Thank you all for your explanations. I've got the electronic bug now and want to know more.

I am trying, (still with the same original layout), to control the IC 7555 using an LDR.
I purchase an LDR 48k/140k, I guess they are values of ON and OFF.
I've looked on the internet on how to connect the LDR and tried many configurations without success.
I tried to connect the LDR to Vo, to Pin 2, then Pin 4, then Pin 7, I placed a Resistor of 10K and then 100k in series connected to the return.
Either the LED stay ON all the time or keeps on flashing as if my LDR and resistor didn't exist.
I don't know how the IC 7555 did survive all this.

So, once more I do need some help and explanation.
1: How do you connect an LDR with its resistor for it to shut down the IC7555 when dark and turn it back ON when light? I know now that I can control the WHEN by fine tuning with a potentiometer.
2: How do you work out the values of the LDR and of its resistors?

Thank you again.

Alain

 

Light dependent resistors (LDR) are wonderful, simple devices with which to play around.

An LDR with specs of 48k/140k simply means that the resistance is 140kΩ in the dark and 48kΩ with ambient room lighting. This is in the lower resistance range as LDRs can be found with resistance characteristics from 10kΩ range to 100MΩ range.

As a simple experiment, replace R1 of your 555-timer circuit with the LDR and see what happens when you cover the LDR with your hand.

As a second experiment, replace R2 of your 555-timer circuit with the LDR and repeat the experiment.

To make the LDR act as a switch, you can control the RESET pin-4 of the 555-timer using an ON/OFF signal. Unfortunately, the LDR on its own does not make a good switch. For that, you need to add a transistor which will be controlled by the LDR.

I am going to be away for a day so another AAC member would be sure to come along and show you how to connect the LDR to the transistor (2N3904) and to the RESET pin.