Need help with dark activated switch

MrChips

Joined Oct 2, 2009
30,980
Try the second circuit first.
Put 6 LEDs in series to replace the single LED.
9V is not high enough to turn on 6 LEDs. So remove one LED at a time until you get sufficient brightness.
 

Thread Starter

sumit231996

Joined Oct 14, 2014
14
I did that for one led and 16k instead of 100k(I don't have 100k right now) with 9v battery. The result was that led lit up even in bright light. Can anybody explain me what is wrong in circuit
 

MrChips

Joined Oct 2, 2009
30,980
16kΩ on the base of the transistor is way too low. Do you have any resistor 100kΩ or greater, even 1MΩ? Put resistors in series if necessary.
Use 3 or 4 LEDs in series, not just one LED.
 

MikeML

Joined Oct 2, 2009
5,444
The optimum value of the 100K depends on the LDR dark resistance; they must be matched....

The 100K also supplies the base current for the transistor when it is time to switch on the LED, so depends on LED currrent and Gain of the transistor.

It is simple circuit; too simple...
 

MikeML

Joined Oct 2, 2009
5,444
Do you own a Multimeter (DMM)?

And you are using MrChips circuit #2, except that the LED is lit all the time?

How are making the LDR dark?

Do you have a part number for the LDR?
 

tracecom

Joined Apr 16, 2010
3,944
it may be way too simple for you but not for me though. You may consider me a complete newbie. I don't know what to do. :(
The first thing that would help is to study how to use a solderless breadboard. I note from your photo that you are inserting more than one wire/lead into a single hole in the breadboard. That is incorrect and unnecesary. There are quite a few guides on the web showing how to use a solderless breadboard. Here is one. https://learn.sparkfun.com/tutorials/how-to-use-a-breadboard
 

MrChips

Joined Oct 2, 2009
30,980
OMG! I didn't notice that. You are going to damage your solderless breadboard by trying to poke multiple wires into a single hole.
 

Thread Starter

sumit231996

Joined Oct 14, 2014
14
Mike the ldr is a common one I found at local store. No part number unfortunately. And no DMM too for now.
tracecom, thanks for that. It was the first time I used it(else direct soldering)
 

Alec_t

Joined Sep 17, 2013
14,379
will 100k with 4 led surely work? What are the chances?
Slim, since you're using white LEDs. A white LED will need around 3V to work well, so allowing for some overhead (voltage loss in the current-limiting resistor) you are unlikely to be able to connect more than two in series with a 9V supply.
 

Thread Starter

sumit231996

Joined Oct 14, 2014
14
is
that so? I can manage with three or less. The point is that what should be the resistance in the base of transistor if I use 9v battery. And does connecting the led with collector or emitter make any difference?
 

Alec_t

Joined Sep 17, 2013
14,379
The resistor value will depend on how much current the LEDs require. Higher LED current = lower resistance of base resistor.
 
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