Table of contents

Details

This module measures light intensity and shows it on a scale from 0 to 782 on the LED display. By multiplying that value with 5, and then divide by 127 you get the light intensity in µW/cm2. If the light intensity rises over or falls below (configurable) the set point; the output activates, with a hysteresis of +-25. The module has two outputs; a constant and a pulse. The constant output can be used to drive e.g. a relay, while the pulse can communicate with other equipment. Powered by: 9-24V.

Set points

The set point is adjusted using the (on)-off-(on) switches, when adjusting; the display shows this instead of the real value. It is stored in EEPROM making it non-volatile, meaning that it is not lost if the module is without power.

Video

LED-Display

In reality only one digit is shown at any given time, but by cycling through them in high speed the human eye sees three stable digits.

LED segment display

When DP x is lit:

  1. The set value is shown, not the real value.
  2. Output is active.
  3. Set point has not yet been saved to EEPROM.

I/O

Inputs

  1. PC.0 Light sensor, analog value
  2. PC.1 Set point up
  3. PC.2 Set point down
  4. PC.3 Active when over or under set point

Calculations

Light intensity

ADC × 5   µW
------- ≈ --- ± 0.039
  127     cm²

Outputs

  1. PB.0 LED-display digit 1
  2. PB.1 LED-display digit 2
  3. PB.2 LED-display digit 3
  4. PB.3 Output constant
  5. PB.4 Output pulse

LED-Display I/O

  1. PortD.0 LED-display A
  2. PortD.1 LED-display B
  3. PortD.2 LED-display C
  4. PortD.3 LED-display D
  5. PortD.4 LED-display E
  6. PortD.5 LED-display F
  7. PortD.6 LED-display G
  8. PortD.7 LED-display DP
LED segment display pin-outs
LED segment display binary values

D-Sub 9-pin

  1. 9-15V +
  2. GND
  3. Output constant
  4. Output pulse

Photos

Testing LED segment display logic
Controller board
Mounting controller board inside module
Inside light sensor
Inside light sensor
Light sensor TSL250R poking out
Light sensor finished and in use

Source code

Schematic drawing

Light sensor schematics

Parts list

  • 1 × AVR ATmega8-16PU, DIL-28, 16 MHz, 23 I/Os
  • 1 × Capacitor, aluminium electrolytic, 10 µF, 25V
  • 2 × Capacitor, aluminium electrolytic, 47 µF, 25V
  • 1 × Capacitor, ceramic, 1 nF, 100V
  • 2 m Control cable, 4-cores, 0.25mm2, 250 V, Ø 4.6mm
  • 1 × D-sub soldering cups, 9 pin male
  • 1 × DIL socket, 28-pin, 7.62mm
  • 1 × Diode, rectifier, 1 A, 400V, 1N4004
  • 1 × Enclosure, plastic (1591 FL), 120x65x40mm, flange
  • 1 × Fuse 5x20 mm, 400 mA, fast-acting
  • 1 × Fuse holder, open, PCB, 5x20mm
  • 1 × Fuse holder, open, PCB, Protective cover
  • 1 × Heatsink, 27.3K/W, 19mm, attachable, TO220
  • 1 × LED display, 3x7-segment, 9.2mm, Green
  • 1 × Light-to-voltage sensor, TSL 250R, 127mV / (μW/cm2)
  • 32 cm2 PCB, stripboard, 100x160mm, 160cm2
  • 11 × Resistor, carbon film, 0.25W, 330 Ω, 5%
  • 6 × Resistor, carbon film, 0.25W, 4.7 kΩ, 5%
  • 3 × Resistor, carbon film, 0.25W, 10 kΩ, 5%
  • 3 × Spacer, round unthreaded, 3mm, Ø6mm, Delrin
  • 22 × Straight pin header, female, Single row, 2.54mm
  • 22 × Straight pin header, male, Single row, 2.54mm
  • 1 × Switch, toggle, 1-pole, micro, (on)-off-(on)
  • 1 × Switch, toggle, 1-pole, micro, on-on
  • 5 × Transistor, NPN, 100 mA, 45V, 0.5W, BC547B
  • 1 × Voltage regulator +5V, 2 A, L78S05CV