IC 555 duty cycle

[mhar]

I'm using a 555 timer to generate a pwm signal to drive a LED driver chip.

It all works except on dim the LED is still slightly on, pwm pulse is still there but shows on a scope as a spike.
I've tried germanium diodes and different C's but you don't get full 0/100% duty cycle.

Maybe another device is the answer, PIC Processor or something.
I'm a bit tight on space in the light fitting.

 

[BrownOut]

There isn't any such thing as 0% to 100% PWM. That is just on/off.

 

[shortbus=]

Hi, try this; RMCybernetics - DIY Homemade Signal Generator with Pulse Width Modulation

By adding the LM393 or LM339 to the 555 circuit you can get the adjustable duty cycle.

Cary

 

[Gayan Soyza]

Another one **broken link removed**

 

[Mr RB]

Put a cap across (parallel) with the LED. Then the very short pulse will produce a smoothed voltage lower than the LED turn-on threshold, so you will get PWM right down to zero (as far as LED brightness goes).

 

[mhar]

Thanks for all the answers.
I'm going to try the revised PWM Signal Generator.

just to clear up a few points:

IC555 pwm does not directly control the LED instead it's applied to a control pin on a driver chip.
The driver chip pulses the high power LED to set the average current@350ma.
The IC555pwm gates the driver with a variable pulse width - this sets the brightness.