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How can I make the LED in this 555 schematic be off when the sound is on? Answered

I am building this laser ray sound circuit using an LDR where the 220 ohm resistor is for a DJ, but thats beside the point. He wants the LED to be off when the sound is playing but on when its not playing how would I modify this circuit to do that?
http://www.555-timer-circuits.com/laser-ray-sound.html

4 Replies

user
mrmerino (author)2012-10-03

Also, not completely sure why you'd want to have it off...

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user
mrmerino (author)2012-09-24

The voltage drop needs to be the same, right?

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iceng (author)mrmerino2012-09-24

Not sure, the 555 is a really tricky.

I would have to build and use a high impedance scope probe
to answer that insightful comment.

Keep in mind any diode only conducts primary current one way,
While the led junction, bigger then a plain diode, can pass inverse
current capacitive pulses.
All this has some performance related involvement not easily
conducive to straightforward mathematical resolution in an
audio context.

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iceng (author)2012-09-23

Replace the led with a non led diode, as the sound is partially derived
by the diode and 470 ohm resistor acting on the voltage control pin 5
through the power output pin 3. Or simply shrink black tube over the LED..

To make an LED come ON  automatically when the sound halts
and off when the sound comes on will require another 555.

A

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