Phototransistor Light Sensor

A phototransistor allows more current through a circuit when as more light strikes the light collecting surface under its clear dome top.  One type of sensing circuit is a supply voltage with the phototransistor connected in series with a resistor.  The voltage between the resistor and the phototransistor is proportional to the current the phototransistor allows through.  With more light, the voltage increases, and with less light, the voltage decreases.

Parts

(1) Phototransistor 350-00029 
(1) 220 Ω resistor (red-red-brown-gold)
(1) 10 kΩ resistor (brown-black-orange-gold)
(1) 2 kΩ resistor (red-black-red-gold)
Jumper wire: Red

Schematic

Wiring

Script

Tests

Direct sunlight is too bright for this sensor.  Close the blinds.  Check the voltage measurement on the micro:bit module’s 5×5 LED matrix display without any shade.  Then, gradually increase the level of shade and monitor the voltage as it drops.  In rooms with brighter light, replace the 10 kΩ resistor with the 2 kΩ resistor.