• SINGLE IC FORMS SENSITIVE MODULATED LIGHT RECEIVER
  The circuit uses a very inexpensive C-MOS IC that is connected to a small photodiode. Using a unique inductive feedback network, the circuit provides high sensitivity under high ambient light conditions. It is a great circuit when you want to extend the range of an optical remote control transmitter.

• TIME TO DUST INDICATOR
  I thought about this circuit when I heard that a lot cleaning personal in hotels were either dusting rooms more often than necessary or not enough. I have not yet built and tested this circuit completely but in concept it should work. The circuit draws very low current from a +3v battery and could be housed in a package similar to a small ashtray. The assembly might be placed in a suitable out of the way area to collect dust. It would alert a maid when it was time to dust the room. The circuit detects dust with an infrared LED that is pulsed so its light shines onto a smooth flat plate. Any dust settling onto the surface is detected by a phototransistor, mounted at a 90-degree angle from the LED. When the dust reaches a particular level, sufficient light is reflected into the phototransistor to change the logic state of the circuit to an alarm condition. The alarm output could be connected to a beeper, a flashing LED or to one of the LED flashing circuits in this hobby circuit section.

• Xenon Lamp Flash Detector
  This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp.  When connected to a flip/flop it can serve as an on on/off Controller

• XENON LAMP FLASH DETECTOR
  This circuit uses a small 2.5mm square photo diode in conjunction with a 100mH coil to detect the short light flashes from a xenon lamp.  The coil makes the circuit immune to normal room lights.  Its 10mv sensitivity can detect light flashes from a range of over 100 feet.  Reflections from a room’s walls and ceiling is usually enough to trigger the circuit.  The entire circuit draws only 3 Microamps from a 6 to 9 volt battery.

Sensing Light with a Programmable Gain Amplifier:  Microchip Application Note - Published 28-Mar-03  (app note added 6/06)

Simple light sensor adapter for Oscilloscope:  (Electronic circuit added 4/05)

Solar Engine Circuit:  Utra Efficient circuit runs small DC motor in a poorly lit room   (added 2/05)

Solar Powered LED Garden Light :  Reverse engineered circuit diagram of a popular mass produced solar light. The light automatically turns on when there is no light falling upon the solar cell, so the parts count is minimal. The charging circiut is not particularly efficient since the battery must charge through a plain diode. (added 2/05)

Sun-Up Alarm:  The Sun-Up Alarm can be used to provide a audible alarm for when the sun comes up or it can be used in a dark area and detect when a light comes on. It can also be used to detect a light beam, headlights etc. The circuit works as follows. The phototransistor is very sensitive to light. (Any phototransistor will work fine) The sun shining on this device will provide a high to one of the NAND gates. This will cause another NAND gate to oscillate which will drive another gate to output a 100hz tone. The transistor provides drive for the speaker. (added 09/05)

Two LEDs Perform Rudimentary Spectral Analysis:  09/01/94 EDN-Design Ideas / (Electronic Circuit diagram added 03/03) A pair of LEDs having different wavelengths can supply the stimulus for a rudimentary spectral analysis during quality-control sorting operations. By measuring the reflectance of each LED, the circuit in Fig 1 can provide information about the spectral response of the object under test. The circuit's operation is akin to the human eye's using three receptors to deduce information about the entire visible spectrum....  

Using the LM555 Timer with Photocell Detectors:  (added 8/03)

Visible Light Photo Detector Circuits (E+F):  (Circuit / schematic design added 6/06)

Visible Light Sensor (AN-28):  Application Note 28 from Microsem  (app note added 6/06)