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1  Using Arduino / General Electronics / Re: Difference between a piezo transducer and a piezo sensor on: February 09, 2013, 06:45:02 pm
I am thinking of driving the piezo disc with 9V using a transistor in order to see if a louder sound is possible:




However it seems that the tone() function of the Arduino is not working in that scenario. Is it possible at all or in that case only a buzzer with a built in drive circuit can be used?
2  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 06:21:50 pm
A newer version of the schematic then, incorporating some of the feedback. The simulator reports 13.5 mA at the LED and 430 μA at the base for an hFE of 32. I didn't change the 10kOhm resistor of the base to 4.7kOhm as that would give an hFE of 16. It is still not entirely clear what the hFE should be and since it is a loose value anyway and the datasheet does not give the value of hFE for 5V i guess an approximate value of 30-70 should suffice.

https://www.circuitlab.com/circuit/gmmqaq/2n2222-led/
3  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 03:54:09 pm
Ok, now i am a little lost. Remember, i am new to electronics...

Quote
The way to deal with this is to calculate the
"nominal" base current required for the app, and then about double it, by using a value
for base resistor R2 of about 1/2 the nominal value.

I am not entirely sure i understand that

Quote
Also, your values above may be wrong. Normally, the hFE value is "lower" for larger
collector currents.

What values are wrong? For the resistors, or the calculated current in the base and the the LED?

Quote
I'm curious why OP chose to switch the high side instead of using the original circuit. With this setup, the transistor is operating in the linear region and will never reach saturation.

So, there is actually a difference if the LED is in the collector side or the emitter side ?? smiley-eek-blue
4  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 10:57:42 am
Thank you all! Just for reference, i eventually used 10kOhm in the base and 100 ohm as a current limiting resistor, for ~100 μΑ base current and ~11 mA through the LED :



https://www.circuitlab.com/circuit/gmmqaq/2n2222-led/
5  Using Arduino / General Electronics / Re: Difference between a piezo transducer and a piezo sensor on: February 09, 2013, 08:08:55 am
So if i understand this correctly, if the same piezo disc is used as a sensor, the 1 Mohm resistor must be add in parallel while if it is used as a speaker, a 100 ohm resistor must be added in series.

What happens though if it is used as a speaker (with the 100 ohm resistor) but someone knocks on the sensor? Isn't there the possibility that too much voltage will reach the Arduino pin (even if the pin is set to output)? Also if i drive the piezo with a higher voltage (for example with 9V through a transistor) shouldn't the sound be louder? Is there a datasheet that is common for all piezos, or each has different characteristics?
6  Using Arduino / General Electronics / Difference between a piezo transducer and a piezo sensor on: February 09, 2013, 05:01:14 am
Hi, i have used a piezo sensor before to detect pressure, following the example at http://arduino.cc/en/Tutorial/Knock. There, in order to protect the pin of the Arduino, a 1 MOhm resistor is used in parallel with the sensor.



Now i want to use the same piezo sensor as a transducer, driven from the Arduino for a home alarm project. But then, according to http://arduino.cc/en/Tutorial/Tone, only a 100 Ohm resistor is needed in series. So, what is the correct way to hook a piezo disc to Arduino in order to get high-pressure sound?

Thank you!
7  Using Arduino / General Electronics / A couple of questions on the 2N2222 transistor on: February 09, 2013, 04:52:42 am
Hi, this is my first post!

I recently started in the world of electronics and i am trying to make myself comfortable with various parts before starting to build real projects.

If this image is taken as a basis, i am struggling to calculate the correct values for R1 and R2:



As with the image, the voltage across the collector is 5V and the current through the Led should be 20mA. The datasheet states that the Collector-Emitter saturation voltage is 0.3V when the current at the collector is 150mA and 1V for a current of 500mA so i assume that for 20mA is even less than 0.3V?

Then if i understand right R1 = (5-0.3)/0.02 = 235 Ohm ?

For the base i am completely lost. The datasheet states that the Base-Emitter saturation voltage is 1.2V when the current at the collector is 150mA but what about a current of 20mA at the collector? Furthermore, the DC Current Gain is 75 for Vce = 10V, Ic = 10mA and 100 for Vce = 10V, Ic = 150mA, but i have a Vce of 5V and a Ic of 20mA. What is the correct gain so that i can calculate the correct value of R2?

Thank you!
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