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1  Using Arduino / General Electronics / Re: Piezo horn, or is it a speaker ? on: August 26, 2014, 02:14:19 pm
Paul__B

Thank You very much for the replies.

I do not understand the RMS calculations that you quoted, so that is on my ' to do ' list to research and grasp.

I do think that it would be easiest of I change the speakers to piezo horns. The circuitry is already in place and I just feel more comfortable working with the lower current.

Regards
Dave
2  Using Arduino / General Electronics / Re: Piezo horn, or is it a speaker ? on: August 26, 2014, 04:04:45 am
I think I understand this now.

The piezo ( open circuit over terminals ) is a high impedance device, and only needs high voltage / low current to work.

The speaker ( 4 ohm closed circuit over terminals ) is low impedance ( 4 ohm ) and requires high current.

The H-bridge is capable of a 1 Amp output current up to 36 V.  I am using 12V.

So while the piezo worked nice and loud on the 12V ( low current ), the speaker can only draw a maximum of 1A from the H-bridge.

The speaker is rated 75W, so at 12V it should draw a maximum of 6.25A.

But limited to 1A by the H-bridge, the speaker will be operating at 1 / 6.25 = 16% of its potential volume.

Does it sound like I am on the right path here ?

Solution would therefore be to use the H-bridge for the single piezo at the finish line, and to user a different component to drive the 3 speakers at the start line.

On second thoughts, the 3 speakers X 6.25A each = 18.75A current required ?  Sounds like a dangerously high number to me. Maybe I need to look at replacing the 3 speakers with 3 piezo horns and keep the H-bridge.
3  Using Arduino / General Electronics / Re: Piezo horn, or is it a speaker ? on: August 26, 2014, 03:26:40 am
Hi Paul

Thank You for your feedback.

Both horns look visually almost identical to the center pic on this page : http://www.prosono.com.tn/TWEETER.php#

They are connected in parrallel ( same wire from H-bridge pin 3 with 150 ohm resistor to the + terminal on each speaker, and H-bridge pin 7 with 150 ohm resistor connected to the - terminals ).

I also thought about the possibility of the 3 speakers / horns being a problem, and disconnected the last 2, so only 1 speaker remained connected - no change in volume.

My supply voltage on the H-bridge pin 8 is 12VDC.

You say : Connected in series : so like this :

H-bridge pin 3 > 150 ohm resistor > horn 1 + pin.
 Horn 1 - > horn 2 +
Horn 2 - > horn 3 +
Horn 3 - > H-bridge pin 7.

This should be no different to the above test I did by connecting only 1 of the speakers.

I also tried using only 1 output from the H-bridge to the speaker positive pin, and the negative pin direct to ground. Same volume.
4  Using Arduino / General Electronics / Piezo horn, or is it a speaker ? on: August 26, 2014, 01:56:33 am
I have a single piezo horn in my system that I need to replace with 3 horns ( swim event starting horn ).

The existing horn spec is :

Impedance  1000 ohm @ 1 kHz
Voltage Nominal  15 V
Voltage Peak  35 V
Frequence Response  3 kHz - 30 kHz
Sensitivity  1W,1m   92 dB

and I am driving it with the attached circuit.

The tone and volume is perfect, but I need to replace this with 3 horns that I can position along the start line of the pool.

I have 3 other horns, but connecting these and testing gives a much lower volume - hardly audiable. The tone is correct, but the volume is way too low.

The only specs that the supplier can give me on the new horns is 75W.

I measured resistance over the terminals.

Old horn : no circuit over the terminals.
New horn : 4 ohms.

So what am I working with here ?

Is the old horn a piezo and the new horn a speaker ?

Any guidance on how to get the new horns working would be really appreciated.
5  Using Arduino / General Electronics / Re: Piezo Horns for swim race starter - flyback diode ? -- Updated on: August 25, 2014, 03:37:48 pm
Update :

I tried the sn754410 H-bridge today.

Connected the existing single horn as in the attached diagram, and used the toneAC library - so simple to use smiley

Code to generate a test sound for 400ms, repeated with a  4 second interval.
Code:
// code adapted from the toneAC library examples
// Connect your piezo buzzer (without internal oscillator) or speaker to these pins:
// Pins 11 & 12 - ATmega2560/2561, ATmega1280/1281, Mega
// Be sure to include an inline 100 ohm resistor on one pin as you normally do when connecting a piezo or speaker.

#include <toneAC.h>

void setup() {
}

void loop() {
   delay(4000);
toneAC(1500);
   delay(400);
   toneAC();
}

Sound volume appears higher with toneAC than when using the tone library and only 1 pwm pin - kids were complaining from the next room.

Tomorrow I will connect the 3 piezo horns and test again.

I am using a 12VDC 5A power supply on pin 8 of the sn754410 , so think I have plenty of power to drive the 3 horns.

Once completed and working, I am thinking to add a second horn at the finish line of the pool, to signal that all lanes times are clear and swimmers can exit the pool. My intention is to use the same H-bridge, and the same PWM pins ( join sn754410 pins 2 + 15, and 7 + 10 ) so both sides of the H-bridge get the pwm outputs from the Mega. Control is with a digital output High to the H-bridge pin 1 or pin 9 ( to enable the relevant side of the H-bridge, so sets which horn is to be activated ) .
6  Using Arduino / General Electronics / H-Bridge for Piezo Horn on: August 23, 2014, 05:18:55 pm
OK. I have tried to understand the info supplied in this thread, as well as the sn754410 IC.

If my understanding is correct, it was mentioned :
It is not a wise idea to feed a piezo with pulsating DC. It may damage the piezo.

this was a bit confusing as the horn has a + and - symbol on the terminals.

However, I later found the toneAC library, and reading seems have me thinking along these lines :

when driving the piezo ( via a 2N2222 transistor ) from only 1 pwm output pin -- pwm is used to set the duty cycle at 50% so that the piezo is repeatedly 'activated' at the required frequency, and after each activation the piezo resets as the output pin shuts down, and the pull down resistor discharges the piezo. At the next frequency 'loop' the output pin again activated the piezo. So it relies on a natural 'fall back' after each pulse.

With the toneAC library, we use 2 output pins, and these are used to activate the piezo with the first pin ( in my mind, 'activate' = a metal disc being pushed up like in a small solenoid ) for the first 50% of the frequency cycle, and then using the second pin to push the disc back down in the second half of the cycle.

Does this sound about right ?

If correct, then I should be able to use the sn754410 as in the attached diagram to control the piezo.

I am guessing that if I wanted to control the volume of the piezo, I would only need to adjust the supply voltage to pin 8.

Any advice is, as always, greatly appreciated.




7  Using Arduino / General Electronics / Re: Piezo Horns for swim race starter - flyback diode ? on: August 23, 2014, 02:13:38 am
Thanks Paul__B and polymorph

You advice is really appreciated.

Would either of you be willing to post a circuit diagram of exactly how this should be done ?

My knowledge on this is so limited, and I am really not wanting to blow the only H-bridge that I have and then have to wait weeks for a replacement ?

Regards
Dave
8  Using Arduino / General Electronics / Re: Piezo Horns for swim race starter - flyback diode ? on: August 22, 2014, 11:40:18 am
Many Thanks Paul

The LM386 is probably 2 weeks away from being able to arrive here - and with our postal service, there is less than 20% chance that once ordered it will actually arrive.

Could any of the other components I have not be used instead ?

They are :
ULN2803 Darlington Driver 8 channel ICs
IRFZ44N 41A 55V N-channel MOSFET
SN754410NE quad half-H drivers
74HC595N shift registers.

Again, Thank You for your advice.

Regards
Dave
9  Using Arduino / General Electronics / Re: Piezo Horns for swim race starter - flyback diode ? on: August 22, 2014, 08:17:24 am
Hint:  Just get an LM386.

Many Thanks for the feedback. Obtaining parts around here is a very long process, but I do have the following in my 'box of bits' :

ULN2803 Darlington Driver 8 channel ICs

IRFZ44N 41A 55V N-channel MOSFET

SN754410NE quad half-H drivers

74HC595N shift registers.

Can you please advise me which, if any, of these could be used to drive the 3 piezo horns, and any advice or diagrams of the best way to use them would be really appreciated.

I have little knowledge or understanding of these components, but my gut feeling is to use the PWM pin 12, connected to a 2N2222, to control the IRFZ44N MOSFET, which would power the piezo horns. My concern is that the horn, when given power ( 12VDC ) makes a single 'click' and only makes a tone when the Arduino Tone is used on the PWM output pin - this has worked with a single horn connected to the 2N2222. Is a mosfet capable of switching the power to the horn at the same speed as the 2N2222 ?

This is a kids charity project I am doing ( which makes it more important to me ) and I am starting to run out of time to get the rest of the project completed and could really use any assistance that you could offer to get these horns working reliably.

The rest of the system with the start timer, android tablet based internet web server, php script for event management and all the whistles and bells is all done - just the horns to complete now.

Regards, Dave
10  Using Arduino / General Electronics / Re: Piezo Horns for swim race starter - flyback diode ? on: August 22, 2014, 01:20:57 am
No, but you can probably drive three transistors from an Arduino pin.
As always, though...we're missing schematic, etc.

Here's the diagram of the existing setup.
11  Using Arduino / General Electronics / Re: Piezo Horns for swim race starter - flyback diode ? on: August 21, 2014, 01:42:36 pm
It is not a wise idea to feed a piezo with pulsating DC. It may damage the piezo.

Is ' pulsating DC ' and the arduino samples ( like http://arduino.cc/en/reference/tone ) not the same thing ?
12  Using Arduino / General Electronics / Re: Piezo Horns for swim race starter - flyback diode ? on: August 21, 2014, 09:23:27 am
1000 ohm impedance divided by 3 is 330 ohms, not unreasonable but at 7 kHz instead of 1 kHz, that is one-seventh or 50 ohms so you could be looking at a current draw of a quarter amp, depending on the other details following:

My main power supply is 12VDC 2A. Apart from the horns, the only other components are the Arduino itself, and a 3.3V reg for the ethernet module.

The piezo driver is a capacitor, of itself it will not generate a back EMF unless it resonates.  You have not specified the pull-up resistor value.  OTOH, to drive it correctly it should have an inductor across it, which will generate a back EMF however that is exactly how you properly drive piezo "screamers".  The whole situation becomes much more complex.

Output pin 12 has a 1K resistor to the transistor, and a 100K pull-down to ground.
The negative pin of the horn that connects to the transistor, also has a 550ohm pull-up resistor to the 12V line.

I hope you are not feeding 9V into Vcc.

Sorry, that should have said 'the Vin' pin of the Mega, which as far as I understand, does not require 5V regulated power.
13  Using Arduino / General Electronics / Piezo Horns for swim race starter - flyback diode ? on: August 21, 2014, 08:16:19 am
I have a piezo horn that I will be using to sound the start of swim events.

I can not locate a datasheet, except for this info :
Impedance  1000 ohm @ 1 kHz
Voltage Nominal  15 V
Voltage Peak  35 V
Frequence Response  3 kHz - 30 kHz
Sensitivity  1W,1m   92 dB

I am using a PWM tone of around 7 kHz.
The piezo horn is connected with a constant 12VDC to the positive pin, and the negative horn pin connected to a 2N2222 or 2N3904 NPN transistor.
The piezo negative pin has a pull-up which I found was needed to 'reset' the horn between pulses from the transistor.
The transistor is driven with a series resistor from pin 12 of a Mega 2560.
The horn is currently working and I am satisfied with the sound and volume.

But since we are likely to have 400 parents shouting for their kids, I want to add another 2 horns so they can be positioned behind the starting line - don't want any parents complaining their kids couldn't hear the start horn.

So my questions are :

1. Should I be able to drive 3 horns from the same transistor ?

2. should I add diodes ( 1N4004 ? ) across the terminals of each horn ( like I need to do for relay coils ) ?

The 12VDC that feeds the horns is also connected to an 9V linear voltage regulator that is used to power the Arduino ( Gnd and Vcc pins ).

14  Using Arduino / Project Guidance / Re: Project too complicated ? on: June 22, 2014, 04:01:39 pm
My thought is "bugger the Arduino, protect the swimmers".
But hey! That's just my POV, as a swimmer.

Are you saying the cabling is safer when connected direct to the Arduino and using the internal pull-up resistors, or when using the 5V powered optocouplers ?

Since both systems are using 5V, would there be any difference ?

And YES, protecting the swimmers is of utmost importance. One of the kids is my own, and he has one heck of a bad a$$ mommy.
15  Using Arduino / Project Guidance / Re: Project too complicated ? on: June 21, 2014, 12:39:03 am
Thinking outside the box. Garden hose, when used normally, keeps water in. Run your cable through a new length of (dry) hose (use talc to make it slide through easier) to keep water out. Seal the ends of the hose with silicon sealant and keep them out of the water to keep your cable dry.

Herny_Best

Thanks for the idea about the hosepipe. Will also help to stop damage to the cable if someone accidentally stands on it, and will also prevent kinks.

I don't understand your need for optocouplers. One side of your switches will be ground, the other side will have a voltage on it, whether you use optocouplers or not.

OK. So the one side of the button / switch will be permanently connected to Ground, and the 'open' side will have a pull-up resistor. My thinking on the optocouplers was to use them to protect the Arduino.

But from your comment, it looks like Ground is Ground, so I can easily use a 5V supply for the optocouplers and the ground to the finish line and back.

Appreciate the input.
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