correct transistor amplification

[diywannabe]

Hi,
i'm building myself a siren to use as an antitefth for my car.

I'm building a very simple 555 based circuit but i have a serious problem with the amplification of the signal.

Actually i amplify the signal coming from pin#3 of the 555 with a NPN (BC547) and then with a darlington (BDW84C or MJ11015).

The speaker i'm using is one made for emergency purposes, 100W and 16Ω.

Unfortunately it is not that loud as i wanted it to be.
I've been told that the way i amplified the signal is wrong but nobody wants to tell me how to do it correctly in order to use all of those 100W.

Any advice???

I attached the scheme i'm using, actually i replaced some of the resistor values to get the correct speed.

The sound (not that important tho) is a faster wail.

Thanks in advance for help.

 

[E-design]

One transistor NPN looks to be drawn wrong in your circuit. The supply voltage and speaker impedance will limit the audio power you can get. W = V^2/R

 

[diywannabe]

Thanks for your answer.

So if i got it correctly i need to get rid of that BC547, right? And then? Should i replace the PNP darlington with a NPN darlington?

I used that speaker because i have a HILO siren from FIAMM which uses that speaker (which is also a FIAMM) and that siren circuit is made for 12v.
It is extremely loud and it uses no transformers at all.

It's hard for me to know what component it uses because it is totally covered with epoxy.
I removed the bottom cover and some epoxy, i can see the PCB on the track side... but i have no idea of the components... i attached a pict... it seems it uses a microcontroller like PIC 16F84 and it only has two final transistors in TO220 cases.
Cannot read the transistor code because i scratched it away trying to remove the epoxy.

As you can see speaker and circuit share the same negative and speaker gets the positive signal from the transistor on the right-bottom side.
two other pins from the main connector are for the PUSH-BUTTON cycle, doesn't matter but just for not making confusion.

That's all i want, trying to retrive the same loudness of that unit.

How can i do that?

Thanks once again.

 

[Audioguru]

If your supply voltage is 13.8V and the voltage loss of the darlington transistor is 2V then the speaker gets a peak voltage of only 11.8V. Its square-wave power is 11.8V squared divided by 16= only 8.7W.
For 100W into 16 ohms the supply voltage for the circuit must be 42V.

If you use a 4 ohm speaker then its square-wave power will be about 29W if the darlington transistor can pass that much current.

 

[diywannabe]

Thanks Audioguru, your name makes me feel sure what you're talking about.

It seems like math has an important role in this project.

Actually i want to keep that speaker (due to its cost) and because it is a great quality loudspeaker.
Having said that i suppose i cannot get 100W from that speaker with that darlington.
In case of 0V loss i would have 13,8V as peak voltage on the speaker, correct?
That would mean 11,9W. Not that difference...

Now i suppose the siren i was talking about doesn't use all the 100W of my speaker.
No way to gain some extra watts from my project?

I noticed that using MJ11015 instead of BDW84C increases the loudness but i've been told that so many amperes would blow the speaker filament.
I'm kinda confused.

Added after 24 minutes:

Thinking about what you said i checked the siren pcb again.
Checking the solder points i marked where it may be possible that a round transformer could have been used to rise the voltage around 45v to reach 100W and that would bring it to your theory.

Ok so let's say that i want to do something like that... beginning to the circuit i made... how can i convert 12V to 45V (easily) and amplify the signal with that voltage to the speaker?

Thanks a lot, that solves part of my mistery on this circuit.

 

[Audioguru]

A darlington transistor always has a voltage loss of about 2V.
If a transformer is used to boost the voltage then the darlington will have a much higher current.

I always talk about real Watts. Many speaker manufacturers talk about phoney Whats.
Your little 100W speaker might not survive 25 Watts.

 

[diywannabe]

I tested my speaker with a Federal Signal PA300 siren, which is a 100W siren and it had no problem, i know it uses a transformer.
All i want is to get as much power as possible that's it.