When I realized that an efficient bandpass design with the woofers I had had a working frequency at about 120 Hz, I knew I would need lots of displacement for the 40 Hz I had aimed. To go all the way down to 40 Hz, the bandpass ported chamber frequency is set to 74 Hz, which is almost an octave lower than the 120Hz. With this setup, the woofers need 4mm of displacement at around 18 watts, 45Hz. It is quite inefficient and can't handle lots of power, but I do not have much space and don't want to order new speakers. There aren't many speaker distributors in Turkey and shipping from other countries is pricey.
Seeing the great displacement requirements and considering volume levels and inputs may be easily misused by careless party goers, I decided to build some kind of protection for the bandpass drivers.
Limiter Circuit
This simple circuit takes the loudspeaker output, applies a low-pass filter and rectifies the output of this filter to drive a muting relay. The low-pass frequency is set at 45Hz. This is the bottom working frequency of the ported chamber and below this frequency displacement rises rapidly. The use of a relay seems inappropriate in such an active circuit but it was an easy design and I had all the materials so I didn't bother other electronics which I do not know very well. Still, I think a MOSFET or photovoltaic relay could do the job in a more reliable way.
Utilities
Patrick has a USB output to charge MP3 players!! It also has a few lighting to give some soft lighting to the control panel and its arms (arms will be carrying drinks). For these utilities, a 5V and a 12V regulator is set up inside Patrick.
Hint: To design a dedicated charger for a USB device, connect the data pins on the USB port together with a 100 ohms resistor. Otherwise your device will think it is connected to a PC.
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