Crystal Controlled FM Transmitter Crystal FM Transmitter The FM transmitter is relatively simple to build and with only one adjustment it is ideal

The FM transmitter is relatively simple to build and with only one adjustment it is ideal for the absolute beginner. But before I continue, let me just make one thing clear. With Wide-Band deviation, the VCO is never locked in phase, only in frequency. It uses a Varicap diode to modulate an oscillator, then lock the oscillator in a Phase Locked Loop, and compare it to a crystal oscillator
Circuit Basics
The circuit follows that of a simple textbook synthesiser comprising a Voltage Controlled Oscillator (VCO), a damped Loop Filter, a Reference Oscillator (Crystal) and a Frequency Comparator.

The only difference is that I have added a frequency divider chip to divide the VCO frequency by 64. This means that if the VCO operates at 100MHz, the output from the divider will be 1.5625MHz. If the crystal oscillator is also 1.5625MHz then the loop will be "in-lock." The control voltage from the filter to the VCO steers the frequency of the VCO so that the output of the divider is ALWAYS 1.5625MHz. Any deviation from this will result in a change of the loop voltage to move the VCO back to 100MHz.

Audio frequencies are then added to the loop voltage that control the VCO frequency. It is in this way the synthesised transmitter is modulated.

Circuit Specific
I will not delve too deeply into the ins and outs of synthesisers. I have already written several pages of information about them and the stages that are needed to make a working synthesiser.

The specific circuit is shown above. The "Prescaler" (divider) chip needs to have a supply voltage of only 5v (+/- 0.25v) so the LM317 has been included. I used the LM317T due to its larger can size (and I have got a lot of them) so it will tolerate a supply voltage of greater than 13.8vDC without burning. All has been somewhat over-engineered.

IC1 (CD4001) is the crystal oscillator with an extra gate used as nothing more than a buffer stage. This feeds the frequency comparator of a CD4046 (IC2). The comparator output is filtered with a "slack- handfull" of resistors and caps to feed the VCO; a BC547. A second BC547 has been used to isolate the VCO from the antenna. Without this device the loop would have the tendency to jump out of lock if you touched the antenna. The VCO is also coupled to the divider, IC3, which can be any one of a selection of chips. MB501, SA701, SP8704 and CA12022 are all the same device.

The filter time-constant is a couple of second or so. This makes it take about one second for the loop to stabilise. If this were not the case then the modulating frequency would be seen as a frequency error and the loop would correct the error (remove the modulation). The modulation input is via a 100K resistor and 3n3 capacitor which provides the little pre-emphasis needed for an FM broadcast transmitter. If your AF input comes from a stereo encoder then remove the 3n3 since the pre-emphasis must occur BEFORE encoding. I hope to post a stereo encoder soon, but I give no promisses.

If you are one of those who likes to dissmantle circuits, then you may notice that I have used the CD4046 Signal and Reference the wrong way around - I have used the signal input for the Reference frequency and the Reference input for the signal frequency. This is because the Reference input is designed to be fed from an external source and so it will respond to small small signals (ca: 200mV) whereas the Signal input is designed to be fed from the CD4046s own CMOS logic level oscillator. IC3 is an ECL device with only 1v output signal and is therefore NOT CMOS compatible. The result of this is that the output sense of the frequency comparator is reversed! That is why the Varicap Diode (BB105) is reference to +5v and Not to Ground.

Audio Controlled Mains Switch

Audio Controlled Mains Switch Circuit diagram. It is often useful for audio or video equipment to be switched off automatically after there has been no input signal for a while. The function of the on-off switch in such equipment is then taken over by switch S2 in the accompanying diagram. It remains, however, possible to  switch off manually by means of Si. Automatic  switch-off occurs after there has been no input  signal for about 2 minutes: this delay makes it possible for a new record or cassette to be placed in the  relevant machine.

 

The audio input to the proposed schema may be  taken from the output of the relevant TV set, amplifier, or whatever. The input earth is held at + 6 V  with respect to the schema earth by potential divider  Ri-R2-R3-R4. The two 741s function as comparators: the output of ICi goes high when the in- put signal is greater than + 50 mV, whereas the out- put of IC2 goes high when the input signal  becomes more negative than -50 mV. Resistors  R6, R7, and R8 form an OR gate that drives transistor Ti. If the output of either ICi or IC2 is logic  1, Ti conducts.

 

Audio Controlled Mains Switch Circuit diagram :

 

Audio Controlled Mains Switch Circuit Diagram

The 555  operates as a retrigger able monostable,  whose period is determined by Rio and Ci. The  device is triggered when its pin 2 is earthed by the  closing of S2. Its output, pin 3, then remains high  for 1 to 2 minutes, depending on the leakage cur- rent of the 555. 

The monostable resets itself as soon  as the potential across Ci exceeds a certain value.  As long as there is an input signal to the schema, Ti conducts and Ci remains uncharged. As soon as  the audio signal ceases, Ti switches off, and Ci  charges until the potential across it is sufficient to  reset the 555. The monostable may also be reset by  closing Si, which connects pin 6 of the 555 to + 12 V.

 

When IC3 is reset, Ci is discharged via its pin 7. Resistor Rrn serves as protection, because without it Ti could short-schema the supply lines. When the output of IC3 goes high, T2 conducts,  the relay is energized, and the relay contacts switch on the mains voltage as appropriate. To counter the induced potential when the relay contacts close, which could damage T2, diode Di has been connected in parallel with the relay coil.