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Tone Generator

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I found a tone generator on the net--see attachment that i want to use as a threshold sound.

My question is, can i apply a sinewave from a different source as well that varies from 200mV Peak to Peak to 0v to this circuit which is already oscillating with a pre set tone.
I want to vary the tone with the sinewave source when it is triggered, the sinewave is constant at 200mV until triggered & depending on how much it is triggered it can drop as low as 0v.

Is this circuit ok to drive a small 8 Ohm speaker as well?

Thank you!


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I should have gave more information,

I have a sensor circuit that changes the tone of a piezo speaker-buzzer when something approaches the sensor, at the moment i have to limit the output to 200mV to the piezo as the sound gets to high pitched to notice small changes with higher voltages.

If i move my hand near the sensor i get a nice quick tone change but if i move my hand closer to the sensor the output to the piezo drops to 0v & of course no sound.

I thought the tone generator attached i may be able to use somehow to create a dip in the tone when triggered with a small 8 ohm speaker instead of the piezo?
The circuit you posted has a very large FLAW! There should be a polarized, 47uF to 470uF capacitor in series with the speaker, otherwise, the power consumption of the circuit will be horrible, the 555 will get hot, and you may even cause the speaker to bottom out in the cone's range of travel. Speakers should always driven with AC only; so the capacitor is needed to block the DC component.
The circuit does not have a battery nor a power supply to power it.

The current into an 8 ohm speaker is too high for the 555 IC.
The 555 IC has a square-wave output like a buzzer and does not have an input for your other signal.
You can try to insert a common-source or common emitter transistor between the rail and the 1MEG resistor. Then feed the signal into the transisotr's gate or base. You'll have to know something about analog design and how the timer's frequency network operates. But you might be able to pull it off.
I'd use a 64Ω speaker (if I could get hold of one) and a capacitor.

What voltage is the power supply?

If it's 6V you'll probably be all right just connecting an 8Ω speaker to 0V via a capacitor.
pin 5 is the control voltage pin which can be used to vary the 555 output but that is a very poor circuit which does have a lot wrong with it..........
Thanks everybody,

I was unsure about the circuit that's why i thought it better to ask, lucky i did, thanks.

Ok, i have an AC sinewave output of around 200Mv peak to peak that decreases when an object nears the sensor & can go to 0v with large or close objects. I can adjust this to around 500Mv output but not with the Piezo tones.

How can i drive a small 8 ohm speaker with a pleasant threshold tone that dips when an object is near the sensor.
Is there a certain type of circuit that i can look up or any suggestions.

The sensor circuit is very sensitive & works nicely & i would like to finish it but i am having trouble with trying to get a good fast responding pleasant tone & one that doesn't die out to no sound with the voltage decrease.

The Piezo is to high pitched, is there a way to make the piezo have a more pleasant tone maybe, besides hitting it with a hammer:D

The circuit power supply is 12v, Frequency of the sinewave is 1kHz.

Any suggestions would be very helpful.

I have thought of a way to do what i need, i think, if i can explain it?

I can get a good oscillation signal from another part of my circuit that will be the threshold voltage with the same frequency as the output to the Piezo- 1kHz

If i use an opamp as an Adder circuit & apply this threshold signal & the output signal to the piezo into it i will get X combined voltage out of it & when the sensor drops voltage it should indicate this at the output of the Adder circuit. I hope!

If i feed this output into a push pull transitor audio amp setup then i should be able to drive my 8 ohm speaker.

Does this make sense?

But i was just thinking that if the output signal to the piezo does drop to 0v the sound will just bottom out to a single threshold tone & not dip as well which is what i want?

What is the maximum voltage an 8 ohm speaker will take?


After thinking some more i think i need to have a set oscillation & use this to create the Threshold Tone & then use the 200Mv to 0v to decrease- pull down the threshold oscillation, this way it will dip the tone when the voltage from the sensor decreases.

The trouble is i have no idea what i require to do this or what a circuit like this is called?

Any Suggestions?
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1) You have a sine-wave signal that has its level change when something moves near a sensor? The level drops too low and stops the signal?
Then you need to design a circuit that limits the reduction in voltage.

2) You have a square-wave signal from a 555 oscillator that you want to play at the same time as the other signal?
Then mix it with the other signal with two resistors at the input to a power amplifier IC that drives the speaker.

3) Do you want the level of the square-wave to change the same as the level of the sine-wave?
Then you need a fairly complicated circuit that measures the level of the sine-wave signal and its output controls the level of a voltage-controlled-amplifier.

4) How much voltage you feed to an 8 ohm speaker depends on how loud you want it to be.
Power= Volts x Amps.
2V RMS= 5.7V p-p so the power amplifier needs to have a supply of at least 9V.
2V/8 ohms= 0.25A RMS.
2V x 0.25A= 0.5W. Look on the datasheet for your speaker to see how loud it will be when it plays at 0.5W.
Thanks audioguru,

I have no data sheet for the Speaker, nothing came with it when i purchased it--cheap Chinese. It is 8 Ohm .25W
"3) Do you want the level of the square-wave to change the same as the level of the sine-wave?"
Yes, i thought if i could like mentioned have a set threshold sound from an oscillator that is controlled by the sinewave then as the sinewave decreases in amplitude it will also decrease or increase the set threshold sound but not decrease it totally.
If i have a set threshold voltage of say 9-12v then when the sine wave decreases to it's maximum the threshold voltage will still be 3v or 6v etc still enough to have a tone in the speaker.

So it just dips the threshold sound as the sine wave dips.

I guess it is complicated but if i start to put things in the Too Hard Basket now at age 14 my future with electronics will look bleak.
I would rather make hard work for myself at this stage to learn more, i guess that's the right attitude to have when you really want to do things?

I am just not sure how to tackle some things--like this!

Thanks, i will keep at it!


It seems i was reading more into this than need be, i now have it working ok with a 555 timer & as the sinewave drops voltage the 555 increases it's frequency & keeps a tone. It's working just with the Piezo at the moment, now to add the speaker.
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I have a TDA2822M IC that i would like to use to drive my single 8 Ohm speaker with a single input into the IC.

I noticed on the IC Data sheet -Output Power versus Supply Voltage- (see attached image) that with a 5v supply the power output is roughly the same rating as my speaker .25W

Is it safe to apply more power into speakers than what there rating says.

For example can you safely exceed this limit of .25W a little if need be.

Thank You


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A tiny speaker with a power rating of only 0.25W is garbage.
The TDA2822 amp has horrible 10% clipping distortion when it has a supply of only 5V and a load of 8 ohms. Its distortion is 10%.
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Thanks audioguru,

I thought the same thing as the speaker being rubbish, i have just found another 8 ohm .5W but it won't help much. The only other "small" size speaker i have is 4 Ohm 15W, maybe i will give this a go with a different IC.

Sorry for the ignorance, could you explain how you got 10% distortion from the Data sheet--Attached


I have a LM380N IC-14 pin that i can connect as a bridge amplifier with a 12v supply & use the 4 Ohm 15W speaker, i understand i won't be able to drive it to full sound but does this configuration seem ok compared to what i mentioned before?


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Hello Codan,
You have been showing the curves from the datasheet for the little TDA2822M (8-pins case), not for the larger and more powerful TDA2822 (16-pins case).

With a 5V supply, an 8 ohm load and operating as a stereo amplifier (not bridged) the output begins clipping at about 240mW. The output is extremely clipped (a square wave) with 10% distortion when the output power is only about 300mW.

The LM380 is not designed to be bridged because it is a single amplifier. You must buy and test many LM380 ICs to find two that have the same offset voltage for them to be bridged. ICs that have two amplifiers on the same chip have matched offset voltages.

A 4 ohm speaker appears like a 2 ohm load to a bridged amplifier because the output current is almost doubled. Then the two LM380 ICs will burn out if the supply is 12V and the speaker is only 4 ohms.

Amplifier ICs made for car radios are bridged and can drive a 4 ohm speaker. With a 12V supply the output begins clipping at about 10W into 4 ohms. With a 14.4V supply the output begins clipping at 15W into 4 ohms. A TDA7240A IC is good for each channel. But the idle current for each IC is high at 120mA max.
I woulnd't bother using a real amplifier IC just to make a speaker go beep.

I'd add a transistor to the output of the 555.


why bother with a decent speaker or amplifier for a tone generator?


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A crappy tiny speaker with a power rating of only 0.25W is garbage. He might as well use a much better speaker and raise the supply voltage so that the amplifier has some power output.
It depends on how loud it needs to be?

A 0.25W speaker is quite loud when driven from the circuit I posted. It'll probably burn out if it's left running got too long though, if the supply is 6V the RMS power in the speaker will be 2.25W but for short bursts it'll be fine, I should know I've done it before. :D

This circuit is easier on the speaker but it drives the speaker with much less than half the power of the previous circuit.


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If you want it to be really loud then use this circuit but make sure the speaker can handle 18W.


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.... if the supply is 6V the RMS power in the speaker will be 2.25W.
The max output high voltage of the 555 is only 4V and the minimum voltage is 2V. With only 2V across it an 8 ohm speaker gets only 125mW for a sine-wave or only 0.25W for a square-wave.

For 2.25W an 8 ohm speaker needs 12V p-p and a 555 cannot supply enough current. The amplifier needs a 14V to 16V supply.
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