555 driving a high current speaker

[rubin]

i'm trying to move a 6 ohm speaker (50W) with a square-wave from a 555 timer (around 10Hz, duty-cycle 50%). on the high-signal, I'd like the speaker so receive a maximal current flow, from my 12V 1A power supply.

I have this high current darlington transistor (TIP121)

I don't know much about electronics, and I guess some caps and resistors might be in order, to avoid damage for the 555 timer. what would be the simplest circuit for doing that?

thanks in advance!
rubin

 

[aljamri]

With this Frequency ( 10Hz ) you will not hear anything from the speaker.

 

[rubin]

i know. i don't want to 'hear' anything. i want to drive its membrane up-and-down, for a demonstration.

 

[Dragon Tamer]

You'll need a resistor on the output of your 555 timer, I would make it about 470 ohms. With 10 Hz square wave you will hear something on the speaker, a bunch of pops from the speaker rapidly "jumping". You'll need a very large isolation cap fo the speaker and some way to slow the charge of the cap to make something to the effect of a sine wave or triangle wave.

You should also check the frequency response of your speaker, most won't go below 15Hz (humans can't hear below 20Hz).

 

[rubin]

thanks. i don't expect to hear a sound, i just want the speaker's membrane to jump up-and-down..
where does the caps and resistors go with respect to the speaker, the 555 output, and the TIP121 ?

 

[colin55]

Just connect the base of the transistor to pin3. Connect the collector to 12v and the emitter to the speaker. The other speaker connection goes to 0v.
Put sand on a piece of paper on the cone and see it vibrate.
Use a pot to alter the frequency.

 

[rubin]

without any extra caps/resistors/diodes ? it won't fry my 555?

 

[colin55]

You can add all these components if you like, it won't make any difference.

 

[Sceadwian]

You will hear it because of the square wave you're feeding it, the mechanical harmoinics in the speaker itself with all the ringing that's going to be produced at that power level will garuntee you hear something. Keep in mind when I said ringing I meant it will go above and bellow where you think a straight stop should be and it will ring back and forth for a short period of time (at a higher frequency) each time it does due to the elasticity, mass, inertia, and velocity of the speaker cone itself.

If what you expect is the physical medium to move from a low position to a high one and back and forth without any other frequencies it will not occur so you need to flesh you what your physical demonstration entails.

If you could feed the same speaker a perfect sine wave of the same frequency it will move up and down gradually and gracefully from one extreme to the other at the same frequency. If you wanted it to go from low to high quickly but without the ringing it gets much more complicated as you have to shape the waveform the speaker gets to avoid the ringing effects. If the mass you're trying to move with the voice coil is anything other than the weight of a needle you'll get more harmonic effects.

 

[colin55]

You will only hear the rattling of the sand on the cone.

 

[rubin]

thanks !!
once more question - how do I add some 'volume' control ??

 

[colin55]

The circuit suggested produces maximum volume. To reduce the volume, reduce the power supply voltage.

 

[rubin]

can't - it's a 12V power supply...
what kind of modification to the circuit should i make?

 

[Dragon Tamer]

Just add a pot in series with the speaker. A value of about 1k will do the trick

 

[colin55]

Add a 3 to 5 watt 3R3 or 8R or 22R wire wound in series with the spkr. It is possibly an 8R speaker.