Theory Of Operation - Ramsey Electronics WCT3 Manual

THEORY OF OPERATION

The WCT3 consists of 4 separate stages, one for each of the wireless control
signals to be detected. The sections are basically identical in operation but
each is specifically designed to detect the signal of interest. In order to keep
things simple we will describe the basic operation of one section and then
briefly describe the differences between the sections.
Looking at the schematic on pages 18 & 19 you will notice that it is divided
into 5 columns titled "INPUT/FILTER", "DETECTOR", "AMPLIFIER",
"COMPARATOR", and "POWER SUPPLY". There are also 4 apparent rows
left to right which correspond to the 315/433MHz, 125KHz, 20KHz and Infra-
red sections from top to bottom respectively. The power supply is common to
all sections and is the right most column. We'll look at the 125KHz section for
this discussion.
Starting on the left side of the schematic in the "INPUT/FILTER" column is the
antenna, "ANT2", which essentially 'collects' the 125KHz signal to be de-
tected. Next is coil L1 and capacitor C4. L1 is a specialized coil called an
RFID (Radio Frequency Identification) transponder coil. L1 along with C4 are
designed, or tuned, to allow a very narrow band of frequencies centered
around 125KHz to pass on to the detector stage of the WCT3. Any frequen-
cies outside the desired band are blocked from passing into the circuit. The
technical term for this circuit is a band pass filter.
Now that we have captured only the desired signal we want it needs to be de-
tected and this is done in the detector section. Diode D2, which is actually a
small package that contains 2 diodes, forms the detector. Without getting into
the details of how a detector works just know that D2 actually converts the
125KHz signal into a DC(direct current) signal. The diode connected between
C5 and R25 provides a positive DC signal to the amplifier, U1B pin 5, thru R25
and the other diode connected to ground prevents any negative part of the
signal from interfering with the operation of the rest of the circuit. C5 and R25
are in the circuit to provide a little more filtering of the 125KHz signal and help
stabilize the captured signal before being detected. Resistor R5 is a very high
resistance, 3,900,000 ohms and provides a very high resistance for the output
of the detector. This is necessary because the detected signal has an ex-
tremely low power level.
Up to this point the circuit design had to take into consideration the frequency
of the signal we were working with. Now that it has been detected by D2 the
signal is a simple DC level which will have an amplitude, or DC level, that is
proportional to the strength of the detected signal. Because the signal is now
a DC level it is much easier to amplify and provide an indication of a signal
being present.
As you have probably guessed the nice DC signal will now be amplified by the
WCT3  5