Vehicle Detector

Posted on Feb 6, 2014

A simple single-transistor Colpitts oscillator whose schematic is shown in figure 2. When a vehicle passes over the loop the inductance decreases and the oscillator frequency increases. A microprocessor which measures the oscillator frequency and takes appropriate action on changes in this value. In this lab, the microprocessor should activate a

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light-emitting diode when a vehicle is detected. The signal conditioning block, shown in figure 3, begins with a unity-gain buffer to ensure that the oscillator output is not excessively loaded by the signal conditioning input. A comparator is then used to convert the signal to TTL levels. The low-pass filter on the comparator reference input allows the reference to track slow drifts in the output of the oscillator. The resistors connected to the comparator input provide hysteresis to ensure that small amounts of noise on the oscillator output do not cause erratic operation of the comparator. Construct the 2N3904 oscillator and use an oscilloscope to measure its output signal. Keep the leads between the detector loop and the oscillator as short as reasonably possible. Construct the signal conditioning circuit and verify that it produces a stable, TTL level signal when connected to the oscillator. Both the TLC2272 and the LM393 operate with a single +5V supply. Remember to use decoupling capacitors close as possible to the device power supply pins. Write a program for the 87C51 which counts the number of oscillator cycles in a 10msec (approx) period (Hint - the 87C51 has hardware which can make this quite simple). The first value obtained should be used as the baseline ( and the `vehicle present` flag should be set to false. This handles the case where the oscillator frequency drifts lower or when a vehicle which was parked...

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