# manual and automatic dimming of leds

#### toekietoekie

Feb 3, 2022
2
hello,

I am building something for which I need to drive 156 leds arranged in 26 strings of 6. They should also be dimmable both manually and automaticaly by flipping a switch. To dim them I am using a 555 timer.
I have figured out how to dim them with a potentiometer for manual control (I think), but I am struggling to make them do it automatically. I want them to become brighter when there is more light and dimmer when there's less light. I'm trying to use a LDR to control the pwm from the 555 but I am struggling to adapt the circuit to the 2 pins of the ldr from the 3 pins of the potentiometer because the capacitor discharges through the potentiometer.
I have also tried to include a circuit to limit the voltage over the 555 to 12V but i'm not sure if this is correct.

#### ramussons

Jun 10, 2014
458
You are planning to use PWM for LED intensity control.
There are 2 ways PWM is done.
In your setup shown above, you are varying the ON/OFF period of the 555 maintaining the clock period the same.
That is, Ton + Toff = constant.
If we plan to keep the Ton the same, and shorten or extend Toff , you are again doing a PWM, but here the clock is not constant. But it will work.

So, modify the above so that only the Discharge (or Charge) resistor is varied over a range (you will not need D1 and D2) to change the intensity.
When a LDR is used, the variable resistor is replaced by the LDR.

#### toekietoekie

Feb 3, 2022
2
Thank you! I have edited the circuit to control the discharge with a potentiometer/LDR. I have also changed the resistor values.
For R1 I used 0,693 * 47.000Ω * 0,000.000.01F. this gives me a charging time of 0,0003257 seconds
For the discharging time I used the same formula and have gotten a maximum discharging time of 0,003465 seconds. This would mean the leds would only be on for about 10% of the time.
The ldr has a light resistance of 16kΩ and a dark resistance of 0,5MΩ, so it should give the same result I think.
Would this be a working circuit or am I doing something wrong?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,336
Looks reasonable to me. Give it a test run on a breadboard. For the test run you don't need the power transistor. The 555 can drive an LED directly (with series resistor, of course).

#### ramussons

Jun 10, 2014
458
The LDR in the discharge path of the timing capacitor will mean that Darker the environment, Dimmer the LED.
You will have to move the LDR to the Charging path of the timing capacitor.

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