The controlling of street lights is usually maintained by an electrical department’s technician on several occasions. This is not only precarious but also sometimes results in wastage of power because of the negligence or unusual circumstances on part of the technician in operating the street lights on and off. Not only in case of street lights, even for controlling the home appliances like garden or outdoor lights, we can utilize the light activated switch for automatic switching of the lights based on daylight’s intensity by using a light sensor.
What is a Light Activated Switch?
Light Activated Switch is a simple electrical project circuit by which we can switch on and off the electrical load appliances like lights, fans, coolers, air conditioners, street lights, etc., automatically based on the day-light intensity instead of manually operating the switches.
By using this method, manpower can be reduced to a great extent. In case of the street lights erected on highways it is not an easy task to manually control them, but, if uncontrolled, the chances of power wastage would increase. To get rid of this situation, the implementation of automatic light switch by using a light sensor that switches lights automatically on and off is the best option.
There are different types of light sensors available, but for better efficiency of the system, LDR (Light Dependent Resistor) is used as sensor light switch in this light activated switch kit. The LDR sensor has some special features as it changes its resistance with the change in the daylight intensity. LDR is rugged in nature, hence can be used even in dirty and rough external environments like outdoor lighting of homes and in automatic street lights as well.
Light Dependent Resistor or photoresistor or photocell is a variable resistor controlled by light intensity. It is made of high resistance semiconductor material like Cadmium Sulphide that exhibits photoconductivity.
In dark, the LDR has very high resistance of around a few MΩ (Mega Ohms) and in the light, its resistance decreases to around a few 100Ω (hundred Ohms). Hence, its resistance is inversely proportional to the light intensity.
As shown in the above figure, the LDR consists of a wave-shaped design on the top surface of it with two terminals similar to a general resistor, and the graph represents the inverse proportionality of the LDR with the light intensity.
The major drawback of the LDR is that, it is sensitive to even artificial light.
Instead of a single transistor, a back-to-back connection of two transistors is used in a circuit which is termed as Darlington pair; this can also be considered as a single transistor with a very high current gained compared to general transistor. The input to the load given through a Darlington pair can be derived as a product of the input current and gain of the transistor. We know that, for switching on a transistor, the base voltage must be greater than 0.7v – but, for a Darlington pair, this base voltage must be 1.4v as there are two transistors need to be switched on.
A relay plays a vital role in the main circuit for activating the load or connecting the load (which is intended to be controlled by using LDR) to the main circuit as well as to the AC mains. The relay consists of a coil which gets energized if it gets enough supply depending on the rating of the relay.
Light Activated Switch’s Working Functionality
A light activated switch kit is an electronic kit consisting of LDR, Darlington pair and Relay as connected in the main circuit, as shown below. A supply of 230v AC is fed to the load (in this case, a lamp).
The circuit shown below requires a DC voltage, which can be supplied from a battery or by using a bridge rectifier instead of the battery. This bridge rectifier converts the 230v AC supply into a 6v DC. The bridge rectifier uses a step-down transformer to step-down the 230v into 12v. The diodes connected in the form of a bridge convert the 12v AC into 12v DC, and the IC7806 voltage regulator converts the 12v DC into 6v DC, and then, this is supplied to the circuit. A continuous 230v AC supply is maintained for both the load and the bridge rectifier.
During the day time, the LDR has very-low resistance of around a few 100Ω, and then the entire supply is passed through the LDR and gets grounded through the resistor and variable resistor as shown in the circuit. This is due to the fact that the resistance offered by the LDR is less compared to the remaining path (Relay and Darlington pair) of the circuit. As we know that the principle current always chooses the low resistance path to flow. Hence, the relay coil does not get energized as it has not got enough supply. Thus, the load remains switched off during the daylight.
Similarly, during the night time (when the daylight intensity is very less), the LDR resistance becomes very high: around a few Mega ohms (approximately 20MΩ). Thus LDR offers a very-high resistance (almost an open circuit type), and hence, opposes the flow of current. Again, according to the principle of current, by choosing low-resistance path, no current flows through the LDR, and thus, the current chooses an alternate path to flow such that it causes the Darlington pair base voltage to increase more than 1.4v. Thus, the Darlington pair gets activated, and then the relay coil gets energized, and thus, turns the load to switch on.
Thus, if the intensity of the daylight falling on the LDR of the light-activated switch is high (during the day time/morning time), then the load will be turned off – and, if the intensity of day light is low (during the night time), the load will be turned on.
Applications of Light Activated Switch
- Useful for automatic outdoor lighting or garden lighting at home.
- Useful for automatic switching of street lights.
- Useful for switching the hoardings on and off automatically.
- Useful for self-switching operation of displaying title hoardings of companies.
- Useful as a light detector circuit.
- Useful as a dark activated switch.
Example: Project by using Light Activated Switch (LDR):
Electronic Eye Controlled Security System
The Electronic Eye Controlled Security System project is based on the electronic eye, which utilizes LDR sensor. The circuit shown below consists of supply circuit with step-down transformer, bridge rectifier, battery and regulator, CD4060 is a ripple carry binary counter connected with transistors and these are connected to relay and buzzer. AC supply is maintained at load which is connected to the relay.
The 14 stage ripple binary counter is used to sense light intensity through the LDR. If light falls on LDR (light intensity is more) then its resistance will be very high as we studied in this article. Hence the transistors are not activated; no supply will be there for relay and buzzer.
If light intensity decreases or some darkness falls on LDR (when a person causes to fall dark shadow on LDR of this circuit) then, its resistance will decrease drastically and thus the ripple counter activates the transistors further these transistors activates buzzer and load (light) connected to relay indicating some theft.
This project can be implemented for security at cash boxes, lockers at banks, homes, etc where security is intended.
Finally, this article concludes with a brief discussion about the light activated switch along with the working of its circuit and its major blocks in brief and also listed a few applications of it too. One application of LDR as Electronic Eye Controlled Security System project is explained above. If you want more information, please post your queries by commenting below.
- LDR Resistance Variation With Light by circuitstoday
- Light Intensity Vs LDR Resistance by electronics-tutorials
- Light Activated Switch Circuit by allaboutcircuits
- Light Activated Switch by michaelsharris