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Simple Oscillator

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EDY19

Tue Mar 11 2008, 09:47PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

For a final project, we have to design a simple oscillator circuit on a small PC board. The driver has to be able to drive a speaker with at least .5W of power, and the signal to the speaker has to be centered around ground (0V average signal). I'm wondering what the best way to do this would be. I could do a Bridge Tied Load configuration with a dual op-amp and 555 for the signal, or a 555 with a capacitor on the output to center the signal around ground. Any other suggestions on methods of doing this? These are my favorite two so far, quite simple and I can design the circuit fairly small. Another possibility could be a 555 driving an amplifier chip such as the TDA7052 (~1W BTL) with the signal ground being a voltage divider at half of the small signal oscillator voltage such that the output is centered around the power ground.

...

Tue Mar 11 2008, 11:52PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

If you are using an 8ohm speaker you will need to get about 250ma of current flowing so you will need some type of amp.

If it were me I would just grab a 2n3904 or similar and build a class A amp, with a nice big dc block cap in series with the output. The power level should be so small that it isn't really worth going into higher power levels...

cheers

Sulaiman

Wed Mar 12 2008, 05:07PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The driver has to be able to drive a speaker with at least .5W of power,
and the signal to the speaker has to be centered around ground (0V average signal)
=================================================
To achieve a signal centered around ground the easiest way is to
drive the loudspeaker from a center-tapped transformer winding
with the center-tap going to ground.
A small audio output transformer such as found in transistor radios would be perfect.

Use a transistor to drive the primary winding with feedback from one side of the secondary to form an oscillator.

If you don't need a sine wave then a simple one transistor blocking-oscillator will be simplest/easiest.

EDY19

Wed Mar 12 2008, 06:26PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
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Posts: 408

I made a mistake in my first post- it doesn't actually have to be centered around 0V, but the average voltage across the speaker must be 0V, implying that I can't use a 555 output to drive it with a 9V signal (avg value for 50% duty would be 4.5V) However, I could use a 9V p-p wave centered around 4.5V, and finally, the size of the circuit is critical. I was looking at some higher powered dual complimentary mosfet drivers (.3A continuous) as well to directly drive the speaker- they can source or sink, so all i would need is a 555 and one of the dual complimentary driver integrated circuits to give around 2.5W

Dr. Dark Current

Wed Mar 12 2008, 07:03PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I would use a KA3525/SG3525 pwm controller, they can source/sink 500mA and you get additional sound effect by varying the duty cycle

They need a minimum of 8V supply voltage.

Proud Mary

Wed Mar 12 2008, 08:08PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

How about a simple square wave oscillator based around the LM386 audio amplifier? Cheap as chips, and only a few discrete components needed. You can also make a Wien bridge oscillator out of an LM386 too.

There are circuits for both sorts of oscillator in the LM386 data sheet, which you can download here:

EDY19

Thu Mar 13 2008, 03:46AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Well, I've found out a little more about the project. Right now, my idea is to use a dual op amp capable of sourcing 1A of current. one amp will be configured as an oscillator with a potentiometer as the frequency setting resistor. The other op amp powered by a 9V battery and used as an inverting amplifier with a gain of 0-1, I should be able to get a voltage swing of about -3.5 to 3.5 volts with a nonpolar cap between the load and the amp. Any defects in my plan? I have 5W speakers, and the signal will be a square wave, so is there a better way of getting closer to a 9Vp-p signal to the speaker? Since it is a square wave, mosfet switching is also an option for lower voltage drops across the switching devices...

Sulaiman

Thu Mar 13 2008, 05:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For the average value to be zero, put a capacitor in series with the loudspeaker,
whatever wavevorm you choose, the capacitor will ensure zero average volts across the loudspeaker
(not counting initial switch-on)

An LM386 sounds like an excellent choice.
9v battery with LM386N-3 or -4 gives 0.5W into 8 Ohms.

EDY19

Sat Mar 15 2008, 07:28PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Does anyone have an example circuit for the SG3525? I've looked at all the datasheets I can find, and every one has a test circuit with many additional features that I don't really need. From the datasheet, I have found that the timing resistor must have a maximum value of approximately 150k with a .1u timing capacitor. However, I can't seem to find equations relating duty cycle to resistor values and frequencies with the timing R/C values. Any example circuits or equations that any of you have used? I'm not asking for values, just pointers or examples of your push pull circuits using this IC.
Thanks!

Dr. Dark Current

Sat Mar 15 2008, 07:32PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

EDY,
here is a schematic from my old site (used as a flyback driver):

Pins 11 and 14 are the push-pull output.

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