Live sound engineering is facing its biggest change ever during the winter 2012...13. Transition from tubes to transistors or from analog to digital was just natural evolution.

The way we do live mixing will be totally different than a year ago. The sound system will be controlled with personal mobile gadgets.

Naturally it was Mackie who made affordable equipment available for iPad based live mixing. I am a one man R&D team. I try to develop simple DIY solutions with a vision similar to Greg's.

Monday, August 13, 2012

Electric brain

I was just called that I have to go working on a construction site tomorrow but please do some homework by making friends with Arduino.




Arduino is the obvious pick as the processor for our Tripping Faders project. Make friends with it. I assure it is more intelligent than watching TV or the action you was planning to perform after watching TV.

H bridge

Motor driver circuits that can run the motor in both directions are called H-Bridge. I really don't know where the name comes from. Any bridge can be drawn in the shape of letter H or it can be drawn as a bridge.

H bridge drawn as a conventional bridge  


Anyone who been thumbling with car window motor control or similar two direction motor control knows that we need a two bit control.

UP  DOWN

0      0      nothing happens
0      1      goes down
1      0      goes up
1      1      nothing happens

We kind of waste one state in the two bit control but you just have to live with that.

An H bridge motor control IC is one of the very basic components used in home electronics like VCR's since forever. SN754410 is a 'modern' dual H bridge. PDF datasheet. It has two 1A bridges that can control a 4.5...36V power rail by 5V CMOS/TTL compatible logic input. Perfect for us.



Playing with the crab was not very surprising. It wastes 1.4 volts as two transistors in serial are expected to waste. A bias current of 20 mA was unexpected. Not so harmful but we don't need it. I assume the bias is for maximum speed that we don't need.

A 16-pin DIP package can handle 2 watts of power in 'normal' conditions. 2 watts / 1.4 volts / 2 motors = around 0.7 amperes each. It seems that we are in the ballpark.


Conclusion

We can use SN754410 as a driver. However it would be good to either
  1. Drive one motor per one SN..blah..blah..
  2. Add some small heat sinking

More current measurements

When measuring the fader motor current, I noticed that the current value drops quite rapidly. That is because of motor winding temperature rise. In loudspeakers that is called power compression.

I made another current measurement iteration. This time waiting some ten minutes for each voltage step and see where the current value sets.


4    210   190    24
5    260   230    30
6    310   265    37
7    360   300    43
8    410   330    50
9    460   350    61
10    510   370    70

Columns:
  1. Voltage / V
  2. Current cold / mA
  3. Current hot / mA 
  4. Calculated temperature rise / C 
Assuming that the motor has copper winding, we can calculate the temperature rise from electric current value shift. Temperature coefficient of copper is 0.0039/C (Aluminium is the same).

We could accept the temperature rise of 70 degrees Celsius with 10 volts in room temperature. That would make 95 C degree in 25 C room temperature but professional equipment must be designed for extreme violent conditions. Direct sunlight may cause some 50 C degree ambient temperature.

9 volts and 50 C ambient temperature would mean 111 C winding temperature. That is absolute maximum in extreme conditions. That is fully acceptable.

Motor windings can take temperatures above 150 C but there are other parts inside the motor made from unknown materials. Anyway the cooler, the longer life.


Hey com'on

We are not going to write a software  that keeps the motor pulling at full force for hours. No, we are not, but we are professionals and we have to consider extreme conditions and situations that can be caused by a malfunctioning component, in hardware or software.


Conclusion

We can keep keep the previous voltage plan and feel safe.

Power consumption

The spec sheet tells us following:

Rated voltage: 10V D.C.
Operating supply voltage range: 6〜11V D.C.
Maximum current: 400〜800mA (Lock the shaft of the motor and the rated voltage shall be applied to the motor.)

Have to make some measurements in order to decide appropriate power supply and driver stage.

Signaali TL-201 educational laboratory PSU from early eighteens. Made in Finland.

I am a happy owner of a vintage power supply that is perfect for the job. It has a 0...20V output with a multiturn control pot and a voltage/current display. Checked its calibration with three different multimeters and it is still very precise. I did measure stopped motor current when applying a voltage between 4 and 10 volts in one volt steps. The slider doesn't move full travel with 3 volts.

V    mA

4    210
5    260
6    310
7    360
8    410
9    460
10   510

 I tried also to imitate an insane user by violently moving the knob back and forth. I saw only some 50...60 mA peaks added to each value. At 10 volts I managed to make the belt slip with that kind of violence. Anyway the plan is to apply a capasitive touch sensing of knob. So a lunatic user has to use a noncapasitive tool in order to cause that extra current.

Naturally the motor speed depends on voltage. I don't think that we would need scary fast faders in live sound. However the voltage must be high enough to ensure reliable movement for years on.


Conclusion

Motor operating voltage between 6...9 volts would be OK.
If you reserve 0.5A per fader in PSU, you are safe.
A standard H-bridge driver IC with 1A current capability is OK.

Sunday, August 12, 2012

Motor fader

My idea is to build a wireless iPad remote control for a small band sound system.

Motor fader is a key component in making existing audio equipment machine controllable. Fortunately they are available now at reasonable cost.

MD100AM2B

I ordered 10k audio taper motorized faders from my regular store, MÃ¥nsteri Store. They cost 20€ a piece which is reasonable but still I minimized the risk and ordered just two pieces for testing first.

The fader is with 100 mm travel and the M3 fixing screws are inline with the slot, so that would easily fit in many existing consoles.

The feel of the fader is just good. The motor turns when when sliding. The motor is coupled to the slider shaft by a teethed belt (morse belt). I tried to short circuit the motor when sliding. You can just feel the difference but that is insignificant.

I am happy that the shaft slot is not upwards but on the side. That keeps shit out of the resistor tracks better.

The PDF datasheet of the fader reveals that it was manufactured in Taiwan by TOP-UP INDUSTRY CORP. I didn't like the lifetime spec on datasheet. It was only 50 000 cycles. We have to keep that in mind when writing software and when using the faders. You can surprise and impress passing chicks with motor faders but not for free.