Sensing Light
by The CHIPAXE Team
Software
The start of the software is the same as the potentiometer setup since you will be using the GP4 pin as an analog input. Using the binary bit designation in PICBASIC PRO allows you to easily set the AN4 bit of the ANSEL register making it analog, while the rest of the I/O pins are zero or set to digital mode.
A variable is established to store the ADCIN result.
The main label establishes the main loop followed by the ADCIN command line where the CDS is read.
After the value of the CDS cell is stored in the adval variable it is compared to the value 150. If it is less than 150 then the LED is off. If the value is greater than 150, then it's dark and the LED is lit using the HIGH command.
Another GOTO statement completes the main loop.
Software Listing
Next Steps
Changing the threshold value from 150 to something higher or lower will determine how dark it has to be to light the LED. The limits are 0 to 255 since you used an 8-bit result. Just like the switch project other sensors can replace the light sensor. A thermistor could replace the light sensor to measure temperature. A potentiometer could be used to create a manual interface. If you remove the pull-up resistor then a Sharp GP2D12 Object Detection Sensor that produces a variable output voltage can be read by the analog pin directly for more accurate robotic obstacle detection.
If you have any questions about this project please email us at support@chipaxe.com.
You can get the parts from Jameco.com and the CHIPAXE module from our website www.chipaxe.com.
If you are new to programming then check out the books by Chuck Hellebuyck. The CHIPAXE Team has contracted Chuck to write books based on the CHIPAXE modules and he has many more that we recommend. His website is http://www.elproducts.com/.
by The CHIPAXE Team
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Sensing light can be useful and easy to accomplish with a resistive light sensor. In this project you'll use an Analog to Digital Converter (ADC) to read a CDS cell (or resistive light sensor) which changes resistance as the light around it changes. The ADC will create a digital value based on the resistance of the CDS sensor. As the light changes the ADC value will also change. The software will test the ADC value and if it's a high value (high resistance) the sensor is indicating it's dark and a single LED light will be driven to glow brightly. Figure 1 shows the final setup.
Hardware The schematic is very simple to build. The CDS cell is connected to a pull-up resistor to form a resistor divider (or voltage divider) to convert the changing CDS resistance into a changing voltage. The pull-up resistor could easily be replaced by a potentiometer to give you a sensitivity adjustment. The hardware only drives one LED on or off based on the outside light level similar to a night light. The connection table and parts list are below along with the schematic in Figure 2. Parts list: Photocell Resistor - Jameco P/N 202403 PIC12F683- Jameco P/N 312514 Red LED - Jameco P/N 94511 330 Resistor - Jameco P/N 690742 1k Resistor - Jameco P/N 690865 |
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Connection Table Micro Pin 1 at C6 Yellow Jumper - a6 to +rail Yellow Jumper - j6 to -rail Green Jumper - j7 to j12 330 ohm - i12 to i18 Red LED - Anode j18, Cathode -rail Yellow Jumper - j22 to -rail Orange Jumper - f22 to e22 Yellow Jumper - b22 to b18 White Jumper - b8 to b17 1k ohm - a17 to +rail CDS Cell - d17 to d18 |
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Software
The start of the software is the same as the potentiometer setup since you will be using the GP4 pin as an analog input. Using the binary bit designation in PICBASIC PRO allows you to easily set the AN4 bit of the ANSEL register making it analog, while the rest of the I/O pins are zero or set to digital mode.
ANSEL = %00001000 ' AN3/GP4 Analog, GP2-GP0 Digital
The comparator is shut down once again.CMCON0 = 7 ' Comparator off
The state of the GP4 has to be set to input mode using the TRISIO register which can also set the rest of the I/O pins to outputs.TRISIO = %00011000 ' GP4 input, GP2 thru GP0 outputs
The ADCIN command requires some setup parameters to be established such as the ADC resolution, ADC clock source, and sampling time. These are easily done with DEFINE statements. The ADC is setup to run in an 8-bit mode.' Define ADCIN parameters
Define ADC_BITS 8 ' Set number of bits in result
Define ADC_CLOCK 3 ' Set clock source (3=rc)
Define ADC_SAMPLEUS 50 ' Set sampling time in uS
A variable is established to store the ADCIN result.
adval var byte ' Create adval variable to store result
The main label establishes the main loop followed by the ADCIN command line where the CDS is read.
main: ADCIN 3, adval ' Read channel AN3 to adval
After the value of the CDS cell is stored in the adval variable it is compared to the value 150. If it is less than 150 then the LED is off. If the value is greater than 150, then it's dark and the LED is lit using the HIGH command.
If adval > 150 then ' Light LED if in the dark
High GPIO.0 ' Light all LEDs
ELSE
LOW GPIO.0
ENDIF
Another GOTO statement completes the main loop.
goto main ' Loop Back to test potentiometer
Software Listing
| '******************************************************************** | ||
| '* Name | : | CDS.BAS |
| '* Date | : | 8/20/2009 |
| '* Version | : | 1.0 |
| '* Notes | : | |
| '* | : | CHIPAXE-8 Pin 1 at C6 |
| '* | : | Yellow Jumper - a6 to +rail |
| '* | : | Yellow Jumper - j6 to -rail |
| '* | : | Green Jumper - j7 to j12 |
| '* | : | 330 ohm - i12 to i18 |
| '* | : | Read LED - Anode j18, Cathode -rail |
| '* | : | Yellow Jumper - j22 to -rail |
| '* | : | Orange Jumper - f22 to e22 |
| '* | : | Yellow Jumper - b22 to b18 |
| '* | : | White Jumper - b8 to b17 |
| '* | : | 1k ohm - a17 to +rail |
| '* | : | CDS Cell - d17 to d18 |
| '******************************************************************** | ||
ANSEL = %00001000 ' AN3/GP4 Analog, GP2-GP0 Digital
CMCON0 = 7 ' Comparator off
TRISIO = %00011000 ' GP4 input, GP2 thru GP0 outputs
' Define ADCIN parameters
Define ADC_BITS 8 ' Set number of bits in result
Define ADC_CLOCK 3 ' Set clock source (3=rc)
Define ADC_SAMPLEUS 50 ' Set sampling time in uS
adval var byte ' Create adval variable to store result
main:
ADCIN 3, adval ' Read channel AN3 to adval
If adval > 150 then ' Light LED if in the dark
High GPIO.0 ' Light all LEDs
ELSE
LOW GPIO.0
ENDIF
goto main ' Loop Back to test light sensor
Next Steps
Changing the threshold value from 150 to something higher or lower will determine how dark it has to be to light the LED. The limits are 0 to 255 since you used an 8-bit result. Just like the switch project other sensors can replace the light sensor. A thermistor could replace the light sensor to measure temperature. A potentiometer could be used to create a manual interface. If you remove the pull-up resistor then a Sharp GP2D12 Object Detection Sensor that produces a variable output voltage can be read by the analog pin directly for more accurate robotic obstacle detection.
If you have any questions about this project please email us at support@chipaxe.com.
You can get the parts from Jameco.com and the CHIPAXE module from our website www.chipaxe.com.
If you are new to programming then check out the books by Chuck Hellebuyck. The CHIPAXE Team has contracted Chuck to write books based on the CHIPAXE modules and he has many more that we recommend. His website is http://www.elproducts.com/.