Automatic Light Controller

     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.

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     This characteristic is utilised in the present circuit. When the common terminal is connected to the ground, the regulator output is equivalent to the rated voltage, and as soon as the terminal is disconnected from the ground, the output increases up to the input voltage.

     The common terminal is controlled by a transistor, which works as a switch on the terminal. For automatic control of light, a light-dependent resistor (LDR1) is connected to the base of the transistor. In this way, the voltage regulator is able to operate a light bulb automatically as per the ambient light.

     To derive the power supply for the circuit, the 50Hz, 230V AC mains is stepped down by transformer X1 to deliver a secondary output of 12V, 250 mA. The secondary output of the transformer is applied to a bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and fed to the input terminal of the regulator (IC1).

     The common terminal (pin 2) of IC1 is connected to the ground line of the circuit through transistor BC557 (T1). The transistor is biased by R2, R3, VR1 and LDR1. The grounding of IC1 is controlled by transistor T1, while light is sensed by LDR1. Using preset VR1, you can adjust the light-sensing level of transistor T1.

     The output of IC1 is fed to the base of transistor T2 (through resistor R4 and zener diode ZD1) and relay RL1. LED1 connected across the positive and ground supply lines acts as a power-‘on’ indicator.

      Normally, the resistance of LDR1 is low during daytime and high during nighttime. During daytime, when light falls on LDR1, pnp transistor T1 conducts. The common terminal of IC1 connects to the ground and IC1 outputs 6V. As a result, transistor T2 does not conduct and the relay remains de-energised. The light bulb remains ‘off’ as the mains connection is not completed through the relay contacts.

     During nighttime, when no light falls on LDR1, it offers a high resistance at the base junction of transistor T1. So the bias is greatly reduced and T1 doesn’t conduct. Effectively, this removes the common terminal of IC1 from ground and it directs the full input DC to the output. Transistor T2 conducts and the relay energises to light up the bulb as mains connection completes through the relay contacts.

     As LDR1 is in parallel to VR1+R3 combination, it effectively applies only half of the total resistance of the network formed by R3, VR1 and LDR1 to the junction at T1 in total darkness. In bright light, it greatly reduces the total effective resistance at the junction.

     The circuit is simple and can be assembled on a small gene r a l -purpos e PCB. Use a heat-sink for IC1. Make sure that LDR1 and the light bulb are well separated.

     The circuit can be used for streetlights, tubelights or any other home electrical lighting system that needs to be automated.

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AUTOMATIC OFF TIMER FOR DVD PLAYERS

      Are you in the habit of falling asleep while listening to music? If yes, you’ll love this circuit. It will automatically start functioning when you switch off your bedroom light and shall turn your CD player ‘off’ after a predetermined time. In the presence of ambient light, or when you switch on light of the room in the morning, the CD player will again start playing. Unlike the usual timers, you don’t have to set this timer before sleeping.

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      The circuit derives its power directly from the bridge rectifiers. When ‘on’/‘off’ switch S1 is closed, LED1 glows to indicate that the circuit is powered ‘on.’

      In the presence of light, the resistance of the light-dependent resistor (LDR1) is low, so transistor T1 conducts to drive transistor T2 into cutoff state and the timer circuit remains inactive.

     The collector of transistor T2 is connected to reset pin 12 of IC CD4060 (IC1) via signal diode D5. IC CD4060 is a 14-stage ripple counter with a built-in oscillator. The time period of oscillations (t) is determined by capacitor C3 and resistor R8 connected to pins 9 and 10 of IC1, respectively, as

follows:

t=2.3RC

where ‘R’ is the value of resistor R8 and ‘C’ is the value of capacitor C3.

     When transistor T2 is cut-off, its collector voltage is high. So pin 12 of IC1 is high and IC1 is in reset condition.

     When light is switched off, the resistance of LDR1 increases, driving transistor T1 into cut-off state. The collector voltage of transistor T1 goes high to light up LED2 (indicating that the timer circuit is enabled) and transistor T2 starts conducting. As the collector voltage of transistor T2 goes low to around 0.2V, ground potential becomes available at reset pin 12 of IC1. The low state at pin 12 enables the oscillator and it starts counting. LED3 at pin 7 of IC1 starts blinking. Its blinking frequency depends on the R-C components connected between its pins 9 and 10.

     The status of LED2 and LED3 in the circuit with light falling and not falling on LDR1 is given below:

LDR1	Timer LED2	Reset pin 12	Count LED3
Light	Off	High	Off
Dark	On	Low	Blink

     During counting, in case the power fails momentarily, capacitor C2 (1000μF) will provide the necessary power backup for IC1. That is, during the period, pin 3 of IC1 is low. When output pin 3 of IC1 goes high, the relay is energised through transistors T3 and T4 and, at the same time, counting is disabled by the feedback from pins 3 through 11 (clock input) of IC1 via signal diode D7. That is, due to the feedback, output pin 3 remains high unless another high-to-low pulse is received at its reset pin 12.

     After the relay is energised, there will be no AC power in the socket. The glowing of LED5 indicates that your CD player has been switched off.

     The desired ‘off’ time period for the timer circuit can be set by choosing proper values of resistor R8 and capacitor C3. If R8 is 680 kilo-ohms and C3 is 0.22 μF, the ‘off’ time period is around 45 minutes.

     The glowing of LED4 gives the warning that your CD player is going to be switched off shortly. In case you want to extend the timer setting for another round, just press reset switch S2 momentarily. LED4 stops glowing and counting starts again from the initial stage.

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How to Make an Automatic Vehicle Headlight Dipper Dimmer Circuit

The circuit described here can be built and used in your vehicle for an automatic dipping and dimming operation of the headlamps, in response to the intense lights coming from an opposite vehicle headlamps.



More innovative circuits HERE.

You must have come across this irritating situation while driving at night when you find the headlight focus from an opposite vehicle falling straight in your eyes, making things difficult to assess, giving rise to a situation of a collision or some kind of possible accident.

Incidentally, the driver of the opposite vehicle might be going through the same situation due to the headlight focus from your vehicle.

Such situations are normally tackled by using manual dipper switch mechanism, where the driver is prompted to "dip" the focus of his headlight, thus giving the opposite vehicle a chance to adjust his vehicle and also an indication that he too needs to "dip" his vehicle lamps.



However, doing the above operation manually, every now and then can become horribly laborious and troublesome, therefore if some kind of automatic system is Incorporated, can help to save this headache of the driver, especially while he is driving in stressful conditions and on dangerous highways.

How to make an automatic vehicle headlight dipper/dimmer circuit

The following diagram describes a simple yet effective auto head lamp dipper or dimmer circuit. The transistor is used as a comparator, which compares the preset resistance level and the LDR resistance level with reference to ground.

Light falling over the LDR from the headlight of the vehicle coming from the front instantly lowers its resistance and allows more current to flow to the base of the transistor.

The transistor conducts and activates the relay, which in turn flips the contacts such that the host vehicles headlamps gets connected with the dipper filament, changing its intensity.

The whole circuit may be enclosed in a small box and installed somewhere near the drivers dashboard area, however the LDR needs to be wired and placed out of the enclosure, in some corner of the wind shield, so that it is able to "see the light from the opposite vehicles just as the driver would see them.

Parts List

R1 = 1K,
P1 = 10 K,
LDR = With resistance @ around 10 to 50 K when illuminated in daylight (under shade).
T1 = BC547,
D1 = 1N4007
Relay = coil 400 Ohms, DPDT, 12 volts    

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