I am working on a project named as automatic night light.The idea is simple; LEDs are on when there is dark then LEDs turn off when the circuit detects light.

However I have constructed the circuit I have mentioned above, I want to add dimmer to the circuit to make it sensitive to the amount of light seen by LDR. I mean I don't want it just to go on and off.

Here is my circuit diagram:

enter image description here

It is normally on and when the light seen, it goes off but it makes instantaneously. What should I add or do to add dimmer here?

  • \$\begingroup\$ What is the resistance range (and change upon illumination) of the LDR? That's the important part here. Have you played around with RV1? Have you considered at (approximately) what frequency your NE555 will run when e.g. at 50% duty cycle? Is it possible that at this frequency, the LDR acts more like a capacitor than a resistor? \$\endgroup\$ – Marcus Müller Nov 26 '17 at 13:53
  • \$\begingroup\$ (oh, and by the way, my eyes are excellent, but I can't say I wouldn't be thankful for a larger font size in your schematic! \$\endgroup\$ – Marcus Müller Nov 26 '17 at 13:54
  • \$\begingroup\$ (oh, and if you have a 50 kΩ potentiometer, why the 1 kΩ R3 in series?) \$\endgroup\$ – Marcus Müller Nov 26 '17 at 13:55
  • \$\begingroup\$ oh, wait, I'm a little confused by your schematic; your NE555 symbol uses input names I've not seen before and I can't read the pin numbers, but wouldn't DC be the discharge pin? Shouldn't that be connected to the a capacitor to ground and the lower side of your LDR to even possibly lead to oscillation? \$\endgroup\$ – Marcus Müller Nov 26 '17 at 13:58
  • \$\begingroup\$ See if you can turn off the grid next time you are taking a screengrab. \$\endgroup\$ – Transistor Nov 26 '17 at 14:05

enter image description here

Figure 1. Variable duty-cycle from 555. Source: Electronics Tutorials.

You should be able to make the circuit do what you want with the addition of a pair of diodes - one to provide a path to charge C1 and one to discharge. By adding a resistor - your LDR - into one or other path you can vary the ratio of on to off time. Provided the base frequency is high enough the flicker will not be visible to the eye.

Form the linked article:

The charging time of t1 = 0.693(R1 + R2)C is modified to take account of this new charging circuit and is given as: 0.693(R1 x C). The duty cycle is therefore given as D = R1/(R1 + R2). Then to generate a duty cycle of less than 50%, resistor R1 needs to be less than resistor R2.

Measure the resistance of your LDR in dark and light conditions and plug the values into one of the many online 555 duty-cycle calculators.


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