On The Inside

On The Inside

Saturday, December 17, 2016

Safe Sound Build Post - Hardware

As previously described, SafeSound is a monitoring and intervention device for mobile passive PA systems. It sits between the amplifier and the inputs and outputs to monitor system usage and intervene when necessary.

SafeSound consists of four AC voltmeters and four relays controlled from a remote embedded microprocessor connected to an ATT cell modem.  The CPU measures the input and output voltages and reports those back to the cloud via the cell modem. The relays are connected to the input and output of the system. On the output side the relays are connected in series with the speakers. On the input side the relays are connected in series with a resistor that is connected in parallel with the input circuit. Engaging the output relays disables the speakers whereas engaging the input relays dims, or lowers, the system sound. 

In this system the speakers and source input connects directly to the safe sound unit. The safesound unit then connects to the amplifier on both inputs and outputs. 

In this post we'll talk about some of the mechanical and electrical details of safesound should you want to build your own version.

Since SafeSound is designed to be built into a rack enclosure, some of the details will be dependent on what you choose for an enclosure and also what input and output connectors your system uses.

For our prototype we used a 2U rack case salvaged from an existing piece of equipment. Unfortunately, rack mount cases tend to be expensive. One option is to repurpose a computer rack mount case like this 2U unit from Roseville.

Whatever case that you choose to use you will most likely need to drill holes to match the jacks for input and output. Most smaller PA systems will use 1/4" inch jacks on both input and output making those a reasonable choice. In the prototype above we use speakon connectors for the output side because they are safer for plugging and unplugging the speakers while live. We use 1/4" connectors for the input side. If you choose to use 1/4" jacks for the output, you will want to use jacks that can be ground isolated such as these jacks that we used on the input side. 

Speakon jacks provide this isolation automatically and do not short the hot side to ground during insertion and removal. They are recommended, however, many smaller systems don't use them so it also may mean purchasing new cables. 

We'll start with the input and output schematic which shows how the jacks are connected to the relays. For a small system up to about 500 Watts at 8 ohms, the Velleman  relay board that we used in the prototype is adequate. 

It will simplify construction to use screw in terminals in place of the inexpensive connectors provided with the kit as shown above.  The schematic is straightforward and is shown below.

I've shown the picture diagram of the velleman board below to show more clearly how the input and output side map to the normally open and normally closed contacts of the relays. In both cases, the default behavior is normal operation is that the relays are disengaged. While this is best for the life of the relays, it actually poses a problem. With this configuration the default behavior with the system powered off is that the sound system will continue to function. If this is undesired, the output side can be wired to the normally open side and the input mapped to the normally closed side. Only one change need be made in the source code to accommodate this change. In this mode, the default behavior when SafeSound is powered off is that the speakers are disconnected and the input to the amplifier is dimmed.

In order to measure the input and output voltages we need AC voltmeters. Our prototype uses a simple passive voltmeter design which works well enough, however, it has some limitations for measuring input voltage. In a later post I'll discuss how you can overcome this limitation by using a pair of sound sensors in place of most of the input voltmeter circuits.

The output voltmeter does not have this problem, however, and we need to be careful to retain the galvonic isolation so that no current flows directly from the speakers into SafeSound. The output meters are isolated with a simple 1-1 isolation transformer. The prototype uses transformers removed 
from old modems.  You can use almost any isolation transformer, a constant in the code may have to be tweaked.

v0 through v3 connect to the analog to digital converter board and vOut/vIn connect to the common points on the jack board as shown above. You can build this on a piece of prototype board.  In the prototype above I built the voltmeter board on a removable subassembly to make testing easier, this isn't really necessary, however.

If you look closely at the picture of the unit at the top of this post you can make out another relay board on top of the cpu. This was a part of the original prototype, however, the current version uses an I2C A2D board which is slightly more flexible and also provides cleaner measurement.  
v0 through v4 connect to A0 through A4 to pin 1 of the connector. vGnd connects to pin 4. The I2C bus must be connected to the shield. The grove header board that I have on hand did not connect the I2C bus to the correct pins. SDA must connect to J4_1 pin 5 and SCL connects to J4_1 pin 6. 

J1_1 pins 1 through 4 provide the digital outputs used to trigger the relays. These pins connect directly to the input pins on the relay board. Additionally, the relay board ground must be connected to ground on the shield. The relay board requires it's own 6V power source. I used an old wall wart that I had lying around. Do not connect the relay power to the shield board power, the two systems can connect at ground only. 


Wiring the AC input is beyond the scope of this document. However, in the prototype unit there is a switch and fuse inline with the AC power which goes to a standard wall jack. 

This is the hardware for safesound. In the next article  I will discuss the Flow based web software that allows remote monitoring of sound system usage.   In the final installment I will discuss the C source code including the driver for the A2D board and calibration.

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