Solving LDR Controlled LED Emergency Lamp Problem

The following post discusses the troubleshooting of an automatic LDR controlled emergency lamp circuit which was requested to me via email by one of the keen followers of this blog. I have explained about the issue and its solution.

Technical Specifications

Dear Sir,

I am following you on your website and I thank you for your valuable contributions as I am a electronic enthusiastic from my childhood.

I love to build projects a for my own use.

I seek your help in building a simple but efficient automatic (overcharge cut off) led base emergency light with LDR sensing. Need to use 12 or more led to light up. Can you send me a simple and efficient circuit with spare details.

Please help me so that I can build this asap. Attached one circuit for your reference I tried to build this but failed.

The led is not stopping when light hits also not stopping when mains on. When light hits led’s just dimming not turning off. If you can please help me. Check if there are any errors on this circuit. Please.

Best Regards

Hariesh

Analyzing the Circuit

Dear Hariesh,

Make the base/ground resistor of the BC548 transistor to 47K and see what happens. If it still doesn't work, replace the mosfet with a 8050 transistor.

A mosfet is absolutely not required for such a simple application.
The given mosfet is an N-channel but the diagram incorrectly shows it as a P-channel.

Regards.

Dear Sir,
Sorry to bother you again.!!!

Thank you for your prompt response . can you illustrate the pin connection of 8050 in place of mosfet.?
Best RegardsHariesh

If you hold the transistor with the printed side toward you with its pins down, the center pin will be the base, the right side pin will be collector and the left side will be the emitter.

The pin toward number 8 will be the emitter, the pin at "0" will be collector...
Base will go to the 10K resistor, collector toLEDs and the emitter to ground.

Dear Sir,You are great..!! finally half success now LED turns off in the presence of light. but not when i plug in mains power. any thoughts on this.? please.

Check the 1K resistor which is connected from base of BC548 to positive between the diodes. If it's connected properly the circuit will surely switch off in the presence of AC mains.

Dear Sir,

Thank you so much for your help. the following changes have led to success as advised by you and it works like a charm.

Replaced MOSFET with 8050.

1k Resistor was not connected properly due to oversight.

Could you kind enough to explain why MOSFET was not working even though I have connected properly. Just out of curiosity and to learn. 🙂

Once again I thank you for your great help.
Regards

Hariesh

Problem Solved

The mosfet should have also worked, there can be many reasons why it didn't work, may be due to some leakage voltage across gate and positive, due to wrong pin connections or simply because the device may be faulty.
Regards.