Re: Need help with op-amp/comparator circuit

From: john jardine (john_at_jjdesigns.fsnet.co.uk)
Date: 01/15/05 

Date: Sat, 15 Jan 2005 00:45:38 -0000

"Anthony Fremont" <spam@anywhere.com> wrote in message
news:glXFd.3432$Ta2.1413@fe2.texas.rr.com...
>
> Hello all,
>
> Danny got me thinking about my op-amp stuff, so I resurrected it for
> another application/experiment. Now what I'm doing is that I'm trying
> to "hear" some tapping type noises (actually an antique clock
> tick-tock). What I want to do is to measure the spacing between the
> beats. I'm sure to be in violation of someone's patent, but this is for
> personal use so on to the show. ;-)
>
> I am currently using the circuit below. The 10K voltage dividers were
> empirically picked so that the idle output voltage of the 741 is 2.52V,
> the divider on the 393 is at 2.50V leaving a 20mV difference.
> Everything works pretty much as expected (i.e. the 393's output rides at
> +5V until a loud enough noise causes the comparator output to flip) A
> "loud enough noise" is anything that causes the 741's output to shift by
> more than about 20mV. What I'm seeing on the scope peaks at about 500mV
> on the output of the 741.
>
> The 393 then swings low on the peaks of the tick sound like I want, but
> the the comparator's output swings high again between the peaks of the
> audio waves (every couple of mS) giving me a stream of pulses. What I
> want is for the output to pull low and stay low until the tick sound
> decays a bit (interpret this as about 50mS'ish ;o). IOW, I want to low
> pass filter the output of the 393. The base pitch of the sound seems to
> be around 600Hz or so. The sharp intial attack is all I'm really after,
> the main jist of the sound is between 40 and 60mS in length.
>
> I tried putting a 1uF cap to ground on the output of the 393. This
> helps, but I still get allot of ripple on the low signal. Bigger caps
> do better, but they also lead to real slow rise times even with the
> fairly stout pullup (2K). What do I do now? I think I need to add some
> RC time constant somewhere, but not sure where.
>
>
>
> VCC +5V VCC +5V VCC +5V
> + + +
> | | |
> | | |
> .-. .-. .-.
> | |10K | |10K | |
> | | ___ 100K | | | |2K
> '-' .---|___|----. '-' '-'
> | | | | |
> | | | | |
> 10uF 1K | | LM741 | | LM393|\| |
> || ___ | | |\| | o--------|-\ |
> |\ __ .--||--|___|---(---o-------|-\ | | | >--o--->OUT
> | | |- || | | >-o----(--------|+/
> | |__|---------------o-----------|+/ | |/|
> |/ | |/| |
> .-. .-.
> Speaker IN | |10K | |10K
> | | | |
> '-' '-'
> | |
> | |
> === ===
> GND GND
> (created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)

Give it a bit of feedback. Adding the C and Rs will (on triggering) force
the o/p low for about 100ms then the output goes high for about 100ms. Then
ready and waiting for the next 'tick' soundburst.

                    LM393
                          |\|
                         -|-\ |
             ___ | >--o---
 741 o/p o--|___|-o-------|+/ |
             47k | |/| |
                  | |
                  | |
                  | ___ || |
                  '-|___|---||--'
                            ||
                    100k 470n

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