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light activated relay

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chandra999777

New Member
hey i want help in light activated relay.plz see the link,in that ckt diagram was given. Dark/Light Activated Relay
Here in my ckt i am not using relay instead want to make white led to be ON when it is dark.It worked but my problem is the output of the transistor is very low(i.e 2.5v) so the led is not brighter. I need at least 4.5v at the output. what kind of ckt will be needed to provide 4.5v at the output in order to make white led to be brighter.plz help me.
 

ericgibbs

Well-Known Member
Most Helpful Member
hey i want help in light activated relay.plz see the link,in that ckt diagram was given. Dark/Light Activated Relay
Here in my ckt i am not using relay instead want to make white led to be ON when it is dark.It worked but my problem is the output of the transistor is very low(i.e 2.5v) so the led is not brighter. I need at least 4.5v at the output. what kind of ckt will be needed to provide 4.5v at the output in order to make white led to be brighter.plz help me.
hi,
So you have connected an LED in place of the relay coil.

What is the forward voltage drop of the LED and what is the value of the series resistor you have connected to the LED.?
 

chandra999777

New Member
series resistor is 120 ohm and the forward voltage drop is approximately 2v. it cannot be turned on with a single 1.5v battery.
 

ericgibbs

Well-Known Member
Most Helpful Member
series resistor is 120 ohm and the forward voltage drop is approximately 2v. it cannot be turned on with a single 1.5v battery.
The circuit diagram shows a 6V supply.
 

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BrownOut

Banned
I think he means he estimated the turn-on voltage @ 2V because it didn't turn on at 1.5V ( 6V - 4.5V measured at the collector )

Measure and post the values of R1, R2, and R3. Measure R3 in darkness; just cover it with something opaque. Post the values for us and we can better analyze your circuit.
 

Hero999

Banned
No LEDs will work off a 1.5V battery without a boost converter.

Well I lied some infrared LEDs might light dimly but you can't see them anyway.
 

sia88

New Member
Hello Sir
I'm Sia need help to find the applications for Light activated relay switch
can you please mention them

thank you
 

chandra999777

New Member
it is used to automatically turn on the lights when it detects dark, this may be a street light or anything that suits your application.
 

sia88

New Member
it is used to automatically turn on the lights when it detects dark, this may be a street light or anything that suits your application.
Thank you for the help i really appreciate it...

But can you tell me the applications for the reverse i.e when Light activated relay switch

this is the circuit diagram:



Here light bulbs are used as load for simplicity

Thank you
 

mneary

New Member
You see the relay has three connections, COM, NO, and NC?

Normally for a night light they used the COM and NO contacts.

You should use COM and NC for a light activated switch.
 

surya.bs

New Member
hello All,

I am using the same circuit has posted in the blog. but i am unable to energize the relay.please help me out. When connected with LED instead of Relay the light is switching on and off properly.
 

audioguru

Well-Known Member
Most Helpful Member
hello All,

I am using the same circuit has posted in the blog. but i am unable to energize the relay.please help me out. When connected with LED instead of Relay the light is switching on and off properly.
Maybe your relay coil needs more voltage or more current than the transistor can supply. How much does it need?
 
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