Aero ‘lectrics

Hangar switch wringout.


This is the last chapter for the little box that lets you turn on electrical things in the hangar by means of an inexpensive cell phone. Now that we have the thing designed and tested, let’s figure out how you can make one of these so that it works the first time, all the time.

As I said last month, unless you have an oscilloscope and are conversant with it, this is not the project for you. On the other hand, perhaps you have a friend who knows more electronics than homebuilding and would trade you ‘scope time for, say, a hand-carved prop for his wall or hangar door. Another source is your local community college; most second-year students are adept with test equipment, and a lot of them need a class project. This one should open the instructor’s eyes!

The locations where all of the waveforms for photos 1 to 3 were taken. Note: There was one error last month on the schematic that has been corrected (wiring from U4B to U4C and U4A).

The locations where all the waveforms for photos 4 to 7 were taken.

Location 1.

Wring It Out

So without further ado, I’ll give you one of the biggest tips of all time: Build this thing one stage at a time and wring it out thoroughly. Don’t be like a freshman engineering major and build the whole thing at once, and then spend double or triple the time troubleshooting it stage by stage. This is far too complex a device to expect it to work by simply slapping parts together and hoping for the best.

Having said that, here is what you should see at each stage of the game. Note that, except for Location 1, the bottom of the ‘scope screen is ground (0 volts), and the top of the screen is power supply voltage (10 volts).

Location 2.

Location 1: This is the raw audio coming from the speaker, and the oscilloscope is set for a vertical sensitivity of 5 millivolts per division. The voltage is going up two divisions and down two divisions (which you can’t see because the bottom line is 0 volts). Thus you have about 20 millivolts peak to peak. You get one of these bursts each time the phone rings.

Location 2: The first stage of amplification takes that 20-millivolt peak-to-peak signal from the speaker and amplifies it to about a volt peak to peak. Once again, you get one burst of signal for each phone ring.

Location 3.

Location 3: The second stage of amplification is set by R10 to give about 5 volts peak to peak, again, one burst of signal for each ring of the phone. This is the end of the analog signal processing. From here on out, we enter the digital world.

Location 4: Here we convert those analog ring pulses into digital pulses. Where the analog signal at Location 3 is noisy, we’ve filtered and converted those dirty ring pulses into nice, predictable square waves, one pulse per ring.

Location 5: Those ring pulses are then counted in U3 pin CP0 (clock pulse zero), a simple 4017 pulse counter. When you get to the eighth ring, Q8 of U3 goes high and starts the ball rolling. This high pulse goes into U4A, a simple “OR” gate. If either (or both) of the inputs of gate U4A are high, then the output goes high. When the eighth ring comes along, R15 and C12 delay turning this gate on for 100 milliseconds (0.1 second). When this gate finally turns on, it resets U3 back to a count of zero at MR, which then waits for another set of rings. This gate also keeps itself on for 20 seconds, through R16 and C13, so that it can’t be triggered again if the phone keeps ringing. (The phone company cuts a cell phone off after about 15 seconds of ringing.)

Location 4.

Please forgive this way-old electronics teacher the only digital joke he knows. See U4D all by itself at the right side of the schematic? That is what we in the business call a “Shakespearean gate.” Input 1 is a value called 2B. Input 2 is a value called /2B (not 2B). So the gate reads 2B OR not 2B, that is the question.

Location 6: Meanwhile, back at the ranch, trusty old relaxation oscillator U1D keeps putting out one pulse a second, set by R18 and C14. As we discussed last time, you can have most any “on” time you want by connecting U4B to one of the Q values, remembering that the on time will be 1 second times 2Q value . Thus, if you connect to Q11, you will have 2048 seconds of on time, or about 34 minutes. If you want much longer on times than a few hours, you can slow down clock oscillator U1D by nearly any factor you want.

Location 5.

Counter U6 can be reset at MR to start counting at zero by any one of two ways through U4B. One way is when U4A goes high at the eighth ring pulse and the other way is when counter U6 goes high at the “Qth” on time pulse.

Location 7: When U3 Q8 went high at the eighth phone ring, it also did something else. It set (S) flip-flop U5A Q output high. This output will stay high until a tenth of a second later, when U4C goes high through R23 and C16, and then the Q output is reset (R) to zero. This flip-flop Q output is also set to zero when U4B signals that U6 has reached its time-out point.

Location 6.

However, when U5A went high, it clocked flip-flop U5B on, and whatever level was on the D input was passed through to the Q output. Presuming that /Q of U5B was high, it clocked a high onto Q, which turned on driver transistor Q1 and pulled in relay K1.

Understand that when Q was set high, then /Q would have been driven low. Thus, if this whole exercise of eight phone rings comes on a couple of minutes after the first set of rings, then the process is reversed. Q1 is turned off, and the relay drops out. This allows you to turn off the electricity to your device as well as turn it on.

Location 7.

Final Touches

A few more tips. The photo shows the cell phone resting on a soft surface (an old sweat sock). This is necessary to keep the phone from vibrating itself on a hard surface and making too much noise for the circuit to handle. You could also wrap it in bubble wrap or something similar.

If your hangar is really noisy, then the little speaker will take that noise and quite properly turn your electrical device on and off randomly. I use a small foam ice chest and have had excellent results, even when my neighbors light off their T-6s and taxi out.

Power relays intended for switching high currents and voltages.

The whole circuit, including cell phone and speaker, in an acoustically isolated beer cooler.

The little relay shown in the photo last month isn’t really intended to carry high currents or high voltage (wall plug) AC voltage. It was meant to turn on one of the relays shown in the photo. Note that you can run this from 24 volts DC if you want by simply making the relays all 24 -volt relays.

Next month we turn to making some more useful aviation electronic devices. Until then, stay tuned.


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