Dear all

can somebody please post a circuit using LDR to turn on 12v bulb following way. bulb should turn on when it detect light and should be turned on until i disconnect the power

Thank you in advance.

 

The basic shape is the 12V supply going through the LDR to the gate of an SCR which powers the light. However, with absolutely no specifications about the LDR and the light bulb current, nobody can know if you will need some amplification stage(s).

 

The basic shape is the 12V supply going through the LDR to the gate of an SCR which powers the light. However, with absolutely no specifications about the LDR and the light bulb current, nobody can know if you will need some amplification stage(s).

hey thank you for your reply. ampere etc are doesent matter since i have a basic electronic knowledge. i just needed a circuit so i can modify it according to my requirements.

 

See

 

The Silicon-Controlled Rectifier (SCR) | Thyristors | Electronics Textbook
https://www.allaboutcircuits.com/textbook/.../chpt-7/silicon-controlled-rectifier-scr/

REVIEW: A Silicon-Controlled Rectifier, or SCR, is essentially a Shockley diode with an extra terminal added. This extra terminal is called the gate, and it is used to trigger the device into conduction (latch it) by the application of a small voltage.

 

See

What happens if you delete the BC517 and R2, increase R1 to 10K, and connect the LDR from the gate to the anode?

ak

 

What happens if you delete the BC517 and R2, increase R1 to 10K, and connect the LDR from the gate to the anode?

ak

I wanted to get a high sensitivity of this machine. I also counted on using a "dull" (with a low sensitivity) thyristor. If you implement your proposals, the sensitivity will greatly decrease. Besides, I'm not sure if such a logic is necessary.

 

Thank you soo much @Bordodynov

 

What happens if you delete the BC517 and R2, increase R1 to 10K, and connect the LDR from the gate to the anode?

That's what I was thinking in post #2, but it's probably because I can't think without a couple of numbers to start with.
But seriously, I haven't designed an LDR circuit since 1985 and all I can remember is they are pretty weak at carrying current. Possibly not enough to fire an SCR. Or feeble enough that a single stage would require careful component selection, like a 5 ma gate current.
One quick spin on the calculator and I see that the last LDR I used wouldn't do more than 2 ma.
Then Bordodynov swoops in with another excellent circuit and there's nothing left to discuss.

 

The mad Russian rides again.

 

Hi. Slight problem. Is there anyway to use BT151 instead of tje above mentioned thyristor ?

 

That depends on what a BT151 is. Since you already know, how about sharing?

ak

 

Br151 is a thyristor commonly available here in sri lanka.
Datasheet - https://www.rapidonline.com/pdf/151290_da_en_01.pdf

 

xiles.
I overdid it with sensitivity in my circuit (0.01 lx).
My circuit will work from moonlight. Without using a transistor and with your thyristor, the trigger threshold is 1000 lx. Therefore, for a sensitivity of about 10 lux, it is sufficient to use a conventional transistor (not a darlington).

 

Thank you somuch @Bordodynov sensitivity to light does not matter that much since a LED (small) will be dirrectly attached infront of LDR.
Actually what i want is..
I have this chinese wireless bell . It has a dancing LED, when someone press button the led starts glowing For few seconds..
I just wanto attach some 12 v led s to light up the surrounding area of the bell when some one press the bell.
All my attempt to connect a seperate circuit to this small led inside the bell failed.. so now im thinking to attache a LDR infront of the dancing led inside the bell to control 12V leds.

 

See

 

I just wanto attach some 12 v led s to light up the surrounding area of the bell when some one press the bell.

I did something similar with a door bell system in about 1975. Phone rings therefore door bell rings. The ring signal is about 105 RMS so it will fire a neon bulb. Detect that with an LDR and you have a primitive opto-coupler. This brings the idea of having your door bell fire an opto-coupler instead of the LED.

All my attempt to connect a seperate circuit to this small led inside the bell failed..

Never mind.

 

If your still looking for a light/darkness detector to switch an output, I made a quick instructable for activating and LED sign using an LDR. If your interested please have a look

https://www.instructables.com/id/Light-Up-LED-Sign-Brightness-Activated/

Regards,
smooth_jamie