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VPDS Circuit Description
Kyle Schickler
Introduction:
Project Description: This project is a vehicle proximity detection system for large-scale semi trucks. The
idea is to have a box mounted inside the cab of the truck which would contain an on/off switch, a series
of 16 LEDs (one warning LED and one danger LED which refer to 8 different locations, four on either side
of the truck). In lieu of an actual truck’s turn signal lever, the prototype box will also have a single pole
double throw ON-OFF-ON switch which will serve as the makeshift turn signal lever for the
demonstration. Outside the cab will be mounted 8 distance sensors (four for the demonstration unit due
to cost). These sensors will be mounted evenly spaced along the sides of the trailer to sense proximity
from other vehicles.
When the turn signal switch is activated, the program will sense which side the turn signal is set to and
read from the distance sensors on that particular side. If there is a vehicle more than 12 feet from the
sensor, then no LEDs will be illuminated. When there is a vehicle 7 feet away, the yellow warning LED
will illuminate for the corresponding location on the display. When that distance drops to 4 feet, the red
danger LED will turn on, as will a small buzzer to alert the driver audibly to the danger.
Circuit Description
Power Supply: Because this circuit is to be placed on a truck, the power to the circuit is provided by the
truck’s battery. Because there is always a chance of batteries being inserted backwards, jumpstarted, or
overvoltaged, the voltage regulator which converts the inputted voltage into a usable voltage of 3.3V,
must be able to withstand any voltage within the range of 9-24Volts. The regulator I have chosen is the
Texas Instruments LM2674N-3.3. This regulator is capable of an input voltage anywhere within the
range of 8-40V and capable of a current output of up to 500mA. However, because the specifications for
my design only need to withstand up to 24 volts (or double the voltage of a normally charged vehicle
battery), the smoothing capacitor I have chosen to use for the input of the regulator is only rated up to
35Volts and the output capacitor is rated to 25Volts (even though the output is only 3.3Volts, the
regulator’s datasheet recommends a 25V output capacitor). Since there is also a possibility of the
battery being hooked up backwards, I have included a protection diode on the input to the circuit which
would protect the regulator from negative voltage input. There is also a toggle switch between the
battery and the regulator which would allow for the driver to turn the system completely off or on at
will. The output of the regulator powers the rest of the circuit which is constructed entirely of parts
which can run off of 3.3Volts.
Microcontroller: There is only one microcontroller in my design which runs everything. This is the
MSP430G2553 in a 28 pin TSSOP package. The reset pin on the controller is tied high through a 9.76K
resistor so that the program never resets itself. The reset pin also has an external jumper as does the
test pin. These are to ensure that the chip can be programmed while it is in circuit. The resistor tying the
reset pin high is insurance that the programmer is able to still pull the pin low when programming.
There are 4 pins that are unused, and these will be set as inputs in the software because that puts the
pins in a high-Z state.
Sensors: Attached to the MSP430 are 8 MB1040LV-EZ4 proximity sensors. These sensors are capable of
sensing objects within 20ft. However, I will only need to sense objects up to 12 feet away, so any value
above that will be ignored by the program.
Each of these sensors is hooked up to the same power supply which powers the rest of the board (3.3V).
These sensors have an option of using an RS232 output (an RS232 formatted signal, but the voltage
levels are 0V to 3.3Volts), PWM output, or linear analog voltage output. The method I have chosen to
use is analog output. This is because the wires which will connect the main board to these sensors will
be very long (over 20feet) in length and I believe that any fast-switching signal will most likely get
distorted along the line. I have tested the analog output through a 50 foot wire, and the readings
remained accurate and not noisy enough to cause major error. While I initially wanted to multiplex
these sensors to one A/D pin of the MSP430, I ran into several issues with doing this. Turning the
sensors on and off by removing their power would leave too much time for the sensors to start up and
initialize (200msec for each, for a total of 1.6 seconds to read values from every sensor). The sensors do
offer a read pin which when triggered will cause the sensors to take a reading which takes only 50
milliseconds. However, when the read pin is not triggered, the sensor’s voltage output is held at the
voltage of the previous reading rather than returning to zero. This would mean that the voltage reading
would always appear as the voltage of whichever sensor is holding its voltage the highest. For these
reasons, each sensor will be connected to its own A/D pin on the MSP430 with a 100pF capacitor to
smooth out any stray noise.
LED Array: Also attached to the MSP430 is an array of 16 LEDs which are multiplexed so that they can all
be individually addressed using only 8 pins. The multiplexing scheme I have chosen is one of ‘rows and
columns’. That is to say that the LEDs are arranged in a grid formation where the anodes of each row of
LEDs are all connected together. Then, the cathodes of each column of LEDs are also connected together
(shown in figure 1).
Figure 1
In this connection scheme, each LED is able to be addressed by its row and column. IE if I wanted to
illuminate the top left LED, I would send a positive voltage to row 1, and a ground potential to column 1.
The only downside to this scheme is that only one LED can be activated at one time. If more than one is
activated, than there is the potential for unwanted LEDs to be illuminated as well.
Transistor Driven Array: Because the microcontroller’s datasheet states that the output pins should not
source more than 10mA of current,there are 2N3904 transistors which switch the 3.3Volt input line to
each row. Each column also has its own 2N3904 transistor switching the ground rail on and off for its
own column. Powering the transistors which switch current on and off requires only 0.2mA
Status LED: There is one LED not connected to the array; this is the Power/Status LED. This LED is
controlled by another 2N3904 transistor to prevent current from being sourced from the
microcontroller. This LED is not in the array because it is to be controlled independently from the rest in
order to indicate the status of the system to the driver or serviceman in case of a failure of the array of
LEDs.
Turn Signal Switch: Also connected to the MSP430 is a single pole double throw switch. The input to the
switch is the 3.3V power line, while the two outputs are connected to P2.0 and 2.1 of the
microcontroller. These outputs will be pulled down by internal resistors to prevent floating. This switch
while being double throw actually has three positions: ON(throw1), ON(throw2), and OFF. The purpose
of this switch is to signal to the microcontroller if the turn signal of the truck is on, and if so, which side
the turn signal is signaling. This is to tell the program which side (sensor bank) should be monitored.
Buzzer: The final device which is connected to the MSP430 is a small buzzer used to alert the driver to
danger. This device is again hooked up to the 3.3Volt power regulator, while the negative side is
switched on and off by another 2N3904 transistor.