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I am a complete novice and I had done it in a hurry with my kiddo around. I have redrawn it, with the values I used.

The circuit works. Also between pins 6 and 7, if I use a 100 K/10 K it works.

Just that I do not see a square wave.

I am just seeing multiple sites - and working with the available values for capacitors and resistors I have.

I read up the theory about the internals of the astable multivibrator too.

I need someone to explain it to me.

About why the circuit works, if the values of resistors are wrong?

Why does the LED stay lit continuously when I remove my finger, in some intensities of light and does not produce a square wave, I assume should keep blinking, if it stays on.

And what should be the right combination of the values of resistors? enter image description here

I connected the pot using the circuit diagram below from 555 timer circuits.

But the way, I connected it was the base of the pot to the 100 K resistor in Pin 4 and the other two ends of the pot to Vcc and GND respectively. (In the picture base is connected to Vcc and one more end of the pot and the other end of the pot is connected to 100 K resistor with pin 4).

Have I connected the pot correctly?

enter image description here


I tried a simple astable multibibrator with the 555 timer. Using LDR at pin 4.

I have tried to draw the schematic to the best of my ability.

Earlier in the morning when I had tried the circuit, the sun was on the east(my room is in the west), hence it was comparatively darker.

Case1: When I used my finger to darken, the LED was turning on and was remaining on for ever, till I turn on the tube light for more light. Case2: The same circuit when I tried in the evening, with more light in the room, or vice versa (now am not sure), it works like a dark detector, the light does not remain on. When I remove my finger it turns off and when I cover the LDR it turns on.

In Case1: My assumption is that when reset, the LED should light up and flicker (the square wave)

In Case2: Seems fine.

I need help in understanding

  1. why the light stays on and
  2. secondly what is the calculation, governing the timing of charging and discharging (my case 10/k and 56K resistors), these were the ones I had and hence I used them.
  3. Is case 2 correct, some more light on it, please.

I have attached a photo, when the LED stays lit. I have a video too, but am not sure it is allowed.

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  • \$\begingroup\$ Your schematic looks really messy. What's that resistor going to nowhere for? Why does it look like the potentiometer is wired incorrectly. Why are the wires so long? \$\endgroup\$ – Bradman175 Nov 9 '16 at 11:30
  • \$\begingroup\$ @Bradman175, I have changed the question and the schematic. Have a look. Thanks. \$\endgroup\$ – electronicsdummy Nov 14 '16 at 1:52
  • \$\begingroup\$ in your drawn diagram, it doesn't work. The resistors on in the wrong place. \$\endgroup\$ – Bradman175 Nov 14 '16 at 2:05
  • \$\begingroup\$ pot connection differs in the two circuits. \$\endgroup\$ – Bradman175 Nov 14 '16 at 2:07
  • \$\begingroup\$ also if the light stays on if you wired like in the second circuit, it's because it's flashing too fast, which was meant for the speaker to vibrate and make a noise. \$\endgroup\$ – Bradman175 Nov 14 '16 at 2:09

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