Connecting PICAXE to low level audio input

Steevo25

New Member
Hi Everyone,

I need a little bit of help with my current project. It's probably a very easy question but not having done something like this before I am really not sure of the answer.

I have created circuit for my aircraft. It's just a circuit that reminds me when certain conditions exist by inserting a bleep every few minutes in to my intercom system. My intercom system has an external audio input which allows you to connect an audio device in to the system which routes through to the aircraft headsets.

From the manual of the intercom, it says the audio input impedance is 1V/600 ohms.

My circuit simply outputs the bleep when certain conditions are met using the SOUND command. Everything in the circuit works on my project board but I have not tried it in the aircraft yet and simply just used 64ohm speaker to test. At the moment the wire comes out the PICAXE pin with a capacitor in series with the wire (as per the PICAXE manual).

Then I found this thread:
http://www.picaxeforum.co.uk/showthread.php?16776-Speakers-connected-to-a-Picaxe

It suggests a different arrangement for connection to an amplifier by using 2 x 1K resistors and 2 x capacitors. I have 2 questions, would this be the correct arrangement to connect to my intercom given the '1V/600ohm' specification. Also, on the diagram from that thread, it shows the diagram twice for the audio amplifier connection but the capacitor values are different (it looks like an error but I don't know which capacitor values are correct).

Lastly, the audio input on the intercom does not have an independent volume control. I can only change the volume of everything connected to it so I wanted to add a volume control for my PICAXE circuit. At the moment, I have just used a 500ohm trimmer on the wire. Would this be the correct thing to do and would that still work with the resistor/capacitor arrangement shown in the thread pointed to above.

Many thanks in advance.
Steve
 

Steevo25

New Member
Many thanks. I understand what the series capacitor does by blocking DC current, but what is the purpose of the 2 resistors and the other 2 capacitors? It's just nice to understand what's going on rather than just doing it and it works. Is the rest just some form of RC filter?
 

rq3

Senior Member
Hi Everyone,

From the manual of the intercom, it says the audio input impedance is 1V/600 ohms.


Lastly, the audio input on the intercom does not have an independent volume control. I can only change the volume of everything connected to it so I wanted to add a volume control for my PICAXE circuit. At the moment, I have just used a 500ohm trimmer on the wire. Would this be the correct thing to do and would that still work with the resistor/capacitor arrangement shown in the thread pointed to above.

Many thanks in advance.
Steve
Steve, the 1V/600 ohm is the standard for amplified aircraft audio inputs. Keep in mind that the Picaxe is going to drive it's output as close to it's supply voltage as it can, so you may be looking at a sound output of 5 volts P-P. This is a pretty heavy overdrive of the intercom input, and while it MAY not hurt anything, it sure won't sound very good!
Standard aircraft headsets are 150 ohms, so the Picaxe can drive them directly (through a DC blocking capacitor), but they will be PAINFULLY LOUD (caps intended, I'm shouting).
You're best bet is a series capacitor (non-polar, or back to back electrolytics, about 1 uF) from the Picaxe output to provide DC blocking, along with a series resistor or potentiometer between the Picaxe and the cap to limit the sound volume and provide a low pass filter response. Start with a 120 ohm resistor, and go up from there until the sound volume is where you want it. You may be surprised at the final value of the resistor (many, many, many kilo-ohms) depending on the gain of the intercom amplifier.
I would also put a 1k-10k resistor from BOTH sides of the DC block capacitor to ground. Otherwise, you're asking the Picaxe pin to discharge the blocking cap every time it goes low, which is not conducive to the longevity of the Picaxe, nor the quality of the sound. Most, but not all, aircraft intercom inputs are capacitively coupled internally, but without knowing for sure you risk damage to either the intercom, or the Picaxe, without this protection.

Don't even think about clipping the signal with zeners or the like. It will sound like crap (not that the standard 300 Hz-2700Hz aircraft intercom response sounds that great anyway), but if your intercom is hi-fi audio capable, it won't be anymore!

Rip
 

hippy

Technical Support
Staff member
From the manual of the intercom, it says the audio input impedance is 1V/600 ohms.
I don't know anything about avionics equipment but that sounds very much like it's possibly using an isolation / coupling transformer as is often the case with telephone systems.

If that's the case then an inline resistor and DC blocking capacitor may do the job, an inline resistor and inline pot giving volume control.

Rather than using SOUND, TUNE or PWMOUT to generate a tone, it may be better to use two output pins and different value inline resistors to bit-bang out a tone which is closer to a sine wave than a square wave. It should be possible to bit-bang a 1kHz tone or similar.
 

wapo54001

Senior Member
Of course, if it is intended as a nagging reminder, a nasty rough beep would perhaps be preferred!
I'm reminded of the pilot who could not hear the tower waving him off from landing because he couldn't hear his radio over the sound of the very loud gear-not-down warming horn. If it's important, use the Picaxe to flash a light on the dash, too.
 

Steevo25

New Member
It will be flashing a light as well. Basically, the circuit is a reminder that the backup fuel pump is on. The back-up fuel pump is switched on during take off and landing as a precaution in case the main fuel pump fails at those critical times. The problem is, I nearly always tend to leave it on for the entire flight. The back up fuel pump isn't really designed to be left on for hours at a time and is supposed to be switched off at a safe height.

The circuit measures the air speed and the RPM of the engine. If you are faster than 50knots and the engine is at more than 2000rpm and the wing flaps are retracted then the circuit assumes you are flying. If the fuel pump is on at this time then a light will flash on the panel and a double beep will play through the headsets every 3 minutes until you turn the pump off.

I had everything else working on the bench but I know very little about audio circuits.
 

wapo54001

Senior Member
It will be flashing a light as well. Basically, the circuit is a reminder that the backup fuel pump is on. The back-up fuel pump is switched on during take off and landing as a precaution in case the main fuel pump fails at those critical times. The problem is, I nearly always tend to leave it on for the entire flight. The back up fuel pump isn't really designed to be left on for hours at a time and is supposed to be switched off at a safe height.

The circuit measures the air speed and the RPM of the engine. If you are faster than 50knots and the engine is at more than 2000rpm and the wing flaps are retracted then the circuit assumes you are flying. If the fuel pump is on at this time then a light will flash on the panel and a double beep will play through the headsets every 3 minutes until you turn the pump off.

I had everything else working on the bench but I know very little about audio circuits.
Is this for a Rotax? If so, you might be interested in this chart which shows that a properly working engine-driven pump and backup electric will overpressurize the carburetor.
 

Steevo25

New Member
This is for a Jabiru engine. Certainly on the Jabiru, according to the fuel pressure, there isn't really a change in pressure. The Rotax fuel system is slightly different to the Jabiru. Jabiru say that you can leave the fuel pump on but from experience I have found that their life is very much shortened. Also, due to the pulsations from the pump, it messes up my fuel figure calculations on my flight information system.
 

wapo54001

Senior Member
This is for a Jabiru engine. Certainly on the Jabiru, according to the fuel pressure, there isn't really a change in pressure. The Rotax fuel system is slightly different to the Jabiru. Jabiru say that you can leave the fuel pump on but from experience I have found that their life is very much shortened. Also, due to the pulsations from the pump, it messes up my fuel figure calculations on my flight information system.
I finished my Challenger in 2005 and immediately had fuel problems and discovered that (at the time) there was no commercially-available fuel pressure monitoring system that was accurate at the 3~6psi required by the Rotax. I subsequently used an O8M with a precision .5~4.5V sender to build a pressure monitor that was accurate to one-tenth of a psi, and interfaced it to the unused water temp input of my EIS. Works great.
 
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