This tutorial shows how to
set up a microcontroller based system that
converts a signal from a light sensor to a 6 bit
digital value. This value can be used by the
microcontroller, perhaps for a robotic
controller, or as in this tutorial, sent to the
PC. It uses the
AT89C2051 microcontroller to collect data
and send it to the PC. A MAX232CPE chip is used
to convert the signals from and to RS232 levels
for sending and receiving through the serial
port. The 2051 microcontroller has a built in
analog comparator that is used to make a simple
analog to digital converter to convert the light
sensor output to a digital value.
This tutorial is
similar to the
Data Collection tutorial.
Refer to the diagram
below to build the circuit. The
Data Collection Tutorial has more detailed
instructions on building the circuit (except for
the resistor network).
The Light
Sensor
Light sensors are one
of the most common types of sensors. They are
used in night lights, street lights, alarms,
toys, cameras, etc. We are using a CDS (Cadmium
Sulfide) Photocell to detect light. The
resistance of the sensor varies based on the
amount of light that hits it. The resistance
can vary from 300K in the dark to 1K in the
light. Our goal is to convert this into a
digital value. We have to convert the variable
resistance into a voltage and then the voltage
into a digital value. To convert the resistance
into a voltage, we use a second resistor Rb.
Then assuming that no current goes into pin 13,
you can find V1. To find V1 you can use Ohms
Law. With Vcc = 5, Ohms Law gives you
(5 - V1) / Rpc =
V1 / Rb
where Rpc is the resistance of the sensor
(photocell).
Then you can solve for
V1 = 5*Rb / (Rpc + Rb)
Then the maximum
voltage is when Rpc is at its minimum, 1K. Then
V1max = 5*Rb / (1k + Rb)
The minimum voltage is
when Rpc = 300K. V1min = 5*Rb / (300k + Rb).
Using the equations for
V1 max and min you can determine that 5.1K is a
good value for Rb. 5.1K gives you a wide voltage
range from minimum to maximum. 5.1K works well
for general light to dark situations. If you are
only interested in bright environments (are you
making an outside robot?) then use a larger
value of Rb to shift the light sensitivity range
towards bright lights (Perhaps 50K). Or if you
are interested in dark environments (are you
making a robotic vampire dog that barks at the
moon and hides from bright lights?) then use a
smaller value of Rb (perhaps 510).
Now we have a sensor
voltage, V1, that varies from about 0.1 volts to
4.2 volts. If you want to take the easy route
you can use the hardware set up in the
Data Collection tutorial to convert this
voltage to an eight bit digital value.
Analog to
Digital Conversion using the 2051
The
Data Collection Tutorial shows how to use a
ready made Analog to Digital converter chip to
get data from an analog source. This tutorial
shows a different method. This method is not as
accurate as the ADC0804 but it is less
expensive, uses less power, and is easily
modified to suit specific needs.
This method uses the built in analog
comparator on the 2051 (not a normal feature of
8051 based chips). The voltage generated by the
sensor circuit is connected to the negative
input of the comparator (P1.1) and we will
generate a voltage to connect to the positive
input of the comparator (P1.0). The output of
the comparator goes to P3.6. P3.6 is not an
external pin on the 2051. It can only be
accessed by the internal software. If the
voltage at P1.0 is higher than P1.1 then P3.6
will be a 1. If the voltage at P1.0 is lower
than P1.1 then P3.6 will be a 0.
By using the other 6 Port 1 pins (P1.2
through P1.7) we can generate a voltage using a
resistor network connected to those pins. By
changing the values of the Port 1 pins we will
get as close as possible to matching the voltage
from the sensor circuit. Then we will have a 6
bit digital value that is a reflection of the
sensor voltage at P1.1.
Each of the 6 Port 1 pins is connected to V0
through a resistor. Setting a pin to 0 or 1
subtracts from the voltage at V0 or doesn't. The
value of the resistor determines how much
voltage is subtracted. If all 6 pins are set to
1 then no current is flowing through the
resistor network and V0 = 5V. The small resistor
on P1.7 (240 ohms) can subtract the most
voltage. When we set it to 0 current flows
through Ra and the voltage at V0 goes down. The
exact amount depends on the value of Ra. The
resistors are chosen so they are roughly twice
the value of the resistor connected to the next
higher pin. (Ideally they would be exactly
double the other value but it is difficult to
get resistors that have exactly the right
values.) By doubling the resistance, the pin can
subtract half as much voltage. When you get to
P1.2 with the 10K resistor, it only has a small
effect on the voltage at V0 when you set P1.2 to
0 or 1.
The actual voltage at V0 is determined by the
resister Ra. To find a good value for Ra look at
what happens when our digital output is about
at the half way point 011111. P1.7 is the only
pin that is drawing current. Starting at Vcc,
the current goes through Ra and then through the
240 ohm resistor to ground (P1.7 = 0). To make
the voltage at V0 equal to 2.5 volts (half of
Vcc), make Ra 240 ohms. But since we know the
sensor voltage V1 only goes up to 4.2 volts you
may want to make the halfway point by 2.1 volts.
Use 330 ohms for Ra to get 2.1 volts for the
halfway digital output of 011111.
Now we can control the voltage at V0 fairly
accurately with P1.2 through P1.7. To make a
small change in voltage, change the lower pins
and to make a large change in voltage change the
higher pins. By starting with P1.7 through P1.2
set to 000000 (P1.7 is on the left and P1.2 is
on the right) and counting up to 111111 you can
get 64 different voltages!
To find the right digital output to create
the right voltage to match the voltage at P1.1
(V1), we start at 000000 and count up until the
comparator output at P3.6 switches to 1 to tell
us our generated voltage is higher than the
sensor voltage. Then we can "track" the voltage
by adjusting the value up and down depending on
the output of the comparator. Since the
comparator only tells us high or low (it can not
tell you if you have an exact match) then one
possibly annoying aspect of this approach is
that the P1.2 bit is constantly switching from 0
to 1 to 0 to 1... as the comparator output tells
us we are low, then high, then low. To avoid
having to watch the 6 bit value oscillate (also
called jitter) we just use the top 5 digits as
our answer. Look at the documentation in the
software for the 2051 in light.asm for more
details on the tracking routine.
The details of communicating with the PC are
covered in the
Data Collection Tutorial. For this project
we are sending the upper 5 digit value (P1.3
through P1.7) to the PC. This can be displayed
on the screen using the
sample light program. After downloading,
double click it to extract the files and then
run setup.exe to install it.
The Software
The basic process of
compiling an assembly language program and
loading it into the microcontroller was covered
in the
first microcontroller project. The
2051 assembly language program for this project
is light.asm. You will need a device programmer
such as the
PG302 to reprogram the 2051.
Make sure the power is
off to the circuit you have built.
Connect the circuit to the
PC's serial port, Comm1. Connect the power to
the breadboard. The circuit should send a
continuous stream of values to the PC. To see
the values on the PC, try this
sample light program. After downloading,
double click it to extract the files and then
run setup.exe to install it. The source code for
the sample program (written in VB 5.0) is on the
CD included with the kit.
The parts for this
project are included in the Sensor Kit. The
Sensor Kit also includes the parts needed to do
the
temperature sensor project and the
data collection project. The kit includes:
