Spy bug FM Transmitter with Integrated PLL in the UHF range 434 Mhz and automatic microphone gain control ACG.


The realization of the micro spy transmitter that I present has been made possible with the advent of new tiny integrated PLL of the latest generation in the UHF range, which perform virtually the entire function of a small quartz FM transmitter with a RF power output of approximately 10 mW, one of them is the Melexis TH72012.2 in SOIC-8 pachage SMD.
In the section of the microphone preamplifier, the built-in motorola MC33111D in the SOIC-16 pachage SMD is equipped with automatic microphone gain control ACG.


The use of the integrated MC33111D ensures effective automatic microphone gain control as it allows you to get an ever constant audio signal at its output, providing maximum amplification of the sounds and weaknesses, and a strong attenuation of those strong signals. The audio signal supplied by the integrator to its pin 2 is always kept stable (within certain limits) on the 100 mV effective and consequently the FM modulation obtained by the varicap diodes is always optimal and well controlled. The two BBY65 varicap diodes are equivalent at SMV1249.
The RF transmission section guarantees a constant frequency obtained with the use of the TH72012.2 integrated that with a few passive contour components realizes a complete quartz fm transmitter with PLL control using a 13.575 mhz xtal integrated multiplies Its frequency is 32 times, so it has 13.575 x 32 = 434.4 Mhz in transmission. The integrated can operate at a frequency range from 380 up to 450 MHz, so it is very easy to vary the transmission frequency by replacing the quartz with other values ​​but remaining always within these frequency limits, it delivers to its pin 7 about 10 mW, RF power that through the condenser and inductance filter reaches the antenna length of 6.299 inches.  This integrated used in this micro spy transmitter can run at low voltage power up to a minimum of 3 volts so that they can be powered by a 3.7V lithium battery, even if the discharge of the battery does not cause the transmission frequency shift to either 4v or 3v, which is very important as once we have memorized the frequency on the receiving radio we should never change it.


The micro spy transmitter was made with SMD components in the 1206 series with the size of the PCB is 2.20 x 1.25 inches so you can enter the small one expected plastic box. Under the power switch on the PCB you must add a piece of insulating tape so that its metal casing can not shorten the underlying slopes. I recommend the use of rechargeable lithium batteries to power the micro spy transmitter, I found some lithium batteries of old cell phones (motorola V70 and C330) or type 603048 that fit perfectly into this plastic case, it is paste back the PCB. The internal battery can only be charged when the switch is in the OFF position by the PLUG 0.13 inch. provided, a 3.7v lithium battery charger must be used for its charging.