06-06-2015, 08:29 AM
This circuit is almost (but not quite!) fine 
The first 555 astable has a period of about 1.5 seconds - 1 second on, 0.5 seconds off - its used to "gate" the second astable - whilst the first 555 output is high, the second one runs at about 680Hz; when the output of the first 555 goes low, the second astable is stopped from running.
One thing I HATE about this design is that the output of the second astable goes HIGH during its supposed off period, i.e. for 0.5 seconds in every 1.5 seconds, 12V is across the speaker, i.e. 18W dissipated for 0.5 seconds out of 1.5 seconds, or 6W RMS of wasted energy - the speaker is dissipating 12.2W RMS on average as the setup stands.
As Mark has also said, you can drop D1 and R3 and take the output of U1 (pin 3) directly to the top of R4 instead of connecting the top of R4 to Vcc - this drops two completely unnecessary components.
To solve the "cook the speaker" issue you need to invert the output from U2 pin3 - a simple small signal NPN and resistor will do this happily - see attached schematic.
This drops the power dissipated in the speaker to about 5W RMS, and stops it cooking whilst maintaining exactly the same audible power output.
Net effect of these changes is that you have the same number of components, use half the power, save the planet and don't kill the speaker
The first 555 astable has a period of about 1.5 seconds - 1 second on, 0.5 seconds off - its used to "gate" the second astable - whilst the first 555 output is high, the second one runs at about 680Hz; when the output of the first 555 goes low, the second astable is stopped from running.
One thing I HATE about this design is that the output of the second astable goes HIGH during its supposed off period, i.e. for 0.5 seconds in every 1.5 seconds, 12V is across the speaker, i.e. 18W dissipated for 0.5 seconds out of 1.5 seconds, or 6W RMS of wasted energy - the speaker is dissipating 12.2W RMS on average as the setup stands.
As Mark has also said, you can drop D1 and R3 and take the output of U1 (pin 3) directly to the top of R4 instead of connecting the top of R4 to Vcc - this drops two completely unnecessary components.
To solve the "cook the speaker" issue you need to invert the output from U2 pin3 - a simple small signal NPN and resistor will do this happily - see attached schematic.
This drops the power dissipated in the speaker to about 5W RMS, and stops it cooking whilst maintaining exactly the same audible power output.
Net effect of these changes is that you have the same number of components, use half the power, save the planet and don't kill the speaker
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