FM transmitter (mod4)

[bananasiong]

It won't be 750mm if it is folded or in a spiral so will have less range.

Oh I see, must be straight and vertical. That is troublesome right? I have to reduce the length of the antenna to reduce overloading, then increase it back to the normal size after getting the right frequency .

Have you checked to see if there's any oscillation from Q2?

I have check the collector of Q2 and the end of C12 by disconnecting it from the base of Q3, but the was no signal.

Thanks

 

[audioguru]

Oh I see, must be straight and vertical.

Yes. It must be parallel to the receiver's antenna and vertical is the easiest.

I have to reduce the length of the antenna to reduce overloading, then increase it back to the normal size after getting the right frequency

It is the radio that overloads so reduce the length of its antenna. My Sony Walkman radio switches a resistor in series with the antenna (the earphones cord) to reduce overloading.

I have check the collector of Q2 and the end of C12 by disconnecting it from the base of Q3, but the was no signal.

The DC voltage at the emitter of Q2 will tell us what it is doing. Measure it and tell us. It should be about 2.0V.

 

[bananasiong]

Yes. It must be parallel to the receiver's antenna and vertical is the easiest.

If I use a wire as antenna, I need to hold the other end by something so that it can stand vertically.

The DC voltage at the emitter of Q2 will tell us what it is doing. Measure it and tell us. It should be about 2.0V.

Okay, but today is Sunday, my school is not opening for me. I will test it tomorrow. Thanks

And, can I know, Super regenerative receiver and Super heterodyne receiver, are they the same? Or regenerative uses the same concept of heterodyne?

Thanks

 

[audioguru]

Use a telescopic whip antenna.

A super-regenerative is a single tuned circuit, single transistor piece of junk. I don't call it a radio. Cheap RC toy cars use it as a receiver with a range of about 3m.

A super heterodyne is a real radio with many tuned circuits, transistors and ICs. RC model airplanes use it as a receiver with a range of 300m or more. There are good ones and not so good ones.

 

[bananasiong]

A telescopic whip antenna is used only for 'straight standing', its antenna is still the same right? How about reception or transmittion?

Thanks

EDIT: super regenerative and super heterodyne is different. I said wrongly, this mod4 FM transmitter was working with my super regenerative radio, but not a super heterodyne.
So, I think super regenerative 'radio' doesn't have the problem of overloading, does it?

What type of transformer should I use for 9 volts regulated supply for the FM trasmitter and receiver? I know 12v-0-12v but how many VA? 8VA? I want it to be small.

 

[Hero999]

That will be fine, either use half the windinng and ignore the centre tap or use two diodes to make a full wave rectifier. You'll also need an LM7809 or LM317 regulator to get the 9V supply.

https://www.tpub.com/neets/book7/27b.htm

 

[audioguru]

My good quality FM stereo tuner and car radio don't overload because their automatic-gain-control and many tuned circuits are used to deal with a wide range of RF levels. My cheap Sony Walkman radio overloads and isn't as sensitive as the others. They are all super-heterodyne.

All super-regenerative receivers I've seen have a "sensitivity" control or the circuit has a fixed low sensitivity to avoid overload.

My FM transmitter draws about 60mA from 9VDC. For a 12VAC transformer to give 60mADC it needs 12VAC x 1.414 x 60mA= 1VA from the transformer. You probably won't find a mains trasnsformer as small as that.

 

[bananasiong]

That will be fine, either use half the windinng and ignore the centre tap or use two diodes to make a full wave rectifier. You'll also need an LM7809 or LM317 regulator to get the 9V supply.

https://www.tpub.com/neets/book7/27b.htm

What's the different between the bridge rectifier circuit (4 diodes) and the 2 diodes (with center tapped)?
If I use a 9-0-9 transformer, I'll get 12.7 peak. If I use the rectifier circuit with center tap, I'll get 11Vdc. Can I use a low drop out 9 volts voltage regulator?
For a bridge rectifier without center tap, I'll get 10.3Vdc, a low drop out voltage regulator can regulate this voltage well?
*I don't know whether there is a 9 volts low drop out voltage regulator in the market.

My FM transmitter draws about 60mA from 9VDC. For a 12VAC transformer to give 60mADC it needs 12VAC x 1.414 x 60mA= 1VA from the transformer. You probably won't find a mains trasnsformer as small as that.

As long as it is rated more than 1VA, then I'll choose the smallest.
If DC voltage at the emitter of Q2 is not 2 volts, means it is not oscillating?

Thanks

 

[audioguru]

If the emitter voltage of Q2 is not about 2V then either the transistor has failed or a part that determines its current has failed or is shorted.

The voltage could be fine but maybe C7 is open so it doesn't provide the transistor with positive feedback for it to oscillate. Maybe C5 is open so the transistor's base is not fixed so that the transistor can oscillate. Maybe the trimmer capacitor or coil is with a bad connection.

 

[Hero999]

What's the different between the bridge rectifier circuit (4 diodes) and the 2 diodes (with center tapped)?

Using centre tapped transformer with two diodes gives similar same results as a bridge rectifier without using a centre tapped transformer, the only difference is there's only one diode voltage drop instead of two.

If I use a 9-0-9 transformer, I'll get 12.7 peak. If I use the rectifier circuit with center tap, I'll get 11Vdc. Can I use a low drop out 9 volts voltage regulator?
For a bridge rectifier without center tap, I'll get 10.3Vdc, a low drop out voltage regulator can regulate this voltage well?
*I don't know whether there is a 9 volts low drop out voltage regulator in the market.

It depends on the current draw but you only have one diode drop so assuming a current of 100mA and 1N4001s you'll get 11.98V.

You don't need a low dropout regulator since an LM317 will have a dropout voltage of about 1.6V at 100mA.

If DC voltage at the emitter of Q2 is not 2 volts, means it is not oscillating?

Yes, it means there's something worng with the transistor or biasing. Did you try replacing Q2?

 

[bananasiong]

It depends on the current draw but you only have one diode drop so assuming a current of 100mA and 1N4001s you'll get 11.98V.

You don't need a low dropout regulator since an LM317 will have a dropout voltage of about 1.6V at 100mA.

Since there is no difference then I'll choose the one with center tap.

LM317 is an adjustable voltage regulator, I've just simply gone through the datasheet, this is the way to calculate [latex]V_{{o}}=V_{{ref}} (1+ {\frac {R_{{2}}}{{R}_{{1}}}) [/latex] and it says that the product of adjustment current and R2 can be neglected.
So can I just do calculation to get 9 volts?

Thanks

 

[audioguru]

The dropout voltage of an LM317 regulator is when it is no longer regulating and its output has dropped 0.1V. Most of them have a dropout voltage of 1.6V at 100mA but with others it is higher but how high is not shown. They have their spec's listed with an input voltage 3.0V and 5.0V higher than their output.

 

[bananasiong]

The dropout voltage of an LM317 regulator is when it is no longer regulating and its output has dropped 0.1V. Most of them have a dropout voltage of 1.6V at 100mA but with others it is higher but how high is not shown. They have their spec's listed with an input voltage 3.0V and 5.0V higher than their output.

So, if I use a 9-0-9 transformer, it might not be working for current higher than 100mA?

 

[audioguru]

If you use a 9-0-9 transformer then the peak is 12.7V and the voltage at the main filter capacitor is 12V. There will be a little ripple but the transformer's voltage will be higher than its rating because it will not be fully loaded. Then there will be plenty of extra voltage to avoid dropout.

Make an LM317 circuit with 470uF for the main filter capacitor for a 100mA load. Use 1000uF or more for higher load current. The 100uF capacitor at its output is already in the transmitter's circuit.

 

[bananasiong]

If you use a 9-0-9 transformer then the peak is 12.7V and the voltage at the main filter capacitor is 12V. There will be a little ripple but the transformer's voltage will be higher than its rating because it will not be fully loaded. Then there will be plenty of extra voltage to avoid dropout.

Make an LM317 circuit with 470uF for the main filter capacitor for a 100mA load. Use 1000uF or more for higher load current. The 100uF capacitor at its output is already in the transmitter's circuit.

Oh, then I can remain the 9 volts transformer, with center tap rectifier. Is 100mA enough for FM transmitter, super regenerative and PIC microcontroller?
Is protection diode needed as shown in the datasheet?

Thanks

 

[bananasiong]

The emitter voltage of Q2 is 5 volts after replacing a new transistor, same as the collector voltage.

 

[audioguru]

Why bother connecting a transmitter and a receiver to the same power supply? If the transmitter can hear the receiver then there will be acoustical feedback howling.

 

[bananasiong]

Why bother connecting a transmitter and a receiver to the same power supply? If the transmitter can hear the receiver then there will be acoustical feedback howling.

Nono.. I mean the same type of power supply but not connecting both the tx and rx together.
Voltage at the emitter of Q2 is 5 volts but not 2 volts..

 

[audioguru]

C7, the pcb or the transistor is shorted since the emitter has the same voltage as the collector. Maybe R7 is open or is disconnected from ground or from the emitter.

The datasheet for the LM317 says that a protection diode is needed from output to input since the output capacitor is big, to protect in case the input is shorted. Add it if you want but I don't think anything is going to short the input.

 

[bananasiong]

C7, the pcb or the transistor is shorted since the emitter has the same voltage as the collector. Maybe R7 is open or is disconnected from ground or from the emitter.

The collector and the emitter is not shorted, I've measured with digital multimeter in null setting. I've also resoldered R7, both ends are connected well.

The datasheet for the LM317 says that a protection diode is needed from output to input since the output capacitor is big, to protect in case the input is shorted. Add it if you want but I don't think anything is going to short the input.

Okay, thanks.