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user9253
Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan
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 Posted: Wed Mar 20, 2024 5:36 am Post subject: LM386 Output Capacitor |
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Radio Shack's Engineer's Notebook by Forrest Mims is my favorite book.
https://www.zpag.net/Electroniques/Kit/The%20Forrest%20Mims%20Engineers%20Notebook.pdf
One page is devoted to the LM386. I made the "Audible Alarm".
I question the value of the 22 microfarad output capacitor because
all of the other LM386 circuits on that page use 220µF output capacitors.
Could that be a typo in the book and the value should be 220µF?
I used three 10µF capacitors in parallel and the alarm works.
Would it work better with a 220µF capacitor?
I increased the value of C1 to 1µF to get a lower pitched tone.
Are there any other suggestion for getting a lower pitched tone?
I have trouble hearing high frequency tones in my old age.
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Joe Gores |
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Ceengland
Joined: 11 Oct 2020
Posts: 391
Location: MS
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| Posted: Wed Mar 20, 2024 7:27 am Post subject: LM386 Output Capacitor |
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On Wed, Mar 20, 2024 at 8:40 AM user9253 <fransew(at)gmail.com (fransew(at)gmail.com)> wrote:
| Quote: | --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>
Radio Shack's Engineer's Notebook by Forrest Mims is my favorite book.
One page is devoted to the LM386. I made the "Audible Alarm".
I question the value of the 22 microfarad output capacitor because
all of the other LM386 circuits on that page use 220µF output capacitors.
Could that be a typo in the book and the value should be 220µF?
I used three 10µF capacitors in parallel and the alarm works.
Would it work better with a 220µF capacitor?
I increased the value of C1 to 1µF to get a lower pitched tone.
Are there any other suggestion for getting a lower pitched tone?
I have trouble hearing high frequency tones in my old age.
--------
Joe Gores |
It's been a really long time since I played with R-C filter formulas, but I'd bet that they went with 22 instead of 220 because they didn't need low frequency audio performance from the circuit. IIRC, the out capacitor sets the low frequency cutoff for the output. It's a 'point design' for a single, high frequency tone.
IIRC, the LM386 is basically a high current opamp. I'd suggest downloading a copy of either the TI or National analog IC 'cookbook' to see circuits. Here's a 1kHz square wave oscillator from the TI LM386 datasheet:
[img]cid:ii_ltzyc0v60[/img]
The square wave will give a 'nastier' more irritating tone, which is what you'd want for an alarm.
 Virus-free.www.avast.com[url=#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2][/url]
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user9253
Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan
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| Posted: Wed Mar 20, 2024 7:57 am Post subject: Re: LM386 Output Capacitor |
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Thanks for that circuit Charlie. I will give it a try.
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Joe Gores |
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user9253
Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan
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| Posted: Wed Mar 20, 2024 3:39 pm Post subject: Re: LM386 Output Capacitor |
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I made the Texas Instruments square wave oscillator, substituting a
speaker for the load resister. It works but with a high pitched tone.
So then I went back to Forrest Mims' alarm circuit and put a 10µF
capacitor in parallel with C1. Now I am satisfied with the deeper tone.
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