Audio level alarm - help please

[HCb]

Hello, thank you for reading my post.

My ex-wife asked me about a device she's interested in but I don't know enough about electronics to know where to go on this. What she wants is a device which will turn on a light if a certain ambient sound volume is reached and turn on another light if a higher volume is detected. I assume that the individual sound levels must be manually adjustable on the unit (she's not going to know what decibel level she's expecting to be produced).

I did some searching online and came up with these:
http://www.aaroncake.net/circuits/vumeter.asp
http://electrosuite.com/audio/lm3915-audio-level-meter.html
http://www.electroniccircuitsdesign...-meter-circuit-design-electronic-project.html

All three center on a LM3915. I checked the datasheet on the chip for an application circuit which might do what I need but I did not see one. In reading the documents above I'm at a loss to understand how I could attach a microphone to this circuit so that the device could detect ambient sound levels.

I'm not sure this IC or these circuits are really what I'm looking for. I think that if I had one of the linked-to circuits running it would be easy to take the 10 outputs and using diodes and/or maybe some op-amp to form an output control so that one light would illuminate at one sound level and another at a second level. But I need the darned thing working at all at first.

I'd like to know if this is even the best direction (the aforementioned circuits/chip) and:

if so, how to enable the device to detect ambient sound levels as well as how to enable manual adjustment for two threshold alarms

and

if not, what circuit would/should I try to build for her?

Thank you for your time, again.

--HC

 

[(*steve*)]

First you need t build a small amplifier for the microphone, then you need t connect the output of that amplifier to the input of the 3915.

Typically you will have some sort of gain adjustment on the amplifier so that the LEDs operate within a range of volumes that you consider correct.

Depending on how you want the system to react, you will probably want to have it hold on to the peak volume for a short time rather than react to the instantaneous level of the sound (which will go up and down with extreme rapidity).

Some of the circuits you have found will probably do the latter part. for the amplifier, that depends on the type of microphone to some extent. Googling "<mic type" mic preamp schematic" will probably lead you in the correct direction (e.g. "dynamic mic preamp schematic")

It sounds like you'll need a bit of help with this, so feel free to ask more questions (even if they sound silly). I've almost certainly glossed over quite a bit here because I'm not sure how much detail you will require.

 

[HCb]

Steve, thank you for your reply.

Glossing over details myself and skipping ahead: your post makes me feel more clear-headed about the amp part. I will do some searching for an adjustable-gain microphone amp. That *sounds* easy.

I understand the dwell time at a certain input level but I'm not sure how to build one from discreet components, if I'm using the term correctly. Using a programmable IC would be easy...if I knew which IC to use...and if I could write code for one. I tried with the PIC from Microchip but, despite countless dollars spent on equipment and hours tinkering, trying to learn electronics on my own has failed miserably so writing the code is shaky and actually determining which chip to start with = null.

My simplistic attempts would focus on some sort of transistor/op-amp/555 circuit to detect the output of one of the 3915's pins (whichever two I selected based on noise testing) and then fire a timer circuit which would output a signal to an alert circuit. It'd be like building a car with Lego...brick by brick and blocky. But it would probably work.

There are several (many?) different types of microphones. Recording ones basically, as I understand, like a speaker but driving not driven and there may be some piezo-style but I could be cross-channeling memories. If I wanted to measure ambient sound levels like outdoors with equipment running or indoors in a crowded environment, which type of microphone would you suggest? I'll try this in a linear fashion and the first piece is the sound detection equipment (microphone).

Once you suggest something I'll look up a schematic for an amp for it and move from there.

My electronics knowledge is limited so glossing over details at this point is good. I'd rather focus on how to do it until I'm happy with a game plan then worry about the details. I'll need a fair amount of specific design help (component recommendations as the microphone above) and general design help (such as the individual functions to be performed). But I do okay looking up the chip data, schematics of simple-purpose circuits. I'll try to do my part here and look stuff up as much as I can.

Again, I appreciate your help and time.

--HC

 

[(*steve*)]

Essentially, the LM3915 lights up the LEDs based on a DC voltage.

Your amplifier will produce an AC voltage.

The first step is to rectify the output of your preamp so you get DC. With clever use of diodes and resistors you can make the voltage climb quickly to the peak output of the amplifier, but decay more slowly -- and this sounds like what you want.

The following circuit shows the general idea. It's not perfect, but it may work well enough.

The AC entering from the left (from your pre-amp) is rectified and used to charge the second capacitor. The resistor allows the charge to slowly drain away.

The actual values of these components would need to be determined...

 

[HCb]

Thank you for the reply and sorry for the delay in my reply; I've been out of town.

Okay. So, first question, will the DC input voltage need to be in the 0-Supply Voltage of the LM3915? Then, all I'd need is an amp circuit for a microphone and then put that output into the schematic you've shown to get a DC output for the LM3915 to "indicate" via its outputs. I assume the various parts would share a common power supply so they're "on the same page" regarding amp output and the scale the LM3915 will indicate.

I can tell you what each of those components in your schematic are, but I have no way of knowing how to calculate their values. If only I was as smart as people seem to think I am. :-/

Thanks for your help. I may be a lost cause.

--HC