555 Circuit Help Needed

May 06, 2008 4 Replies

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Howard 18 years ago

Hello to all and thanks for reading. I am attempting to build a 555 circuit that will "listen" to audio output from a computer speaker (hard wired to the speaker) and fire a relay and hold it for 5 seconds after the music stops.

I put a Bowden 555 circuit together that fires the relay when a push button is pressed and holds it for a 5 second time period.

I am looking for suggestions on a circuit that will replace the manual push button, thus allowing the relay to remain energized while music is playing.

Any help appreciated.

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Tim Wescott 18 years ago

Search around the web for an "audio detector" circuit. You need to rectify the audio and compare the DC level with a threshold. The output of the comparator will "push the button". You may find that you'd want to filter the audio first, to get the best missed event vs. false alarm rate.

Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

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JeffM 18 years ago

It is good netiquette, before posting, to READ some of the previous threads.

Bob Monsen laid out almost this identical project barely 2 weeks ago in the BEGINNERS' group:

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*-*-bird-is-*-noisy+The.sound.detector&fwc=1

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Jim Gregory 18 years ago

You will need to modify /lengthen the 'audio' presence after amplifying it to 'crushing' and rectifying its envelope to a DC level. Then, by a RC time constant with electrolytics, make sure it ramps up fast and discharges (decays) slower. This 'stretched' DC level will have to be amplified enough to hold a relay on for about 5 secs. That's the gist. The 555 timing function might be superfluous when audio is punchy and continuous. Jim

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LVMarc 18 years ago

5 seconds is pushing the slow side and the R values to thelimimts. You may want to add counter or use a time delay chip more suited to ##seconds, LSI logic makes a fine series of these delay timers...

Marc

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