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Does this circuit look correct? (Audio related)

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Rorut

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I have a small circuit with a piezo transducer that I can connect an audio signal to. It also has output for sending the signal to a amplifier.
The piezo is like a small drum making sound from the input hitting it.

I accidently damaged it and need a new one. I tried to draw a diagram from the circuit board but I'm not sure I got it together correctly. Maybe someone can point out any directly visual mistakes a made?

(Had the transistor the wrong way first, thats because it looks a bit wierd after correction)

Thanks
 

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audioguru

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1) A piezo transducer has a high impedance so your 10k volume control is severely overloading it and is reducing its output level and might be reducing its low frequencies even more.
2) The pin 3 input of the first opamp is not biased so it will severely distort the drum beat.
3) If the piezo output goes negative then it will destroy the input pin 3 that has a maximum allowed voltage of only -0.3V. You do not know if the wiring of the piezo allows its output to be positive, negative or both.
4) The base of the transistor is not biased so it will severely distort the audio.

I guess the output of the first opamp charges the 3.3uF capacitor during positive drum beats and the charge biases the input of the second opamp so its output can supply a positive collector voltage to the transistor so it can produce distorted audio for a moment until the capacitors discharge.
 

Rorut

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Thank you audio guru!
As volume control, do you mean the 10K resistor?
(There is no 10K potentiomter there)

What do you mean with "biased" ?

About piezo and if it goes negative and destroys pin 3. How can i fix this?

Trim pot is for sensitivity of piezo other pot is for time it will sound
 
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audioguru

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As volume control, do you mean the 10K resistor?
(There is no 10K potentiomter there)
Sorry, the 50k trimpot is the volume control and its value is much too low for a piezo transducer to feed. Add a Schottky diode to clamp negative input voltages then use a 1M trimpot.

What do you mean with "biased" ?
Basic electronics. Audio and the drum beat are AC signals that swing their voltage up and down. You have the (+) input of the opamp (pin 3) biased at 0V so it can swing up but it cannot swing down so half of the drum waveform is cutoff. Make a voltage divider with two resistors in series so that half the supply voltage is fed to pin 3 then it (and the opamp output) can swing up and down. A coupling capacitor is also needed. The transistor also must be biased so that its base and collector can swing up and down.

About piezo and if it goes negative and destroys pin 3. How can i fix this?
Add a Schottky diode (1N5817) from pin 3 to 0V so that it conducts and clamps the voltage so it cannot go more negative than about -0.2V.
 

Rorut

Member
Thanks! Understand but not everything. Will try to add the adjustments to the diagram that you explained.

If that many errors I probably missed things when I tried to make the diagram from the circuit board. Circuit was working very well before some of the copper path got loose from the board.

The 50k pot that you mentioned as volume cotrol is for adjusting "hit" sensitivity for piezo. (Could adjust it without problems with the 50k pot) Maybe that is the same as volume in theese terms? Thought volume used log pots.

Im such a beginner. But Im learning from your advises (I hope :) )
 

audioguru

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Yes I noticed that your sensitivity control (I call it a volume control) is LIN and not LOG.

What do you want the circuit to do?
I think each time the piezo is hit then the audio output will produce a loud POP at the beginning of a timed duration of very distorted audio.
 

Rorut

Member
The circuit/synare. It is made for sirens. You connect your siren output to the input where the transistor is. And then you connect the output to a anplifier. When you hit the piezo it makes a sound from the connected siren. With the 1M pot you can adjust how long time you here the effect fading out

Like this:

Siren and that piezo circuit in one box
 

audioguru

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Maybe your siren sounds better when it is severely distorted by the transistor.
 

Rorut

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Im unsure...severely distorted sounds like something bad :) Just want the circuit to be functional like the one I have/had. Dont know if they did things by purpose like they are or maybe you are more skilled and pointed out some weak points. I dont have much experience in electronics and can not tell whats best because I dont know how things work. Very basic knowledge
 

Rorut

Member
This is how the board looks like.
Reflected one image for easier matching.
There is some other stuff that that is not used. Only the components I have in my diagram
Did I miss something or does it look like what I draw?
 

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audioguru

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Sorry, I did not look at the pcb. Does it work like what you want?
 

Rorut

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I breadboarded from the diagram and it works same as the original circuit. Does not sound distorted. But now Im very curious to test your solution audioguru. It seems lile you found some serious problems in the circuit and would be fun to improve it
 

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audioguru

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I looked in Google for some good piezo e-drum circuits and I found hundreds of them designed wrong, only a few were correct.
You know what a sinewave looks like and its smooth sound, like a flute? Here is a properly biased transistor, its good sinewave output and its high gain. Here is the same circuit without bias, its input needed to be increased 50 times for it to produce this small buzzer sounding (very distorted) output:
 

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audioguru

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Yes, R1 and R2 will bias the input of the opamp if there is a coupling capacitor between the slider of the sensitivity control and pin 3 to block the 0V from the control.
R4 and R5 will not work without an input coupling capacitor and an emitter resistor since the base-emitter diode of a transistor changes when the temperature changes and each transistor is a little different even if they have the same part number.
Where is R3?

EDIT: The 10k resistor to 0V from pin 2 of the opamp needs a capacitor in series with it to prevent the opamp from amplifying its DC bias voltage.
 

Rorut

Member
Yes, R1 and R2 will bias the input of the opamp if there is a coupling capacitor between the slider of the sensitivity control and pin 3 to block the 0V from the control.
R4 and R5 will not work without an input coupling capacitor and an emitter resistor since the base-emitter diode of a transistor changes when the temperature changes and each transistor is a little different even if they have the same part number.
Where is R3?
yes where is r3 :) my brain is a bit tired...
is a coupling capacitor blocking DC and letting AC through and should I use ceramic or electrolyte?

How do I know value on the coupling capacitors and the emitter resistor?

Should R1,R2 and R4,R5 have same values?
Im learning something now :)
(R3 can be emitter resistor)
 

audioguru

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A ceramic capacitor acts like a microphone and can cause noise or feedback. Its value changes when the signal voltage changes which distorts low frequencies. I try to use a film capacitor for audio coupling but an electrolytic must be used for values higher than about 1μF.

R1 and R2 should be the same value so that the input and output of the opamp can swing symmetrically up and down. R4 and R5 are determined by how much is the supply voltage, collector current and collector and emitter resistor values. Look at my example with the 9V supply, the 10k collector resistor and the 1k emitter resistor your R4 is 75k and your R5 is 15k. Then the average collector voltage is 4.5V so it can swing equally a few volts up and down.
 
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