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What is the easiest way to fade in and out/pulse leds? Answered

Consists of 21 blue leds connected in parallel taken from one of those cheap toy lightsabers. Before disassembly, the leds were powered by an on/off switch and 3 AA batteries. All options considered - 555 PWM, PIC, or even descrete components. Finally, circuit to be powered from 5V-9V.

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11010010110

9 years ago

connect leds in parallel with big capacitor - gives fade out if you add resistor in series with all of that (including the capacitor) you get fade in too for better fade try 555 and big capacitor on entry 5 (instead of the 100 nF used there normally). you may need to invert the output of the 555 with a transistor if it fades the wrong way

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clint288011010010110

Answer 9 years ago

would the 555 circuit look something like this?...

555_throb_sch.gif
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11010010110clint2880

Answer 9 years ago

it should be like any 555 flicker circuit entry 5 of the chip is the reference voltage and it determines when the led is on or off. usually it just has a small filtering capacitor of 10 - 100 nF. but if you play with the voltage on it you can affect the work of the 555 it does not directly affect the frequency or duty cycle but is more complex. if you have the components just build the circuit on a matrix board and go trial and error with it

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clint2880clint2880

Answer 9 years ago

built the circuit as described above, substituting the BC517 for a 547 (as already had the 547, and the 517 was out of stock) Only trouble is, using a 9v battery and the 6v power supply (which will be the actual supply) the LEDs are very dim at maximum brightness. How can I increase the brightness - less LEDs (preferably not) or change the transistor?

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11010010110clint2880

Answer 9 years ago

maybe the problem is that the 555 cannot be used the way i suggested lets try to use external component to do the actual switching of the pwm like in this circuit the 555 is wired like any flicker circuit at high frequency. note that we take the output from the run capacitor (entry 6) and not the normal output. there is a bit warped triangle wave there that goes between 1.6 - 3.3 V (if its on 5 V) (or 1/3 - 2/3 of whatever voltage your supply is) the 358 is comparator (actually 2. we use only 1) that compares the 555 voltage with a dc voltage you give on the input if you use more leds then replace the 1 led on the 358 with transistor

pwm2.png