Masters of electronics come here pls :D

[baohead]

I'm recently working on a Remote control for a car. I have do tons of research on the web and have search through the forums. But I'm a total N00B in this and electronics. My knowledge is only limited to all basic boolean gates,transistor. And what does capacitor,inductor in theory does.
I've found many schematic diagrams and websites that teach how you do it. But it doesn't tell how each parts work, like "capacitorB is used for blahblah" . Like the one below :


*P.S Is this a radio transmitter? *
**broken link removed**


Is there any math or more theories I should know before starting on this?
If so, can you guys recommend some websites where I can learn it?

Thank You and don't laugh at me

 

[Russlk]

Q1 is an audio amp, modulating the oscillator, Q2. C4 is an RF bypass, needed to make the oscillator work. the triangular symbol on the right is the antenna. I don't know the function of D1.

 

[baohead]

what does C1 do?

 

[bmcculla]

C1 removes the DC part of the signal comming in from the mic. R2 and R3 set the DC level on the other side of the capacitor. The AC part of the signal should be the same. Having the DC level correct for the transistor is important for making the transistor amplify the signal in the correct way.

Brent

 

[Dean Huster]

C1 is a "blocking" (aka "coupling") capacitor used to pass the AC audio signal from the electret condenser microphone on to the input of the amp while blocking the DC. R1 is the resistor that feeds the DC power to the electret to make it work.

D1 is in series with the "-" battery connection and the coil/collector and simply sets up a 0.7v (approximate) bias that's "filtered" (RF bypassed) by C3.

R2 and R3 set up a voltage divider to bias the base of Q1 to the desired operating point. I'd guess that C5 is stuck in there to form a low-pass filter to knock off higher audio frequencies or more likely the RF generated by the transmitter portion so that you don't end up with a lot of nasty RF feedback problems.

A1 is operating as a standard common emitter amplifier. R5 sets up the emitter bias while C5 bypasses any audio around the resistor to eliminate any negative feedback that would otherwise be generated, keeping the gain of this stage pretty high. R4 is the collector load resistor where the output voltage of the amp is developed.

I'll leave the rest for the RF geeks so I don't embarass myself!

Dean

 

[baohead]

C1 removes the DC part of the signal comming in from the mic. R2 and R3 set the DC level on the other side of the capacitor. The AC part of the signal should be the same. Having the DC level correct for the transistor is important for making the transistor amplify the signal in the correct way.

Brent

sorry,I'm a n00b. I only know the capacitor charges up at +ve side of C1 and release charges on the -ve side.

How does it remove "DC part of the signal coming from the mic"?

what do you mean by "DC part of the signal coming from the mic"?

Thanks

 

[gregory hansell]

C4 blws things up, athough that may be differnt to the C4 in your diagram.

 

[baohead]

C1 is a "blocking" (aka "coupling") capacitor used to pass the AC audio signal from the electret condenser microphone on to the input of the amp while blocking the DC. R1 is the resistor that feeds the DC power to the electret to make it work

How does it block the DC while R1 is supplying Vcc to the capacitor and mic?

D1 is in series with the "-" battery connection and the coil/collector and simply sets up a 0.7v (approximate) bias that's "filtered" (RF bypassed) by C3.

Why do you say D1 is in series with GND?
what do you mean by bias? (n00b question)
What do you mean RF bypassed?How does it bypass by C3?

R2 and R3 set up a voltage divider to bias the base of Q1 to the desired operating point. I'd guess that C5 is stuck in there to form a low-pass filter to knock off higher audio frequencies or more likely the RF generated by the transmitter portion so that you don't end up with a lot of nasty RF feedback problems.

Why do I need R2 and R3, isn't the charge is released from C1? Do you mean that the transistor need a proper voltage level to operate? But why do we need 2 resistor, why can't we just place one?

What is low-pass?

What is RF feedback?

A1 is operating as a standard common emitter amplifier. R5 sets up the emitter bias while C5 bypasses any audio around the resistor to eliminate any negative feedback that would otherwise be generated, keeping the gain of this stage pretty high. R4 is the collector load resistor where the output voltage of the amp is developed

Don't understand this part:
"R4 is the collector load resistor where the output voltage of the amp is developed"


Thank you so much for answering

 

[bmcculla]

Capacitors can be used to prevent DC voltage on one side from getting to the other side. If you apply two different voltages to either side of a capacitor the capacitor will charge up and then stop. As long as these two voltages stay the same there will be no current flowing between the two voltages and they shouldn't affect each other at all - this is what is ment by blocking DC. But if one of the voltatges changes suddenly current will flow out of the capacitor on one side and into it on the other. Even though the electrons going into the capacitor arn't the same ones coming out on the other side it works like current is flowing across the capacitor. So as long as the voltage on one side keeps changing it will affect the voltage on the other side by adding or removing current but only in proportion to the change in voltage and not the DC level.

You need R2 and R3 to set the DC level on the other side of the capacitor because no DC current can get through the capacitor to set the voltage current flowing out or into the transistor would set the voltage and its hard to figure out what that voltage would be.

R2 and R3 set the bias voltage for the first transistor. Bias just means the DC voltage at the input to an amplifier. Biasing is important because transistors will only amplify a certain range of voltages if the voltage is too low nothing will happen.

"R4 is the collector load resistor where the output voltage of the amp is developed"

Transistors allow a current through them based on the input voltage. The current flows through R4 and generates a voltage. So R4 takes the current flowing into the transistor and turns it into a voltage.

Low pass means that the circuit only allows low frequencies to get through. The simpleset lowpass is a resistor and capacitor. the risistor charges the capacitor from the input. Because capacitors only allow current to flow when the voltage is changing (like I talked about above) it works like an open circuit at DC (no DC gets through) but as the voltage changes faster (the higher the frequency the faster the voltage changes) the cap works more and more like a short. You can use this idea to analize circuits. For DC you can just imagine removing all the capacitors and that is how the circuit will function with DC. For high frequencies you can imagine replacing all the caps with wires to see how the circuit will function.

Hope this helps your understanding
Brent

 

[baohead]

Thank you so much for spending time answering my questions.

Capacitors can be used to prevent DC voltage on one side from getting to the other side. If you apply two different voltages to either side of a capacitor the capacitor will charge up and then stop. As long as these two voltages stay the same there will be no current flowing between the two voltages and they shouldn't affect each other at all - this is what is ment by blocking DC. But if one of the voltatges changes suddenly current will flow out of the capacitor on one side and into it on the other. Even though the electrons going into the capacitor arn't the same ones coming out on the other side it works like current is flowing across the capacitor. So as long as the voltage on one side keeps changing it will affect the voltage on the other side by adding or removing current but only in proportion to the change in voltage and not the DC level.

I think I have some wrong concept with the capacitor. What I think is that when the capacitor is charging up on +ve side, current flows through. Where the capacitor is fully charged, no current flows through. Am I wrong with this?

What is the difference between DC level and Voltage? I thought they were the same thing? (n00b question )

when you say voltage keep changing on one side, do you mean the voltage(signal) from the mic?

You need R2 and R3 to set the DC level on the other side of the capacitor because no DC current can get through the capacitor to set the voltage current flowing out or into the transistor would set the voltage and its hard to figure out what that voltage would be.

Don't understand

Thanks[/quote]

 

[panic mode]

Well you are right... Capacitor charges up and then stops. From that moment
it is like open circuit and it does not affect the potential on either side.
When mic picks some noise or sound voltage accross mic will change.
Depending on direction of change, C1 will be either further charged or
discharged until it's in ballance again.

This charging/discharging current is proportinal to signal comming from
microphone. This charging current will temporary create imbalance on the
side connected to transistor. Because audio signal is too fast for capacitor
to fully charge or discharge, the changing audio signal comming from mic
is transferred through capacitor. Note that only CHANGE is transferred.
Change in signal is considered the "AC component" of the signal.

"Full signal" on the microphone side is combination of DC signal
(ca 2-3V depending on model of the microphone) and the changing signal
(few milivolts comming from microphone).
Full signal on the base of the Q1 transistor is combination of the
DC voltage (ca 0.7V) and AC component which is "comming through" C1
(few milivolts).

C1 is sized to be large enough so the AC component of the signal can
pass.

Impedance of capacitor is calculated like Xc=1/(2*pi*f*C) where
pi = 3.14152
f = frequency of the signal
C = capacitor value


If the frequency increases, impedance is lower. In other words, the
higher the frequency of the signal, the smaller capacitor you can use
to pass the signal through.
C1 is used to transfer audio frequencies. C4 and C5 are also used in
a similar way - to transfer some of the AC signals to ground
(high frequencies such as audio noise and interference).
That's why C5 is much smaller than C1. If you increase values of
C4 and C5, the audio signal will be shorted to ground as well and you
will hardly hear anything from your radio.

Q2 works as radio frequency oscillator (much higher frequency)
so the capacitors arount it are much smaller.

As for D1 and C3, I'd just get rid of them and put jumper.

 

[baohead]

Well you are right... Capacitor charges up and then stops. From that moment
it is like open circuit and it does not affect the potential on either side.
When mic picks some noise or sound voltage accross mic will change.
Depending on direction of change, C1 will be either further charged or
discharged until it's in ballance again.

This charging/discharging current is proportinal to signal comming from
microphone. This charging current will temporary create imbalance on the
side connected to transistor. Because audio signal is too fast for capacitor
to fully charge or discharge, the changing audio signal comming from mic
is transferred through capacitor. Note that only CHANGE is transferred.
Change in signal is considered the "AC component" of the signal.

"Full signal" on the microphone side is combination of DC signal
(ca 2-3V depending on model of the microphone) and the changing signal
(few milivolts comming from microphone).
Full signal on the base of the Q1 transistor is combination of the
DC voltage (ca 0.7V) and AC component which is "comming through" C1
(few milivolts).

C1 is sized to be large enough so the AC component of the signal can
pass.

Impedance of capacitor is calculated like Xc=1/(2*pi*f*C) where
pi = 3.14152
f = frequency of the signal
C = capacitor value


If the frequency increases, impedance is lower. In other words, the
higher the frequency of the signal, the smaller capacitor you can use
to pass the signal through.
C1 is used to transfer audio frequencies. C4 and C5 are also used in
a similar way - to transfer some of the AC signals to ground
(high frequencies such as audio noise and interference).
That's why C5 is much smaller than C1. If you increase values of
C4 and C5, the audio signal will be shorted to ground as well and you
will hardly hear anything from your radio.

Q2 works as radio frequency oscillator (much higher frequency)
so the capacitors arount it are much smaller.

As for D1 and C3, I'd just get rid of them and put jumper.

So the main purpose of C1 is to block the voltage from R1, so only RF from mic only goes through C1 and reach the transistor?
Thanks

 

[panic mode]

Yep, except it's not the RF but AF signal going through C1.

 

[baohead]

Thanks

Also, I want to know which part of the circuit translate the MIC signal to the oscillator and can be regonized by the radio. I know oscillator includes inductor and capacitor, but how does the Signal become related with the oscillator?

 

[baohead]

what does Q2 do?

 

[baohead]

hmm, alos what odes c6 do?

thanks