speaker resistance

[michael boyd]

Hi

In manual 3, the speaker in the illustration shows a resistance of 40 ohms.
All the little speakers I can source are either 4 or 8 ohm, or am I not understanding this correctly

regards
michael

 

[inglewoodpete]

Michael, It helps to give a section or page number when referring to information in the manuals.

I presume you're referring to "Output Devices 4 - Piezo Sounder & Speaker". Earphones are usually 35 to 40 ohms.

If you want to use a speaker, you may find it difficult to find a 40ohm one. So you'd have to use a small audio matching transformer - available from hobby electronics shops. Personally, if I were to go down that path, I'd try to salvage one from a discarded transistor radio.

My choice would be to use the piezo tranducer option. They put out quite a bit of sound with 5v input. Don't confuse it with a piezo beeper, though. You need a a transducer that reqires an external drive circuit Eg this or this, both available from Tech Supplies.

Whoops. I see you're in OZ. You can order a piezo transducer from MicroZed

 

[westaust55]

The manual DOES indicate that if you only have an 8 Ohm speaker to install a 33 Ohm resistor in series.
See Manual 2 page 239

A 40 or 80 ohm speaker can be connected with two capacitors as shown.
For an 8 ohm speaker use a combination of the speaker and a 33R resistor in series (to generate a total resistance of 41R).

The circuit also shows some capacitors for use with a speaker.

As IWP has indicated, MicroZed (Queensland) sells some piezo speakers.
If you have the space, I recommend the larger PZ002 (at 90mm diameter) as good value and since it has a “cone” does produce a better volume than many of the thin flat types.

 

[Technical]

The problem is that a 4 ohm speaker across an output is effectively just a short circuit, not a good idea as the maximum current output of the PICAXE chip will be exceeded.

The circuits in the tune section of manual 2 show how to work around this. The capacitor cuts out the DC in the signal to prevent this short circuit.

 

[curry87]

Whats the preferred way of hooking a small 8ohm speaker .3a to a picaxe output pin via a cap or a transistor so the speaker can be operated within human hearing threshold frequencies ?

Can you treat a speaker in the same way of a piezo by using various tune commands in basic ?

What commands could be used with the speaker to generate a preset frequency ?

 

[gengis]

Technical's series capacitor is the easiest to implement with a low impedance speaker.

They do make high impedance speakers but I only see them pop up on the surplus market from time to time. They are employed in some TV sets and small portable radios.

A couple of noise making links I found interesting:
Murata application guide to piezo transducers and drive electronics. Real handy if you want to get max volume from bare discs or packaged transducers.

http://www.aurelienr.com/electronique/piezo/applic.pdf

DB products electromechanical buzzer schematic - similar to the old Bulava Accutron wristwatch driver circuit.

http://www.db.com.hk/en/technical.php?tID=2

 

[westaust55]

Whats the preferred way of hooking a small 8ohm speaker .3a to a picaxe output pin via a cap or a transistor so the speaker can be operated within human hearing threshold frequencies ?

Can you treat a speaker in the same way of a piezo by using various tune commands in basic ?

What commands could be used with the speaker to generate a preset frequency ?

Basically if you can use a Piezo sounder/speaker, you can also use a standard coil type speaker - you just need to ensure you do not overload the driving circuit and burn out the speaker coil - hence the capacitor.

I have a 90mm piezo direct connected to a PICAXE output
but have also on other modules using an 8 Ohm 0.4 Watt speaker with a series 30 ohm resistor driven by a simple transistor circuit with a simple filter (see circuit in the TUNE command in manual 2) into the transistor base - and no capacitor in series with the speaker.

The output from the TUNE command is always within human hearing range (unless the hearing aid has flat batteries)

 

[boriz]

Nice PDF Flooby. Stored for reference. Thanks.