Aero ‘lectrics

Beep, beep, beep.


This is your basic beep-beep collection of designs. You want an electronic stall warning horn? Or how about a low oil pressure audio alert? A buzzer to let you know you’ve left the master switch on? Anything that you want to call attention to, these are the ways of doing it.

How you sense stall, how you sense low oil pressure, or how you sense that the master switch has been left on is not the subject of this series of articles. All I need you to tell the circuit is that one of these conditions has happened. It is the circuits job to then start giving out audio tones to alert you to these conditions. The sensor must give a digital “high” or “low” (+ battery or ground) output to the circuit, usually generated by a switch of some sort.

How you drive your headphones and/or speaker are also not the subject of this series of articles. It is the circuits job to drive a standard audio panel line level input with a volt peak-to-peak of audio. We can give you more output voltage if your audio circuits need more drive, but we cant drive headphones or speakers directly.

Sounding Off

These, then, are what can be broadly described as tone generators. What sorts of audio tones can we generate? From so low your ear cant detect them to way above where your ear can listen to them. Most of them, though, are in what we call the “audio” range from 300 to 3000 Hertz (Hz). Say what? (The saying as you get older is that 50 Hz, but 60 really Hz.) OK, let me relate this to the piano. 440 Hz is A above middle C. Not too conversant with the piano? Can you remember what the whine of a commercial airliner in the audio system sounds like? Thats exactly 400 Hz. How about the outer marker on an instrument approach? Thats also exactly 400 Hz. The middle marker? 1300 Hz. The inner or fan marker? 3000 Hz.

So how do you pick a frequency to do your alert? By trial and error. What tone just grates on your eardrums? Thats the one you want for your alert.

Let’s start with the easy stuff: a plain, steady tone that will get your attention. Schematic B shows such a device. It uses the popcorn-simple LM324 op-amp that is still available at most Hobby Shack stores (and at all of the usual mail order suspects) and costs anywhere from a quarter to a buck apiece. A few small parts later and you have your continuous tone oscillator.

The Concept

Without boring you with all the details, let me describe what you are doing here. As we’ve said for years, an op-amp is nothing more than an electronic “chunk” of voltage gain. Take a look at Schematic B and lets presume we’ve just turned the juice on and the output (right side of the op-amp) is at zero volts. But wait a minute. The (+) input of the op-amp (lower left side) is at 3 volts (9 volts into the voltage divider consisting of R4/R5/R6). The op-amp, trying to please, immediately drives the output up to the supply voltage. Now we’ve got that same voltage divider, but with two feeds to the 9-volt supply, and the (+) input rises immediately to 6 volts. Meanwhile, C3 is charging through R3, and when the charge on C3 exceeds 6 volts, the op-amp (in its never-ending battle to try and please us) drives the output to zero volts (ground) and the (+) input goes back to 3 volts. When C3 discharges through R3 into the output and the voltage on C3 drops below 3 volts, Pow-Wham, the output goes back to 9 volts and the merry-go-round just keeps on going with the output slamming between ground and the positive supply.

So how fast does all this happen? It is controlled entirely by the “time constant” provided by R3 and C3. The actual equation for the time it takes for this to happen approximately is given by t = 1/(1.4 x R x C) where t is the time it takes for one complete “cycle” of up-down. It so happens mathematically that the frequency is the reciprocal of time, so that we can say that f = 1/t. Let’s plug some real numbers in and see what we get. In the schematic Ive given you, I made R3 = 100KΩ (in engineercalcspeak, this is 100E3, or 100 x 103). I also made C3 10 nanofarads, or 10E-9. Inserting these into the equation for time, we find that t = 1/ (1.4 x 100E3 x 10E-9), which calculates out to be 1.4E-3, or 1.4 milliseconds.

But we also said that the actual frequency was given by the reciprocal (thats the little 1/x button on the calculator) of this number, and that calculates to a frequency of 714 Hz. Thats pretty close to where the female voice starts to sing, “Row, row, row your boat…” If you want a higher frequency, use a lower value for R3 and vice versa.

Now let me pass on some of the secrets for designing this sort of stuff. While it isn’t a hard and fast rule, we like to keep op-amp resistor values between 1KΩ (1000 ohms) and 1MΩ (1 million ohms) and capacitor values between 100 pf (picofarads) and 10 f (microfarads). We do this for a couple of reasons. First, those values are easy to buy. Second, “second order effects” or things that don’t make the op-amp quite the perfect little player start to rear their ugly heads at values above and below these. The good news is that sound generators can easily be made within these constraints.

Spike Killer

Schematic A bears some discussion. I could have run this whole circuit directly from the 12-volt battery bus. However, the frequency is somewhat a function of supply voltage, and I wanted a well-regulated supply to keep your chosen frequency close to the actual frequency of oscillation. Further, if you’ve never had the pleasure of putting an oscilloscope on the avionics bus and watching the gawdawful spikes that happen during the starter process or when running any heavy user of the battery such as the landing gear, you would never again use anything but perhaps an incandescent light bulb on the raw battery bus. This little circuit uses about 50 cents worth of parts to take the battery bus down to 9 well-regulated and filtered volts and keep those killer spikes out of the equation.

So what do we have with this simple circuit? Beeeeeeeeeeeeeeeeeeeeeeeeeeeeeep. Hmmm. Not what I wanted. I wanted Beep…beep…beep and so forth. No worries. Next month we take another section of that two-bit op-amp and make not only a beep…beep, but a bip…………bip……….bip or a tick-tock-tick-tock. Stay tuned. Its going to be a fun project.


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