqrRoot(LC)), but your problem is knowing the values you have!. In particular the capacitance will be greatly affected by 'stray capacitance' - it's far easier simply to adjust it to the frequency you want.

 

[audioguru]

Hi MS,
You don't need an LDR to change the RF frequency of that extremely simple transmitter. It will change its frequency by something getting near its antenna (changing the tuned circuit's capacitance since the antenna is connected directly to it), temperature change or battery voltage change (changing the transistor's capacitance.).

What does your receiver's LED indicate? Stereo? This isn't a stereo transmitter, and when your receiver detects its signal it won't indicate stereo.

Why connect the output of two oscillators together? An FM receiver's capture ratio will show that it "locks" onto the strongest or first signal and ignores the other.

 

[mstechca]

You all are lost.

what I want to do is transmit a tone. The frequency of the tone itself is based on how much light has fallen on the LDR. I want the tone to be high audible tone if the LDR picks up no light and I want the tone to be low if the LDR picks up bright light.

I know that F=1/(2*PI*SqrRoot(LC)), and that determines the frequency I want to transmit the tone to. But I need to know the values of the parts that make up the audio frequency, not the jamming radio frequency.

My LED detects music. Its more like a "Color organ". and what I want to see in the receiver is the same amount of light that the transmitter picks up. Like I said, I want to make a wireless remote light detector.

I forgot to add... If an FM transmitter throws you off, then tell me how to make a simple oscillator using only 1 transistor and no IC's, and tell me the equation that determines the tone frequency. If possible, remove the transformer and inductors from the circuit.

 

[audioguru]

Hi MS,
Nope, we aren't lost and the FM transmitter doesn't throw us off.
Your FM receiver will be lost and thrown off by the RF frequency drift of the extremely simple FM transmitter.
You should have said that you wanted to replace the transmitter's mic with a variable frequency audio tone in the first place.

A phase-shift or Wien bridge oscillator can be made with only 1 transistor but the frequency-determining resistors are more than one.

 

[panic mode]

hi mstechca,

the transmiter in the circuit you ware presenting is using microphone to
pickup sound. if you ant to generate audio signal to modulate this oscillator
that's fine but you need additional circuit. make sure to pick something
with nice sine output.
this can be opamp or single transistor oscillator with phase shift (3x RC network).