Automatic lamps with photocell

This is a photocell circuit for detecting the light intensity. At full light the resistance of the photocell will be few ten ohms and at darkness it will rise to several hundred ohms. IC1 Op amp uA741 is wired as a comparator here. At darkness the resistance of photocell increases and so the voltage at the inverting input of the IC1 will be less than the reference voltage at the non inverting input. The output of the IC1 goes to positive saturation and it switches ON the transistor to activate the relay. By this way the lamp connected through the relay contact glows. The diode D1 works as a freewheeling diode.



A light sensor (photo detector) that varies its resistance between its two terminals based on the amount of photons (light) it receives. Used for photographic light meters, automatic on-at-dusk street lights and other light-sensitive applications, it is also called a “light dependent resistor” (LDR) and “photo resistor.”

The photocell’s semiconductor material is typically cadmium sulfide (CdS), but other elements are also used. Photocells and photo diodes are used for similar applications; however, the photocell passes current bi-directionally, whereas the photo diode is unidirectional.

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Simple Automatic Headlight Brightness Switch Circuit Diagram

Going by car the highway with your high-beam headlights can actually increase your visibility, but can be a blinding hazard for other drivers. This easy circuit can be connected into your headlight swap to provide automatic swaping between high and reduced beam headlights when there is oncoming traffic. It does this by feeling the lights of that traffic. In this way, you can propel securely with your high-beams on without blinding other drivers. 

 Automatic Headlight Brightness Switch Circuit Diagram

1. Q1 should me mounted in such a way so it points toward the front of the car with a clear line of site. 
2. Suitable places are on the dashboard, in the front grill, etc.
3. Adjust all the pots for proper response by testing on a deserted road.
4. S1 enables and disables the circuit.B1 is, obviously, in the car already.
5. Before you try to connect this circuit, get a wiring diagram for your car. 
6. Some auto manufacturers do weird things with wiring.
7. Connection A goes to the high beam circuit, B goes to the headlight switch common and C connects to the low beam circuit.
   

Part	Total Qty.	Description	Substitutions
R1	1	5K 1/4W Resistor
R2, R3, R4	3	5K Pot
Q1	1	NPN Phototransistor
Q2	1	2N3906 PNP Transistor
K1	1	Low Current 12V SPST Relay
K2	1	High Current 12V SPDT Relay
S1	1	SPST Switch
B1	1	Car Battery
MISC	1	Case, wire, board, knobs for pots

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Automatic light activated switch circuit

The circuit can be used for switching OFF a particular lamp or group of lamps in response to the varying ambient light levels. The unit once built can be used for switching OFF a lamp when dawn breaks and switching it ON when dusk sets in. The power supply is a standard transformer, bridge, capacitor network, which supplies a clean DC to the circuit for executing the proposed actions.

Automatic light activated switch circuit schematic

The LDR must be placed outside the box, meaning its sensing surface should be exposed toward the ambient area from where the light level is required to be sensed. The circuit can be used as an automatic street light controller system or a simple light activated switch.

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