Help with circuit design

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billybob

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This seems like it can be a very simple circuit, but I can't figure out what it should look like.

Basically I want a setup where after a quick (maybe a fraction of a second) 4 to 5 volt pulse to the gate of a mosfet, BJT or something a buzzer will sound for about 3 seconds then return to neutral. I'm assuming a capacitor is mandatory in this circuit.
Any help is appreciated.
-Ben
 
Search for "555 timer one shot circuit". The 555 has been around longer than you're been around (twice as long at least), and it's still going strong with something like a billion a year being sold today.

A one-shot circuit is triggered by a pulse or a button press, and stays active for a time period set by a capacitor and a couple resistors. Some one-shot circuits are retriggerable – a pulse coming in during the active time will extend the active time. Others are non-retriggerable, meaning that pulses coming in during the active period will be ignored.
 
“One-shot circuit” That is exactly what I was needing. I think I found a good circuit in a book I had off of the good ol 555. Thank you.

What this is for is a small portion of a circuit I’m making for my aunt. She wants a meter that has a few LEDs indicating the noise level of the room. When my 10 year old cousin gets to loud it will flash the top red Led and trigger the One-shot circuit to set off an alarm of sorts to let him know he is being too loud. XD
Its been an interesting project so far.
I can send what I have so far later and see what y’all think.
 
The circuit you want is called a monostable multivibrator. Lotsa letters, but you already have the concept. There are four basic types: retriggerable, resetable, a "true" monostable, and whatever-the-hell a 555 is.

Your description is a bit light on details, but based on them, what you described might be done with as little as one transistor, one resistor, one capacitor, and maybe one diode, depending on the trigger source.

The most important factor in determining which type is right for you is the relationship between the trigger signal and the output. Once the trigger signal has changed state causing the output to change state, does the trigger:

a) always return to its initial state before the output times out.

b) always stay in the trigger state until after the output times out.

c) could be either one.

The electrical characteristics of the trigger signal are the next important detail, so, yes, please post your circuit.

ak
 
I was planning on making this schematic. Then adding the one shot circuit at the output of the last led on the string. I will send a full schematic when I get the chance. Let me know what you think.
 
You need to check what your sound sensor does - as has been discussed here recently it's NOT a preamp, and won't do what you were hoping. I also seem to recall that the LM3914 is wrong, it should be an LM3915 for audio.

Also your transistor is completely wrong, wrong type (PNP) and connected wrongly - no bias and 'weird' components in the emitter.
 
Ah yes, the infamous MrDEB sound sensor module! These come in a number of variations, but the important point is that the pot isn't a volume control – it sets the threshold level on a comparator, and the output is a digital output when the sound exceeds the set level.

This is exactly what you want here, as you don't need the LM3914 unless you want an LED indication too. The sound sensor already handles the comparison. There's a description below.

Nigel is right about the LM3914 vs LM3915 Chips. LM3914 has a linear output (i.e., the steps are equal intervals) while the LM3915 has a logarithmic output which more closely resembles how the ear reacts.

 
Although the LM3914 is an analog ic I have found many different schematics on audio level meters with it so I may have already purchased a few (a little to hastily.) I see the weird factor in that schematic I posted and am looking for another. I never thought that the mic module was a small preamp of sorts, but I was a little confused on the digital aspects of it. I had a few already and thought they might contribute a little to the circuit than a regular mic. I will check the board on it as the article you posted suggested and find the analog out.

I may look into a op amp from the mic to the signal of the ic for better results. I can’t remember if the lm386 was an op amp but I know I have a few of those.
 
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There are a ton of mic preamp modules on ebay that are actual preamps. The one you show is a comparator. By itself, it will deliver 1/2 of your requirements, the part about triggering a beeper when the ambient sound is above a certain level. To get the lower-level LED indications, you have two basic options.

The 3914/15 gives lots of indications, and these can be spaced apart linearly, or logarithmically like a VU meter. There are bar-graph LED modules on ebay for less than you can buy the parts in ones.

If you want to build from scratch, for less cost and bit a bit more work an LM339 quad comparator (or LM324 quad opamp) will drive up to 4 LEDs at any threshold levels you want.

If it were me, I'd use an LM339 like this:

1 section - microphone preamp

2 sections - low and medium level detectors - LED drivers

1 section - high level detector - monostable to drive a piezo beeper

Full disclosure - I'd do it this way partly because I already have a bunch of these parts. Where are you located?

ak
 
billybob said:
I can’t remember if the lm386 was an op amp
Click to expand...
It is not. The 386 is a low power audio amplifier designed to drive a speaker. BUT - many ebay "mic preamp" modules use this part because it requires very few external components. Poor noise and distortion performance, but cheap.

ak
 
Perhaps a simple op-amp or preamplifier circuit feeding into the 3914.
The more I look at the mic module the more useless it seems.
I know there are a lot of cheap VU meters, but my goal isn’t neccicarily to have a working project on the first try, I enjoy the process of building my own as well as the experience I can gain.
There is however a point when I have to weigh the options of more money and knowledge or quick functionality XD

Im in Georgia by the way.
Thank you,
Ben
 
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The ebay Sound Sensor Module has a +5V or 0V digital output, to show sounds or no sounds. It shows a transistor that "might" be a preamp as shown in post #7 but the transistor has no output connection and if used as a preamp it will need a DC-blocking coupling capacitor and the display driver IC will need a resistor to 0V on its input.

The PNP transistor shown in this thread has no function and should be NPN then it is absolutely useless.

The logarithmic LM3915 is obsolete and not made anymore. Fake or defective ones are available on ebay.
The LM3914 is still made and used as a linear voltmeter that cannot show 10 audio levels, just loud or quiet.

Years ago I made a Sound Level Indicator circuit with a microphone, an LM3915 and opamps. It is 15 years old, has worked 24/7 and still works perfectly.
I used a peak detector circuit that shows actual levels instead of the blur of many levels at the same time in the simple circuit in this thread.
 
This site looks promising and easy to understand. https://circuitdigest.com/electronic-circuits/vu-meter-using-lm3914

To clarify a few things, I don’t need a accurate VU meter with rolling LEDs to the sound (if that makes any sense) I just need something that will let my cousin know his voice is getting a little to loud and to alert him when it reaches a threshold.

Thank you audioguru for your insight, if you happen to come across the schematic for the one you used, that might be a possibility.

-Ben
 
Circuitdigest is made by student kids in India like many other online sites. They have many errors because they have not learned about electronics yet.
They post many videos because YouTube pays them. Their VU meter circuit has some errors.

I did not find my schematic, instead I designed the circuit to do what I want. It is here:
 

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China and India only comprise 36% of the world population. Audioguru still has nearly ⅔ of the world's population to disparage.
 

The note on the schematic is inaccurate. This is a half-wave rectifier with a gain of 100. If the intent is to envelope-detect the audio waveform, it needs a diode.

ak
 
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