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Dark Detector operating ways(using 555timer)

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catherine

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I have a dark detector circuit asssignment at this moment and I am confused with how this circuit operate?I choose monostable state to give an indication when a sudden shadow falls on LDR.

Can someone tell me how this circuit operate during on light and offlight.
Through which pin when those condition occurs.

Thanks for help.
 

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I have a dark detector circuit asssignment at this moment and I am confused with how this circuit operate?I choose monostable state to give an indication when a sudden shadow falls on LDR.

Can someone tell me how this circuit operate during on light and offlight.
Through which pin when those condition occurs.

Thanks for help.

hi,
If you look at pin4 of the 555 [datasheet] you will see its the RESET pin for the 555.
While this pin is LOW the 555 is held OFF.
An LDR when illuminated by ambient light will have a low resistance, so pin4 is LOW.
When the LDR is dark its resistance is high, so pin4 goes high to a point where the RESET is off and the 555 monostable operates.

If the LDR goes light again the RESET is applied..

Do you follow. OK.?
 
When LDR is saturated with a lot of dark, it has a lot of resistance. This causes Reset to go high. When a light turns on, it sucks the dark out of LDR so it's resistance can go low.
**broken link removed**

Problem is, Reset on a 555 works at .7V, so the light has to suck a LOT of dark out of the LDR to get it to work. Even a little residual dark will contaminate the LDR and give it enough resistance to stop Reset from going below .7V. Check the voltage on the Reset line. If it doesn't drop below .7V when you suck the dark out of it, try a higher resistance for R1, like 470k or 1M.
 
Last edited:
When LDR is saturated with a lot of dark, it has a lot of resistance. This causes Reset to go high. When a light turns on, it sucks the dark out of LDR so it's resistance can go low.
**broken link removed**

Problem is, Reset on a 555 works at .7V, so the light has to suck a LOT of dark out of the LDR to get it to work. Even a little residual dark will contaminate the LDR and give it enough resistance to stop Reset from going below .7V. Check the voltage on the Reset line. If it doesn't drop below .7V when you suck the dark out of it, try a higher resistance for R1, like 470k or 1M.

To be polite, reading that link, its a load of cr*p, you dont honestly believe what it says, do you.:rolleyes:
 
I honestly believe the reset voltage is around .7V. :D
 
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