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Picaxe LDR and LCD

This is the AQA version closing after June 2019. Visit the the version for Eduqas instead .

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The LDR

The LDR and the 1kΩ resistor form a voltage divider.
The output from this voltage divider is connected to the PIC chip ADC0 input.
ADC0 measures the input voltage and converts it to a number between 000 and 255.

A Subroutine to Convert 123 to "123"

This is a tricky concept for a programming beginner.

The LDR sensor produces a voltage.
The ADC converts this into a binary number like 123 = 01111011
To display that as text, you need a "1" character, a "2" character and a "3" character.
Writing the code to convert this is tricky.
PICAXE BASIC has a built-in bintoascii subroutine. (Binary to ASCII)
The ASCII code represents all the usual letters, numbers and punctuation marks on a keyboard for American English.

Build the Circuit

Build this circuit and copy and paste the code below.

PX LDR LYT.jpg

The Code

    setfreq m4        ; 4MHz Internal Clock

    call    blank
start: 
    call    readadc0  ; readadc0 is NOT an AQA instruction
                      ; Read the ADC and store the value into B0.

    DEBUG             ; Comment this out if everything works OK

    call    displayb0

    jmp     start
	
; ==========================================
; ===== These are NOT AQA instructions =====
; ==========================================
; bintoascii converts a number like 135 into
; "1", "3", "5". These ASCII codes can be 
; copied to the LCD display.
; ==========================================
displayb0:
    bintoascii b0,b1,b2,b3
    serout 7, N2400, (254, 128)   ' Blank First Line
    serout 7, N2400, ("LDR = ",b1,b2,b3,"     ")
    ret
; ==========================================
; A subroutine to blank both LCD lines.
; ==========================================
blank:
    serout 7, N2400, (254, 128)   ' Blank First Line
    serout 7, N2400, ("                ")
    serout 7, N2400, (254, 192)   ' Blank Second Line
    serout 7, N2400, ("                ")
    ret
; ==========================================

This is not an accurate light meter but it demonstrates the principle.

This has only two "active devices" so, as it stands, it is not suitable for an A2 project.

Making an accurate light meter might be an ideal project.