Light sensor schematics

This device allows one or more lamps to illuminate at sunset and turn off at dawn.Q1 and Q2 form a trigger device for the SCR, providing short pulses at 100Hz frequency. Pulse duration is set by R2 and C1.When the light hits R1, the photo resistor assumes a very low resistance value, almost shorting C1 and preventing circuit operation. When R1 is in the dark, its resistance value becomes very high thus enabling circuit operation.... [read more]

This circuit shows how to use an ordinary LED as a light sensor. It makes use of the photovoltaic voltage developed across the LED when it is exposed to light. LEDs are cheaper than photodiodes and come with a built-in filter, which is useful when the application involves colour discrimination. The photo-voltage of a red LED (its bandgap voltage) is typically about 2V. The source impedance of this voltage is about 800MΩ in daylight, rising to infinity in darkness. A TL071 JFET input op amp is used to amplify and buffer this extremely high impedance signal.... [read more]

The circuit can be divided into inverter and charger sections. The inverter section is built around timer NE555, while the charger section is built around 3-terminal adjustable regulator LM317. In the inverter section, NE555 is wired as an astable multivibrator that produces a 15kHz squarewave. Output pin 3 of IC 555 is connected to the Darlington pair formed by transistors SL100 (T1) and 2N3055 (T2) via resistor R4.... [read more]

Voltage regulator ICs (78xx series) provide a steady output voltage, as against a widely fluctuating input supply, when the common terminal is grounded. Any voltage about zero volt (ground) connected in the common terminal is added to the output voltage. That means the increase in the common terminal voltage is reflected at the output. On the other hand, if the common terminal is disconnected from the ground, the full input voltage is available at the output. This characteristic is utilised in the present circuit.... [read more]

The finish line circuit detects the first of three cars to cross the line and illuminates a 25 watt 120 VAC lamp indicating the winning lane. Three photo transistors are used which can be embedded into the track with a light shining down onto the finish line so that as the car crosses over the sensor, the light is blocked, activating the relay and lighting the lamp for the appropriate track. The light source should be an incandescent type, florescent lights may not work due to low infra-red content. The circuit was tested using a 100 watt incandescent light fixture about 3 feet above the photo transistors.... [read more]

This is basically a Schmitt Trigger circuit which receives input from a cadmium sulfide photo cell and controls a relay that can be used to switch off and on a street lamp at dawn and dusk. I have built the circuit with a 120 ohm/12 volt relay and monitored performance using a lamp dimmer, but did not connect the relay to an outside light.... [read more]

The 555 is proved to be the most versatile and ubiquitous IC all over the world.This is a possible use: simple inverting schmitt trigger.... [read more]

This handy little circuit can tell the difference between darkness and light, making it very useful for switching on and off signs, porch lights or other things when it gets dark or light.... [read more]

Here's a circuit that takes advantage of the photo-voltaic voltage of an ordinary LED. The LED voltage is buffered by a junction FET transistor and then applied to the inverting input of an op-amp with a gain of about 20. This produces a change of about 5 volts at the output from darkness to bright light. The 100K potentiometer can be set so that the output is around 7 volts in darkness and falls to about 2 volts in bright light.... [read more]