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IC 555 duty cycle

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mhar

New Member
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.
 

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Mr RB

Well-Known Member
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

New Member
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.
 

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