light dark sensor

trymyx · Oct 1, 2009

need help in explaining the circuit.

birdman0_o · Oct 1, 2009

1) The potentiometer and the LDR form a voltage divider, which, when it reaches Above 2* 0.65V at the base of the first transistor, it slowly turns on the darlington pair of transistors which will turn on what looks like a relay. The diode is to protect the back EMF from the relay when it turns off.

To summarize, when there is enough light / or not enough, the relay will turn on.

trymyx · Oct 2, 2009

birdman0_o said:
1) The potentiometer and the LDR form a voltage divider, which, when it reaches Above 2* 0.65V at the base of the first transistor, it slowly turns on the darlington pair of transistors which will turn on what looks like a relay. The diode is to protect the back EMF from the relay when it turns off.

To summarize, when there is enough light / or not enough, the relay will turn on.

can u give me detailed explanation.

mneary · Oct 2, 2009

trymyx said:
can u give me detailed explanation.

That *is* a detailed explanation. Perhaps you can tell us what part you do not understand.

Feel free to research key terms first, such as LDR, voltage divider, transistor, relay, and darliington.

trymyx · Oct 2, 2009

thanks for the help from both of u.

there is some more questions i would like to ask.
when i did the circuit in bread board there was a chattering sound from the relay when the light is not stable to the ldr. to eliminate the chattering sound i have put a capicitor valued 10uf to the base of the tr1 and the emiter of tr2(ground). and the chattering sound is solved.
but i would like to know how to calculate the correct value for the capacitor? ( i tried with a high valued capacitor(22pf) and low valued(1.4uf) capacitor which i had. but only the 10uf didnt make any noise and stable in switching. )

"thanks for the help in advance"

trymyx · Oct 3, 2009

help

when i did the circuit in bread board there was a chattering sound from the relay when the light is not stable to the ldr. to eliminate the chattering sound i have put a capicitor valued 10uf to the base of the tr1 and the emiter of tr2(ground). and the chattering sound is solved.

How to calculate the correct value for the capacitor?

( i tried with a high valued capacitor(22pf) and low valued(1.4uf) capacitor which i had. but only the 10uf didnt make any noise and stable in switching. )

trymyx · Oct 3, 2009

Help!!!!!!!!!

ericgibbs · Oct 3, 2009

trymyx said:

hi,
What is the problem if it works with 10uF cap.?

Its a very simple circuit and it will chatter at switch over., dont worry.

trymyx · Oct 3, 2009

hi

want to know how to calculate the capactor. !!!!!????

trymyx · Oct 3, 2009

help

can anyone help me .........
how to calculate the capacitor value.

birdman0_o · Oct 3, 2009

It's more of a guess and check thing, and you found what works for you, leave it be!

sheldonstv · Oct 3, 2009

trymyx said:
can u give me detailed explanation.

be careful- txt speak is not allowed on this forum- you will have your post jumped on by audioguru and nigel-im the only txt speak poster here according to audioguru.....

birdman0_o · Oct 3, 2009

There is a difference between:

Hi can you please help me with my project

and

hi cn u plz hlp me wit prjct

It is to ease the communication barriers. It's hard enough to give all schematics codes and datasheets, but if you don't know what they are asking for!!!

Mr RB · Oct 3, 2009

It's a terrible circuit. The darlington will have a really soft turn-on and light changes very slowly. The relay will be pulled in/out unreliably with chattering etc.

For 2 NPN transistors and similar circuit complexity (maybe adding 3 resistors) you can make a schmidt type version of the circuit with some hysteresis that will switch very cleanly.

trymyx · Oct 3, 2009

Mr RB said:
It's a terrible circuit. The darlington will have a really soft turn-on and light changes very slowly. The relay will be pulled in/out unreliably with chattering etc.

For 2 NPN transistors and similar circuit complexity (maybe adding 3 resistors) you can make a schmidt type version of the circuit with some hysteresis that will switch very cleanly.

where do ineed to connect the resistors and which values???

trymyx · Oct 4, 2009

circuit with all values

this is the circuit with all values to the components

Mr RB · Oct 4, 2009

Ok, I added 2 more resistors and 1 small cap.

The relay transistor is the detector, the other transistor is inverse operation and provides hysteresis via the 1meg resistor.

To reduce the hysteresis use a higher value than 1meg, but I think the 1meg will work ok. It's been a few years since i built a light operated switch like this but those parts values look ok. It should switch clean and fast.

The value of the small cap can be anything, probably 0.1uF or less should be ok.

Note too if you are using a 9v battery and 6v relay you might want to put a resistor (R ?) in series with the relay to keep it's operating voltage at about 6v on the relay coil, this will use less battery power.

trymyx · Oct 6, 2009

hi

Mr RB said:
Ok, I added 2 more resistors and 1 small cap.

The relay transistor is the detector, the other transistor is inverse operation and provides hysteresis via the 1meg resistor.

To reduce the hysteresis use a higher value than 1meg, but I think the 1meg will work ok. It's been a few years since i built a light operated switch like this but those parts values look ok. It should switch clean and fast.

The value of the small cap can be anything, probably 0.1uF or less should be ok.

Note too if you are using a 9v battery and 6v relay you might want to put a resistor (R ?) in series with the relay to keep it's operating voltage at about 6v on the relay coil, this will use less battery power.

hi. is there any way that i can keep the the darlington pair and put the resistors to make the circuit stronger.

"in my earlier circuit i put the one without the capacitor. now there is the circuit with the capacitor to eliminate the chattering."

thanks for the help you are giving

Mr RB · Oct 7, 2009

Yep, you can keep the darlington pair as the right hand transistor, and just add the left hand transistor and other parts as seen on my schematic.

light dark sensor

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