A License Plate Recognition and Speed Detection System - Index of

I will discuss the pathway the input signal takes through the readout circuit from the input jack to
the 7-segment LED display. For brevity, I will ignore the headphone, calibration, and gain
control circuitry.
Two six-volt dry cell lantern batteries connected in series powers the readout display. The
input power is separated by a voltage divider circuit (formed by resisters R1 and R2), to give a
12 volt supply and a 6 volt supply needed by the integrated circuit (IC) chips.
The signal from the microwave oscillator enters the readout circuit via the signal jack, J1.
The signal is amplified by passing through all four stages of an LM324 Quad Op-Amp. The
input signal is applied to the non-inverting pin of each amplifier stage and negative feedback
loops are used to control the gain and prevent the op-amp from going into saturation. The gain
(G) in a negative feedback loop is controlled by the values of the resistors in the voltage divider
configuration in the specific loop. The gain (G) = 1 + R2/R1.
The amplified signal leaves the last stage of the LM324 Op-amp at pin 7 and is directed
to the input pin of a 4093 Quad Schmitt trigger NAND IC. A Schmitt trigger has the unique
ability to convert any type of input waveform into a square wave output and at the same time
acting as a noise filter. The graph below (figure 34) shows an arbitrary waveform at the top of
the graph and the resulting output after passing through a Schmitt trigger. The Schmitt trigger is
a type of comparator IC that uses two independently set thresholds. When the input waveform is
below the lower threshold, the Schmitt trigger’s output is low. When the input waveform is
above an upper threshold, the Schmitt trigger’s output goes high. The Schmitt trigger’s output
changes only at those times when the input waveform’s trailing edge crosses the upper threshold
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