Sound amplifier

oracacle

Senior Member
I have had a search through and there are plenty of threads making use of the LM386 to amplify the sound output from a PICAXE. However I like to be difficult, and I have some spare op-amps on an LM224.
As it stand there is a single op-amp used for a photodiode light detector which works great, and was thinking that I should either put the other three to use of find a single supply single unit op-amps package.
I have tried a basic single op-amp non inverting (2 resistors, 1k from inverting input to ground, 100ohm from output back to inverting input) amplifier connected to a 0.2w speaker, this is fine but not greatly louder than just the picaxe running it.

I am no analogue circuit master I suspect that there are some here who have dabbled in this sort of thing who can point in the right direction. A net search shows up lots of option for dual rail amplifiers but no much in the way of single rail.
I have a couple of 1W 80ohm speakers from an old computer monitor which would be great to use, although I am aware more voltage maybe needed, ruling it out as an option
 

SteveDee

Senior Member
...A net search shows up lots of option for dual rail amplifiers but no much in the way of single rail...
I doubt that you will get satisfactory results from an LM224, but if you are looking for other low-cost suggestions, then take a look at the NJM2073D.

You should be able to get this device for less than £1, and I think I've seen it as low as about 50p somewhere (maybe Amazon or eBay?).

It is a dual amp in an 8pin package, and capable of good sound quality from a single supply rail.

Check out my notes: https://captainbodgit.blogspot.co.uk/2016/03/the-njm2073d-amp-for-pi.html
 

hippy

Technical Support
Staff member
I am no op-amp expert either but simplistically, P = IV, and I would guess an LM224 and most other op-amps are no better at delivering the I and V which a PICAXE can.
 

techElder

Well-known member
...to amplify the sound output from a PICAXE....
You don't need an audio amplifier to amplify the digital output from PICAXE. All you need is a higher voltage, a mosfet (2N7000) and your higher power speakers.

Add some capacitance to help tone down the high frequencies in the digital output, and I certainly would consider using a capacitor to couple the sound to the speaker.

Set the maximum current in the speaker circuit to align with the speaker's power capability (and voltage applied) when the mosfet is driven full on.

Think digital; not analog.

That's my opinion WYLION.
 

erco

Senior Member
That's my opinion WYLION.
WYLION=Whether You Like It Or Not.

Ha, I had to look that up too.

Back OT, I've made several one-transistor amps which work fine. But when I found these amplified speakers at the Dollar store. They come in a nice ball housing, have a great stereo amp (I rewire for mono) and a loud PM speaker. They run off two AAA batteries, but I wire to my Picaxe 4-5V supply. Plenty of room inside for other circuitry too:

 

sghioto

Senior Member
I have tried a basic single op-amp non inverting (2 resistors, 1k from inverting input to ground, 100ohm from output back to inverting input) amplifier connected to a 0.2w speaker, this is fine but not greatly louder than just the picaxe running it.
Not that it will make any difference but from your description you have the resistors reversed. Tex has a better solution but you don't necessarily need a higher voltage supply. Using a mosfet or even a transistor you can drive the 80 ohm speaker directly getting about 300mw of power. What exactly are you trying to amplify?
 

oracacle

Senior Member
The project will have a 7.2v supply. not greatly more than the 5v the rest of the circuit will run on, but more non the less.

I am using a simple sound command to generate beeps as an audible notification of something happening. The idea is that the project can be left running for some time, or needs to inform of an event I am at some distance away.
 

geezer88

Senior Member
The idea is that the project can be left running for some time, or needs to inform of an event I am at some distance away.
This portion of your project reminds me of my mail box alert that I recently finished. A picaxe monitors the door on the mail box, 1 km away from my house. It sends a call signal with a family radio transmitter. This signal when the mail arrives eliminates unnecessary trips to check for the mail. It's been working great for the last two months.

I have identified a weakness in my planning that you may want to avoid. That is the event when I'm making some kind of noise that prevents hearing the alert. Or if I've stepped outside for some reason. I will add a latching and reset circuit, so when I return, I will see the led flashing that the mail has arrived.

tom
 
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