Need to control PWM in NE555 timer IC.

I have a blinker circuit which turn ON & OFF 100 yellow leds using NE555 timer IC. It is working properly. Now I need to lower the intensity of yellow leds during night and increase the intensity during day. The power to the circuit is given using a solar panel. Solar panel is connected to a solar charge controller and a battery is also connected. I have recently made this solar charge controller using arduino and its working fine.

I want to know how can I reduce the intensity of leds at night and bright them up in daylight. First I thought that during night, solar panel output voltage will be very low. So I can simply read the solar panel voltage. If its high, that means its a day time and we have full sunlight and if voltage is low, that means its night or no sun light is available. In this way I can determine sun light availability, but then how to control leds intensity using this. I connect my blinker circuit as a load to the solar charge controller. So I thought of giving PWM pulses to load which will reduce the amount of voltage going to blinker circuit so leds intensity will be reduced. But I don't think that would be appropriate solution. Is there any good way of doing this.?
Please help

You may modify this circuit with an LDR to give a low duty cycle during night time.
**broken link removed**

I already thought about using LDR. But the circuit which I am talking will be placed inside a black metal body which will be fully covered and there will no provision for LDR as LDR needs to be outside. Is there any way I can control this pwm using arduino only.
Here is the image

The ctl (5) pin on the 555 is a handy method to alter duty cycle (per link in post #2). Send the PV output voltage to this pin. No doubt it's more complicated than that because it's likely you'll need to apply gain, level-shift, etc.

To attenuate use a resistor network, or zener, etc. A 12V rated panel might rise to 20V or more with light load. It probably is bad for the 555 IC if you expose any pin to a voltage greater than the supply voltage.

Amplify voltage (or level-shift) through an op amp. Etc.

It will help if you experiment by applying various volt levels to the 555 ctl pin. See how it affects duty cycle. More tests should produce the desired brightness of the led's, for daytime and night. The aim is to tap the PV voltage, and use it to alter brightness.

czabhinav said:

Is there any way I can control this pwm using arduino only.

Click to expand...

Rather than using the Arduino to generate an analog signal that controls the 555 which generates the PWM signal, can you use the Arduino directly to generate the PWM? The common way for a digital microcontroller to generate an analog signal is to use either a DAC or generate a PWM signal and filter that. If you are using the Arduino PWM method then why not use that directly.
The other issue you will have if the device is to be mounted inside an enclosed container is how to determine when it is night and day.This will require either an externally mounted sensor (which you have already effectively rejected with the LDR idea), an accurate RTC of some sort, or an external clock source that can reset an internal clock (e.g. a network interface to an NTP server).
Susan

Depending how you drive or switch the LEDs you may be able to sense a day/night condition by using the photo-electric properties of an LED. The sensing can be done during the off cycle.

My approach would be to use a small 8-pin PIC such as the 12F683 or 12F1822. They have built in ADC and PWM generators and their own on-board clocks. A few lines of simple code would read the PV voltage, control the PWM ratio and flash the whole lot on and off periodically. By the time the extra control circuits are built around the 555 it would probably cost less to use a PIC and the circuit would be less complicated.

Brian.