Hello all!

I have an app on my phone (Interval exercise timer) which can be set to notify over a predetermined span of time with interval notifications. For instance, I can set it up to work for a whole hour with 10 min notifications.

My initial cheap solution was to use cheap target PC speakers ($11) to amplify the noise file that comes out of the line out from the phone. This works well, except that the volume is not there. The reason I need this much louder is that we are using it during a racquetball shuttle (grown men banging a small ball against a wall as hard as they can, generating tremendous amounts of noise to overcome).

I imagine an op amp driving a small relay that will close the circuit to a 110v capable siren (to be sure about the db level that I want to achieve).

Of course I want to insure that there is plenty of isolation between the phone and the 110v source.

Does anybody have some example circuits that I could use with as cheap as possible components?

If there is any further clarifying information that I can provide please let me know.

Thank you,
Tim

 

Hello, I noticed this is your first post, welcome to the forum!

As for your project, I would probably use a small opamp to drive a 32Ω speaker as it will be coming from the headphone jack in your phone (assuming this) so if you find a speaker or buzzer of choice, you can build a small amplifier circuit around this opamp to boost the signal to help drive that device. There may be a different, or better way to do this, but I know it is an option for you.

I don't know how loud you need it, but try to keep it about 80 decibel or so, you don't want prolong exposure anything more than that as it can cause hearing damage.

A good idea is to look up the desired buzzer or speaker, and post their specs for the more inclined pro's to give input.

 

To start off with, I guess I need some suggestions as to what buzzer to get and then I can go from there.

I am inclined to use a buzzer that is about as loud as one could stand without causing
discomfort at 5ft. The courts we play in are 20'Wx40'Lx20'H with a 5' opening on the rear wall, up high. A little less than a door alarm buzzer is what I am looking for.

Lets say I want a circuit that would drive:
http://www.radioshack.com/product/index.jsp?productId=2062404

This 95dB Panel Piezo buzzer operates on 12VDC. Buzzer rated at 3,200Hz and carries 7mA current at 12V.

I will be looking to power it with 110vac with proper isolation.

Thank you,
Tim

 

I will see what I can find for a circuit that will drive that buzzer.

 

This one would be better because I would then be able to use just a 9VDC battery.

http://www.radioshack.com/product/index.jsp?productId=2062405#inTheBox
Operates on 6-14 VDC, with 150mA Current at 12V. Buzzer Tone rated at 2,000-4,500Hz.

Any suggestions as to a better way to do this?

Thanks!
Tim

 

You should buy a modern buzzer from a real electronic parts distributor. Then it has a detailed datasheet from its manufacturer.
RadioShack doesn't make anything and does not list the manufacturer nor a detailed datasheet.

Most little piezo beepers (buzzers) have their loudness rated at a distance of only 10cm (you are nearly wearing it on your ear). The "102dB" siren is 96dB at 20cm, 90dB at 40cm, 84dB at 80cm and only 74dB at 160cm (5' 3") which is not loud. The "95dB" buzzer has much less loudness.

 

Ok well I found this circuit from an onlie source via the google machine. It uses a 555 timer IC that drives an 8Ω, 0.5W speaker. The values can be changed to suit a piezo buzzer.

Hope this helps.

 

Ok well I found this circuit from an onlie source via the google machine. It uses a 555 timer IC that drives an 8Ω, 0.5W speaker. The values can be changed to suit a piezo buzzer.

A piezo buzzer is small and uses a small amount of power. That is why it is not very loud. A 555 has lots of output power to drive an 8 ohm speaker but it is not used by a little piezo buzzer.

An 8 ohm speaker is not efficient. A piezo buzzer is efficient.
They might produce the same output loudness when they have the same signal voltage. But the speaker needs a high current and the piezo needs a very low current.

 

A piezo buzzer is small and uses a small amount of power. That is why it is not very loud. A 555 has lots of output power to drive an 8 ohm speaker but it is not used by a little piezo buzzer.

An 8 ohm speaker is not efficient. A piezo buzzer is efficient.
They might produce the same output loudness when they have the same signal voltage. But the speaker needs a high current and the piezo needs a very low current.

I found another one that uses a more complicated circuit but is supposed to be very loud using a piezo beeper. This might work better for you.

 

The Kobitone beeper transducer produces an output of 85dB at 30cm (one foot) with a 9V p-p drive. Then with a drive of 40V p-p its output is about 100dB at 1 foot, 94dB at 2 feet, 88dB at 4 feet and about 86dB (not very loud) at 5 feet. Like a fire detector.

 

Like a fire detector.

Hmm, interesting, that makes me think I could cannibalize a smoke detector for its noise making ability.

Any thoughts on taking a typical smoke detector and have the signal that turns it on an audio output jack from a smartphone?

Tim

 

I have an old smoke detector in my hand. It uses a Motorola house-numbered IC but has no transistors. It has no inductors and was made in 1988. The piezo transducer has 3 terminals including one for positive feedback. The resonant housing is 4cm in diameter.

 

so i have figured out a piiezo siren hooked directly to a twlelve volt dc wall adapter. i have a reed coil switch that requires a five volt signal to activate with a resistance of twohundred fifty oms witha twenty milliamp nominal current draw. what kind of small circuit could i use that would take the line out from a phone to trigger the reed coil switch? for whatever reason my phone in this version of firefox is not letting me do numbers. tim

 

Your reed relay uses 5VDC at 20mA. We don't know the level and current of the AC sound from the phone that will need to be rectified and filtered into DC.

 

What is the best way to measure this?

 

An oscilloscope can show the output voltage, waveform and frequency. It can be loaded and the voltage drop can be seen and then the current can be calculated. Then a suitable amplifier can be designed.

 

After a little digging I came up with what I think are reasonable assumptions:

http://en.wikipedia.org/wiki/Line_level

Use Nominal level Nominal level, VRMS Peak Amplitude, VPK







Consumer audio −10 dBV 0.316 0.447
and 2 reasonably close circuits I could use:

http://www.discovercircuits.com/PDF-FILES/AUDIODETECT1.pdf
http://artengine.ca/~pflemming/flemmweb_current/schem/opamp_linear/741_comparator_audio_relay.pdf

I certainly like the small current draw of the first, yet the simplicity of the 2nd is more appealing.

Any ideas as to the pot that I would need to be able to have for the comparator circuit to trigger properly?

Thank you!

 

Line lever is not a fixed level. It can be at a low level, a medium level, a high level or anywhere in between.

The first circuit is probably too sensitive but it properly rectifies and filters the tone.
The second circuit does not rectify and filter the tone so its output will quickly turn on and off at the tone's frequency which is bad.
The output voltage of the very old 741 opamp might not go low enough to turn off the power darlington transistor. But you do not need the high power darlington to drive your little siren.

 

Well, I got the parts in from mouser and the between the combination of the parts being so small (surface mount) or mistakes on my part, I can't get the circuit to work.

I didnt even think that the form factor of the comparator was going to be the size of a tick!

Tim