Light Dependent Switch Using ORP12 Photocell

Figure 1 

This light dependent switch (dark activated switch) was constructed around the operation of a ORP12 photocell where light is the most important factor.

• Photocell – a type of resistor wherein the current flows more freely with the presence of light in the cell while resistance increases significantly with the absence of light in the cell; a vacuum tube with a concave cathode called emitter, made from a material that emits electrons easily
• ORP12 – is a light dependent resistor made of cadmium sulphide photoconductive cells

The ORP12 photocells are designed to sense light in the spectral range from 515 to 730 nm and have a peak wavelength of 515 nm. These light dependent resistors offer high sensitivity and high stability. The resistance of the ORP12 can be as low as 80 ohm in bright light while the resistance increases by over 1M ohm in darkness of around 50 lux. The function of the UA741 op-amp is to note the difference the pins 2 and 3. The output of the op-amp will reach 2 volts when the relay turns off with the adjustment of the control voltage at resistor R1.

The difference in the input voltage is amplified by the op-amp while the resistance of the photocell increases because of deficiency in the source of light. Because of this, the output will fluctuate towards full supply and power the relay and transistor. The circuit switches on and off with slightly different light levels because of the small amount of hysteresis produced by the 270K ohm resistor to get rid of relay chatter due to unwanted rapid switching. This is because hysteresis can be used to filter signals so that the output reacts slowly by taking history into account.

Due to wide variations in CdS photocells, it is usually best to install the cell and then measure its resistance under normal lighting conditions.

The photocell is at the middle of the many applications of the photoelectric effect which includes the flash of a camera or light camera exposure, used as photocell detector in medicine, used in garage door openers or shutter controls, used in movie films, light controller switches, brightness controls, night lights, low light level detection, light dimmers, auto headlights, sun sensors for air conditioners, security systems, office machines, audio equipment non-contact volume control, other visible light sensor applications, used in robots and toys, and automatic street light controllers.

Source:www.zen22142.zen.co.uk/Circuits/Switching/darksw.htm