MrRcSound unit volume control

[Hangarqueen]

In my BH Spitfire, I installed a sound unit, for that extra touch.

It is a wonderful Merlin sound, it really adds to the experience, but the problem is that on the ground, it is often too loud to be comfortable if you set the volume to a level that is perfect for in flight.

The volume control on the MrRcSound is manual, through a small rotating knob on the side of the bottom PCB.  As the sound unit is usually well buried inside the fuselage, it isn't easy to experiment with the volume.  Ideally, the volume is controlled from a spare receiver channel, so it can be adjusted in flight.

I thought about different solutions to achieve this, like using a digital potentiometer instead of an analog one (but I would have to add even more electronics to control it through ppm).

Mounting a small servo on the knob posed two problems:

- the rotation axis of the knob is inside the edge of the PCBs, and there is no room for a servo between the PCBs.  Mounting it on top of the PCBs isn't an option.

- a standard servo rotates over only 90 degrees; I want to be able to vary between lowest and highest volume

And so I de-soldered the potentiometer from the PCB (caution, this requires some experience and patience, ans it is soldered through the PCB) and connect it through a servo cable to the unit.  I could now mount it away from the sound unit.

I then modified a servo by adding series resistors to the feedback potentiometer of the servo.  By experimenting, I found the required resistor values to get pretty close to a 180 degree rotation range of the servo.  As there was little room inside, I had to mount them outside of the servo housing.

I then hot-glued everything together for testing, and it works like a charm

 

Next step is to mount everything inside the Spitfire and take it outside for testing.

[Shaun Walsh]

But surely if you were to stand next to a real spitfire with the engine running it would be uncomfortably loud! So you were true to scale?

[Paul De Tourtoulon]

Does the sound vary with voltage, would an old brushed speed controller do the job ?.,,

[Hangarqueen]

5 minutes ago, Paul De Tourtoulon said:

Does the sound vary with voltage, would an old brushed speed controller do the job ?.,,

 

I have no schematic of the sound unit, but a potentiometer is a voltage divider, while a brushed controller does "chopping" of the current to a motor.

So I don't think you could achieve this by using a brushed controller.

[Denis Watkins]

1 hour ago, Paul De Tourtoulon said:

Does the sound vary with voltage, would an old brushed speed controller do the job ?.,,

 

1 hour ago, Hangarqueen said:

 

I have no schematic of the sound unit, but a potentiometer is a voltage divider, while a brushed controller does "chopping" of the current to a motor.

So I don't think you could achieve this by using a brushed controller.

 

That is a nice idea from Paul, and a logical choice, but as HQ states, it does not work with the pot.

This is because the stick position for proportional control through the brushed controller for say fast idle, sets the power to " more off pulses, than on power pulses " and for greater speed from the motor, more On than Off. Am trying to keep it simple but also to make sense.

[Paul De Tourtoulon]

my idea was to change the input voltage on the sound module It wasn't to replace the potentiometer,,,

 

 Potentiometers are cheap and cheerful that would be the obvious choice finding one with the range that you need with a swing of 90° and run it

with a servo like the throttle,,,  I used a couple to change a computer joystick to a useable 'single-stick' for a handicapped flyer,,,

[Hangarqueen]

1 hour ago, Paul De Tourtoulon said:

my idea was to change the input voltage on the sound module It wasn't to replace the potentiometer,,,

 

 Potentiometers are cheap and cheerful that would be the obvious choice finding one with the range that you need with a swing of 90° and run it

with a servo like the throttle,,,  I used a couple to change a computer joystick to a useable 'single-stick' for a handicapped flyer,,,

 

I'm sorry Paul, but I didn't quite understand what you meant.

In general, and depending on the type of amplifier, you could vary the supply voltage of the amplifier and that would have an impact on the output power, but only within a limited ranged range I think.  If the supply voltage drops too much, the power transistors wouldn't be able to function within their linear range.

I've never looked at an amplifier this way, as it is designed with a fixed supply voltage in mind.  It would be interesting to know what happens when you lower this voltage, although I'm not going to try it on mine ?

 

[Paul De Tourtoulon]

We did try it with success a helicopter sound module and it did work pretty well till he crashed it,,,?