LED TESTER

Published

Introduction: LED TESTER

This device will allow you to:

1. Test low power LEDs, including surface mount types,

2. Display its’ intrinsic voltage drop (VLED),

3. Adjust its’ brightness by changing the current through it (iLED),

4. Choose any voltage up to 9V that you plan to use the LED for in a project (Vtarget), and

5. Based on the above parameters, will display the proper resistance to use for that LED (RLED).

6. Detect shorted contacts during testing.

7. Control contrast/brightness of the LCD.

Step 1: Giving Credit Where Due

Firstly, complete credit for this idea and core circuitry
goes to the author of robotroomTM (please see his original article at http://www.robotroom.com/LED-Tester-Pro-1.html). I adapted his idea for use with the PIC 12F683, using the very easy (and powerful) Great Cow Basic for coding. To accommodate the low pin count on the PIC, I used the 2-wire LCD circuit by Myke Predko (see http://www.hobbyprojects.com/quick_circuits_reference/microcontroller_circuits/2_wire_lcd_interface_using_PIC16CF84.html).

Step 2: Materials Needed and Parts List

Eagle for Schematic & Layout

Great Cow Basic for coding

Superfine hobby saw

Parts list:

Part Value

C1 0.1uF CAPACITOR

C2 1uF POLARIZED CAPACITOR

C3 0.1uF CAPACITOR

C4 0.1uF CAPACITOR

C6 0.1uF CAPACITOR

C7 1uF POLARIZED CAPACITOR

C8 0.1uF CAPACITOR

D2 1N914 DIODE

IC1 PIC12F683 PIC12F683P

IC2 74LS174N Hex D type FLIP FLOP, clear

R1 1K RESISTOR

R2 10K POTENTIOMETER

R3 500 POTENTIOMETER

R4 10K POTENTIOMETER

R5 47 RESISTOR

R6 10K RESISTOR

R7 10K RESISTOR

R8 47 RESISTOR

R9 100 POTENTIOMETER

FEMALE HEADERS FOR LCD, EXTERNAL LEDS

SMALL SPDT SWITCH FOR ON/OFF

LM317 VOLTAGE REGULATOR

MCP1702-5V VOLTAGE REGULATOR

BACKLIT 8X2 LCD WITH 16 PIN MALE HEADER

Step 3: Schematic

Step 4: Layout

Step 5: CODE

;LED tester, derived from Robot Room project
;original project from http://www.robotroom.com/LED-Tester-Pro-1.html
;Uses 2 wire setup for LCD
;uses PIC 12F683
;Since need 3 analog inputs, will use An0,AN1,AN2, and
;will use GP4 & GP5 for the 2 wire output to the LCD.
;-----------------------------------------------------------------------
;Hardware Setup:
;INPUTS  - LED high from circuit to AN0 (pin7)
;          LED low from circuit to
AN1 (pin 6); this is the current sense resistor
;          10K Target voltage pot
wiper to AN2 (pin 5), ends to +5V & GND
;          GP3 (pin 4) to +5V so
not floating.
;OUTPUTS - GP4 (pin 3) to LCD DAT
;          GP5 (pin 2) to LCD
CLOCK
;-----------------------------------------------------------------------
;Chip Settings
#chip 12F683,8
#config MCLRE=OFF          ;no
external reset
;2 wire LCD setup
#define LCD_IO 2
#define LCD_DB    GPIO.4   ;shift register data on
GP4, pin 3
#define LCD_CB    GPIO.5   ;shift register clock on
GP5, pin 2
;Main Program
Dim ledhigh, ledlow, Vtarget as long
Dim Vled, Iled, Rled as word
;inputs
#define high AN0
dir AN0 in
#define low AN1
dir AN1 in
#define target AN2
dir AN2 in
;Define custom characters arrays ("ma" and "ohm")
dim index as byte
          ;custom character
bytes
lcdcmd 64
                     ;go to base address of character 0 in
CGRAM,
                                       ;writing continues for
subsequent characters
Dim AA(8)
AA() = 0x0A,0x15,0x11,0x04,0x0A,0x0E,0x11,0x00
                           ;"ma"
character, written into CG RAM address 64 (= ASCII 0)
gosub write
AA() = 0x00,0x00,0x0E,0x11,0x11,0x0A,0x1B,0x00
                           ;"ohm"
character, written into CG RAM address 72 (= ASCII 1)
gosub write
goto resume
;Write each character into CGRAM of LCD circuitry----------------------
write:
Set LCD_RS On
for index = 1 to 8
   LCD2_NIBBLEOUT Swap4(AA(index))
   LCD2_NIBBLEOUT AA(index)
next
return
resume:
;Main program loop
do
;-----scale inputs
ledhigh  = ReadAD10(high)
ledhigh = ledhigh * 5000
ledhigh = ledhigh / 1023
ledlow   = ReadAD10(low)
ledlow = ledlow * 5000
ledlow = ledlow / 1023
Vtarget  = ReadAD10(target)
Vtarget = Vtarget * 9000
Vtarget = Vtarget / 1023
;-----calculated results:
Vled = (ledhigh - ledlow) * 2
Iled = ledlow/47
ledlow = ledlow * 10
ledlow = ledlow/47
If  (ledlow % 10) >= 5 then Iled ++
ledlow = ledlow * 47
ledlow = ledlow / 10
Rled = (Vtarget - Vled) / Iled
;Printing to display:
 If Vled/1000 = 0 then
   locate 0,0 : print "SHORTED "
   locate 1,0 : print "CONTACTS"
   goto resume
 End if
 If ledlow < 50 then
   locate 0,0 : print " Touch  "
   locate 1,0 : print "  LED   "
   goto resume
 End if
 Locate 0,0
 Print Vled/1000 : Print "."
 Print (Vled % 1000)/100 : Print "V"
 locate 0,6
  If Iled < 10 then
     locate 0,4 : print "  " :print
Iled
  else
     locate 0,4 : print " " :print Iled
  End if
 locate 0,7 : LCDWriteChar 0
 Locate 1,0
 Print Vtarget/1000 : Print "."
 Print (Vtarget % 1000)/100 : Print " "
 Locate 1,5
  If Rled < 100 then
     locate 1,4 : print " " : print Rled
  else
     locate 1,4 : Print Rled
  End if
 Locate 1,7 : LCDWriteChar 1
loop

Step 6: Screen Shot

Step 7: Construction Notes

Construction notes:

Þ Solder the wire connections (red, yellow, and pink in the layout) first, then the components which will be covered by the LCD (see photograph).

Þ The 2 copper pads are actually a single piece of PCB with a superfine slit in the copper layer to separate them electrically. The pads are soldered by wires to underlying PCB traces. The pads are also mechanically connected to the underlying board by 2 screws; this allows for replacement if the pads wear out from repeated use.

Þ Note the 16 pin female header accepts the LCD with presoldered 16 pin male header.

Þ 6 pin female header between pads for testing LEDs via wires if desired.

Step 8: Notes About Battery Power


- A 9V battery will work for most LEDs until it drops below ~6.5V.

- Use a fresh battery for blue LEDs; won’t work if drops below ~8.2V.

- Need new battery if cannot raise current to desired level with adjustment of iLED pot, or current drops when test a blue LED.


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