Hi Guys,

As part of a project, I'm required to build a sound activated device to start a tape recorder. This is the first electronics course I have done, so I'm still pretty much electronics-illiterate. Anyway, I've been looking through books and on the net (seems there are quite a few similar projects on these forums), and I think I'm getting somewhere.

It must,
- Filter out frequencies of below 100Hz
- Have a timer to record from 1-10min (User adjustable)
- Be battery operated

So far, I've figured that I need
- Condensor Mic
- Op-amp (Non-Inverting amp)
- High Pass Filter
- Some sort of timer
- Transistor switch

I'm still unsure about how I will provide a user-adjustable 1-10 min timing option.

Am I on the right track? I'd appreciate any comments or ideas. Thanks.

 

Hi,
You are on the right track.
For a timer the "classic" approach is to use a "555" timer IC. The newer 7555 CMOS version is better for longer delays. Do a web search for 555 timer, NE555, LM555, ICM7555.

HTH,
Robert G8RPI.

 

As a Starter, You could use a MC2830P.
It is a "Voice Activated Switch", Integrated Circuit in an 8 pin Dip Pkg.

It may not be available from normal sources anymore, But I can supply it.

Gary

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The 555 timer by itself is not good for 10 minutes. You need a count-down circuit which could use a 555 oscillator as clock input. Use a 1 Hz clock and divide by 600 (600 seconds in 10 minutes). U need 10 stages in the divider. To figure the taps, subtract the largest power of 2 possible from 600 then subtract the largest power of 2 from the remainder, etc until nothing is left. For shorter time delay, speed the clock up.

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