Electronics Project - audio mixer for headsets

[sain]

Hi All,

 

This is a little project I was working on a while back. Its a simple audio mixer that allows input from 2 sources (i.e your aircraft's radio and an mp3 player) to be played through your headset. Audio from the primary source (normally the radio) will cause the secondary source to be muted, so any radio calls will cause the mp3 audio to go very quiet until the radio call finishes.

 

I've pretty much ceased working on it, as I discovered the radio isn't quiet often enough to enjoy a good song. At least not on the melbourne centre frequencies. The other problem is the cost came in above what I thought is a worthwhile $maxcash level.

 

If there is enough interest I will design a circuit board to go with this, and if there is heaps of interest I'll consider making pre-built versions available through Ian's store - assuming that he is willing. I've supplied it here in case somebody else wants to build it, or take over development of it.

 

It comes with a couple of quick caveats which I'll give before going into further details:

 

This has not been tested with an aircraft radio. This is simply because I don't have access to one for testing.

 

The audio amplifier may be capable of generating enough volume to cause hearing loss if run at a high level. which is a pretty standard warning i think - basically don't crank it all the way up when your listening to your favorite tunes.

 

I'm a beginning hobbiest - not an electronics engineer. nuff said.

 

The schematic for the circuit is attached below.

 

Parts list coming when i figure out how to do a table with this...

 

The first part of the circuit is the sound activated switch based around the tl072 op amp (IC1a and b). This compares the incoming audio from the radio with a voltage level set by VR1. Once this level is passed, this causes Q1 to switch on, which causes Q2 to switch off - stopping the mp3 audio from being passed to the output amplifier (IC2 - a lm386). Once the radio audio level drops below the set level again c3 and r7 cause a small delay before allowing Q1 to switch off (and q2 back on).

 

The vox circuit was pretty much taken straight from N1HFX's basic vox circuit (http://www.rason.org/Projects/basicvox/basicvox.htm) with just some small additions to have Q1 invert the output for Q2.

 

The second part of the circuit is the output amp (IC2, a lm386). This is used to amplify the combined the audio inputs from the radio and the mp3 player (volume level for each is set by vr2 and vr3 respectively). Examples of this type of audio amp can be found all over the net.

 

The final part is the voltage regulator (ic3 and associated components)- which should provide a nice clean voltage for the circuit to work with. The 4 diodes form a bridge rectifier, and is a bit of overkill, but allows the power supply to be hooked up either way around without causing any problems.

 

The circuit should (in theory - untested) work fine off 4AA batteries (6v) if you don't want to run it from the airplane's power supply. If you want to do this, omit the voltage regulator section and connect the + side of the batteries to the vcc points on the circuit.

 

Cost of the components comes to $24.77 based on jaycar prices. As jaycar supplies resistors in packs of 8 (for $0.38 a pack) there will be a few left over resistors at that price. Dick Smith prices may work out cheaper.

 

The finished product should just plug in where your headset does currently, and then your headset should plug into it.

 

Any comments would be welcome.

 

 

 

[sain]

Sorry for the double post.

 

Parts list:

 

Qty Part Nos Description Cost/Unit Total Cost

 

5 c1,c2,c4,c5,c8 100N MKT Polyester capacitor $0.30 $1.50

 

2 c3,c7 47uF 16VW Electrolytic capacitor $0.28 $0.56

 

1 c6 22uF 16VW Electrolytic capacitor $0.20 $0.20

 

2 c10,c12 470uF 16VW Electrolytic capacitor $0.55 $1.10

 

1 c9 10uF 16V Tantalum $0.95 $0.95

 

1 c11 1000uF 16V Electrolytic $0.70 $0.70

 

1 d1 1N4148 Signal Diode (or similar - 1n914etc) $0.32 $0.32

 

4 d2,d3,d4,d5 1N4004 1A Rectifier Diode $0.40 $1.60

 

1 ic1 TL072 Dual Lown Noise JFET op-amp $2.35 $2.35

 

1 ic2 LM386N Low Voltage 1W Amplifier $2.10 $2.10

 

1 ic3 78L12Z 12V 100MA Voltage Regulator $1.00 $1.00

 

2 LED1,LED2 LED 15MCD 3mm or 5mm Some Colour $0.20 $0.40

 

2 Q1,Q2 PN100 General Purpose NPN (or similar) $0.25 $0.50

 

5 r1,r8,r10,r11,r13 1k 1/2watt 1% metal film resistor $0.38 $0.38

 

2 r2,r4 47k 1/2watt 1% metal film resistor $0.38 $0.38

 

1 r3 100k 1/2watt 1% metal film resistor $0.38 $0.38

 

2 r5,r6 27k 1/2watt 1% metal film resistor $0.38 $0.38

 

1 r7 4.7k 1/2watt 1% metal film resistor $0.38 $0.38

 

2 r9,r15 2.2k 1/2watt 1% metal film resistor $0.38 $0.38

 

1 r12 330 ohm 1/2watt 1% metal film resistor $0.38 $0.38

 

1 r14 10 ohm 1/2watt 1% metal film resistor $0.38 $0.38

 

3 vr1,vr2,vr3 10k Log Potentiomenter 16mm $1.75 $5.25

 

1 mp3 input 3.5mm stereo switched pcb socket $1.00 $1.00

 

1 radio input black 6.5mm mono phono line plug $0.85 $0.85

 

1 head output 6.5mm mono chassis socket - unswitched $1.35 $1.35

 

Total:

 

$24.77 Based on Jaycar (http://www.jaycar.com.au) prices

 

 

[Ross]

The early model radio supplied with my J160 kit has an input lead to which I have fitted a mono jack for external "music". So it mutes the external input when the radio is active. Presumably the upgraded versions still contain this feature.

 

Regards

 

 

[sain]

Yeah, I'm aware now that most current radios and even some headsets have facilities for doing this. Again, thats one of the reason why I'm not going to bother going any further with this unless there is a lot of interest. It works (at least on my test setup), but I don't think its that useful.

 

Origionally it was more of an exercise for my self, to learn how to do this sort of stuff again after playing with it as a kid. I have more ambitious projects planned - this just seemed like a good place to start.

 

My idea for the next project is a 4 channel CHT gauge. For the display I was thinking either a OLED screen (the very high visibility lcds in new mobile phones) showing 4 colour coded bar graphs or 4 arrays of led bargraphs (main difference will be price). If anybody has any input or suggestions for features they would be welcome. A source for the thermocouples normally used would be useful as well :big_grin:

 

 

[Ross]

Hi Sain

 

I admire your fortitude. It is apparent to me that there must be a fair bit of opportunity for development of instruments especially for the experimental market and a number of people are starting to show an interest in developing more competitive electronic instruments for this market.

 

Your idea on the CHT front sounds good but with plenty of competition already unless you can get the price down dramatically. Another one I had thought of was a continuous record/display of the cylinder head pressures over the compression and firing cycle of a cylinder in the engine and/or even of all the four strokes of each cylinder in the engine.

 

I don't know if pressure transducers are available that can respond to the cycle speeds and temperatures of the engine. Then there is the problem of inserting one in each head and the effect that might have on the combustion chamber. From a dim memory I think the combustion pressures can be over 1,000 psi or approximately 7,000 kPa.

 

One of my pet gripes about Bright LEDs and other electronic displays is that they get very annoying when there is low ambient light outside. They all need adjustabe/automatic brightness and many are nowadays to allow for viewing from a bright sunshine condition to almost darkness so that they do not affect the vision of the outside of the vehicle.

 

Regards

 

 

[sain]

The per cylinder pressure gauge idea is very interesting. The sort of pressure sensors I've been able to find certainly have the capacity to respond that quickly, but wont work in those temperatures. I havn't been looking hard though. I'll keep thinking on it.

 

The display brightness is certainly a problem I've been considering. I was just going to provide a brightness control on the front panel (probably 2 push buttons) to allow the pilot to change the setting in flight. The other option is to have a few preset brightness levels - i.e bright daylight, lowlight daylight, night, etc etc. I think the continually variable range is a better option personally, and, happily, easier to implement.