Light Sensing

Light Alarm

This is a simple Photo-sensing circuit. When light hits the photo-transistor, triggers the 4011 that drives a loudspeaker. Easy to make and can be very usefull too...

CAR STOP LIGHT

Capacitor Cl is connected continuously through the supply of 3 volts to 10 megohm resistor Rl. The capacitor is charged (relatively slowly) to 3 volts. The wink SWI is closed, it connects the charged capacitor (Cl) in series with C2 and R2. The capacitor C2 begins to charge, by placing a positive voltage during the gate of the SCR and turn it on. The two parallel "self-flashing" bulbs I1 and 12 on Tum. They flash on and off the SCR and the circuit is turned off until the vehicle is driven off the switch and C1 can recharge. ..

Photo detector using LED

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. ..

Schmitt Trigger Photo switch

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. The photo cell should be shielded from the lamp to prevent feedback and is usually mounted above the light on top of a reflector and pointed upward at the sky so the lamp light does not strike the photo cell and switch off the lamp...

Optical switch detector with 555

When the phototransistor is stroken by IR light it conducts and the voltage between the 1Mohm resistor(arbitrary) and the phototrans drops from VCC to lower values. When the voltage drops lower than VCC/3 the 555 is triggered and goes high (from 0 TO VCC). The amount of light that strike the phototrans necessary to bring his collector to VCC/3 is determined by the resistor (Vdrop = Icollector * R , so , if Vdrop= 2*VCC/3, the resistance needed to set the threshold on current is R=2*VCC/(Icollector*3)). ..

Solar powered robot

Make your own solar powered robot at home using things you probably already have. Transistors, resistor, capacitor, solar battery and flashing LED are available at any electronic store, if you don't already have them. Solar cells out of calculators work as well, you might want to use more than one since they are really small. If you do, connect the positive wire from each solar battery together. A small motor should be used like the ones in walkmans. Solder everything together and watch it come alive. Put it under a light and let it charge, it should take couple seconds for each burst of energy. Works best in sunlight...

This circuit is a simple visible light or IR detector for testing any equipment which outputs light. The circuit can be also used for measuring ambient light levels. This circuit is based on one photodiode which is used in photocell mode so that the photodiode generates a voltage and current signal when it get light...

Light Barrier Detector

This simple circuit using a single transistor turns ON the relay when light falls on the LDR. The potentiometer is adjusted for the required sensitivity. The power supply is 6V. Be careful about the impedance of the relay. Its impedance should not be less that 60ohm. Its working can be explained as follows: With the light falling on the LDR,its resistance is low and the transistor is saturated and turns the relay ON...

Daylight alarm with 555

The circuit presented here wakes you up with a loud alarm at the break of the daylight. Once again the 555 timer is used here. It is working as an astable multivibrator at a frequency of about 1kHz. The circuit's operation can be explained as follows: When no light falls on the LDR, the transistor is pulled high by the variable resistor. Hence the transistor is OFF and the reset pin of the 555 is pulled low. Due the this the 555 is reset...

Sun Light Chaser

With RFT illuminated, point R goes to 0 volt and LM runs. With LFT illuminated, point R goes to 0 volt and RM runs. With RFT and/or LFT not illuminated, points R/L go to +9 volt; P1 and P2 controls threshold point when RM/LM stop or start running. Remember to seperate FT1 and FT2 with a vertical cardboard wall in order to produce a shadow over the other FT whenthe first is illuminated by a lateral incident light...

Light/dark relay switch

This kit is the most basic, practical circuit to build using an LDR to turn on a relay. The two transistors connected as a Darlington pair give the circuit enough sensitivity, while the trimpot give sensitivity adjustment. The switching point of the relay is dependent on the supply voltage and temperature. This circuit is satisfactory if the changes in light level to be detected are large and the transition is quick - for example, a person walking past a doorway. But an inherent problem of the circuit is chattering of the relay for slowly changing light levels just at the transistion point between turning on/odd and vice versa. ..

Music Light Show

The circuit connects to the speaker outputs of your stereo or to the back of your speaker. The music passes through the transformer and the volume level is adjusted by the 5k ohm pot. Each light bulb is turn on by a different frequency of sound based on the resistor & capacitor combination in the gate circuit of the SCR. If the resistors R1, R2, or R3 are changed, the frequency of sound that will trigger the SCR will change. The isolation transformer is for protection...

Sun - Up Alarm with 4011

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...

Simple Day/Night switch (40106B)

This is a simple circuit that does the day/night switchings you have in mind. POT1 is used to set the light level at which the circuit switches from enable to disable...

Power Failure Light

This is a very basic power failure lighting circuit based around a relay. This simple circuit has many uses, from lighting up rooms and walkways in the case of a power failure, to monitoring and security uses. There are many different power failure circuits out there based on 555 timers or transistors but they all have different problems including limited input voltage, price and complexity, and poor backup power. This unit has been designed to work with mains power all the way down to 5 volts, and power 3 LEDs to provide light for a hallway or a child's room in the event of power failure. The PCB includes many simple add-ons and modifications too. ..