# Automatic Street Light using LDR Project ( Need someone's assistance)

#### Arthur Rey

Mar 16, 2017
2
Good day everyone, firstly i am a fresh college student taking up civil engineering profession. My professor wants us to make an automatic street light with the use of ldr, the problem is i don't know how it works so i ask some help with electronics engineering student here in our school, they gave me a circuit design for the pcb and power supply. I can't understand a thing on it but we were able to make the circuit with the help of my other group mates. Now i'd like to know how it works but my classmates can't explain it completely so if anyone is willing to explain how the design of the circuit works i would be really thankful. There are also a lot of question i'd like to be answered like if how the transformer transform 220volts into 9volts, what formula should i use?

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#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,760
From left to right:
To the left is your AC input, a voltage somehere between 9 V and 12 V, typically.
The 4 diodes form a bridge rectifier to convert AC input into DC.
The capacitor is a smoothing capacitor to create a smooth Dc from the pulsating Dc after the bridge rectifier.
The LED is useless. It is the wrong way round and lacks a current limiting resistor.
The component labeled 7806 is a linear voltage regulator. It creates a regulated 6 V DC output from the unregulated input. Note that the typical application note for this type of regulator asks for a e.g. 100 nF ceramic capacitor at input and output. These capacitors are missing here. See the datasheet of an 7806 fpr more information.
The LDR (top resistor) and the fixed resistor (bottom) form a resistive voltage divider to drive the base of the NPN transistor.
When it is bright, the resistance of the LDR is low which will create a high base voltage and turn on the transistor. Th current through the transistor will turn on the relay and the relay output in turn will activate or decativate the light depending on the connection of the light to the relay contacts: as the relay is a switchover relay the light can be turned on or off when the LDR is lit.
The diode across the relay contacts is a freewheeling diode or flyback diode to protect the transistor from the inverse back-emf of the relay coil when turned off.
Again the LED as shown in parellel to the diode is useless as it is connected in the wrong polarity and lacks a current limiting resistor, see above.

#### Audioguru

Sep 24, 2016
3,656
LDRs are not used in solar garden lights anymore because they get sunburned and corroded.
If they are in a streetlight then bird droppings cause the light to remain turned on all the time.

My city replaced all the streetlights with LED ones and each has a radio-controlled circuit, you can see the small antenna on top.

#### Arthur Rey

Mar 16, 2017
2
Thank you, i will try to review it.
I have another question, i am trying to compare the power usage of the automated one with conventional street lights. Is there a difference between them? i know some formulas regarding with the computation of the power but i still have doubts if the other components would affect the circuit's power usage.

Would the LDR affects the electricity consumed since it is use as a sensor compare to conventional street lights?

Last edited:

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,760
Any add-on to a conventional light will consume power which is in addition to the power used by the lamp itself.
• What if the lights are on for exactly the same length of time?
• What if the automatic optimizes (minimizes) the time the lamp is on compared to the time a conventional light is on?
• What if the automatic is badly adjusted and increases the time the lamp is on compared to the time a conventional light is on?
To express this in percent you'd have to know the power dissipated by the sensor circuit and that dissipated by the lamp.
Note that you will have to take into account also the time where the light is off. A converntional lamp would not require any power in this state, the sensor will require power to keep alive.

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