The Alternate Scientist

Improvising A Wired-Wireless Intercom

"It's about time!" I said, picking up the phone.

"Thees ees de bonk," came a strangled voice. "Ve vant to talk aboot yoor overdrafty."

"Mike," I replied. "I've been waiting half an hour."

"Sorry," said Mike, half meaning it. "My dad was on the phone. He just got off."

"I hope he didn't break it, standing on it that long. Well, c'mon over. I have something to test." I couldn't help grinning.

"Ho, Ho," said Mike. "I'll see you in a bit."

I knew that meant he would have a sandwich, get his bicycle out of the garage and push it leasurely up the mile of hill from his house to mine. I had half an hour. I spent it modifying the first of two intercoms on the workbench, leaving the remaining, dismantled intercom lying upside down.

I had just finished when I heard a clunk and then a clatter as mike kicked down his kick-stand which promptly folded back up as he rested the weight of the bicycle on it. A few seconds later he burst in the basement door.

What'cha got?" he demanded. And, "What's that?" as he poked a finger at the open plastic case on the workbench.

"That," I responded pompously, "is a wireless intercom and it's going to solve the problem of you not being able to use the telephone."

"How does it work? Is it like a C.B. radio?"

I rolled my eyes heavenward as any right-thinking amateur does at the mere mention of C.B. "No, Mike. It is not a radio at all, though similar in all respects except the antenna."

Mike looked dubious.

"Look," I said. "A radio uses a transmitter and receiver usually switched between a common antenna. These wireless intercoms have the exact circuitry of a low power transmitter and a not-very-sensitive receiver. But, instead of an antenna, they are coupled to the power line.

Mike looked dubiouser. "They use the power line for an antenna like those gadgets you can buy for your TV?"

"Please," I begged, "Don't get me started on those. And no, they don't use the power line as an antenna. They use it as the medium."

Mikes face went from dubiouser to bewildered.

"It's simple." I said. "Instead of using antennae, you could connect a radio transmitter and receiver with a piece of wire and they would work the same, even better. Right?"

"I guess so."

"So that's what we do with these intercoms."

"Then why are they called wireless intercoms then?"

"Wired-wireless would be more accurate." I corrected him. "Instead of using a separate wire, they use the power lines to which they are connected anyway."

"I see now." he brightened. "Well, give me one and I'll take it home and plug it in so we can talk."

"Not so fast." I cautioned. "Wireless intercoms usually operate in the 100 to 200 KHz area. These frequencies pass quite well through the wiring in a building but are severly autenuated by electric meters and stopped dead by pole transformers."

"Oh well. It was a good idea while it lasted. You got anything to drink?" he said drily.

"In the refrigerator. And it is a good idea. Look. Here's how a wireless intercom couples to the power line." I sketched a quick circuit as Mike came back to the bench with a soda in his hand.

"Here is the A.C. plug connected directly to the power transformer. This is a ferrite core coil with the primary resonated to 160 KHz by a capacitor. In talk mode it uses this small tickler winding to provide feedback to a single transistor modulated oscillator. In listen the primary is connected to a simple diode detector and an audio amplifier. The secondary is coupled to the power cord through these two .1 microfarad capacitors."

"What are the capacitors for?"

"They are small enough the keep the 60 Hz line corrent from burning up the secondary winding but large enough to couple the 160 KHz into and out of the relatively low impedence of the house wiring. By the way, the secondary has a small number of turns relative to the primary to better match into that low impedence."

"So the radio waves..." he corrected himself, "No, the radio frequency current flows through the house wiring directly from one intercom to the other. That's why you don't need a big fancy transmitter or receiver. Right"?

"There's hope for you yet," I smiled.

"Thanks," he said, putting down his soda on my drawing and leaving a water ring.

I pointedly moved the can to the back of the workbench and shook the water off my schematic. I made a few quick changes.

If we disconnect these two wires and bring them out the back, we can connect the intercoms together without using the power lines." I then picked up my pencil iron and quickly made the changes in the remaining intercom while Mike noisily slurped his soda.

I slipped a five-foot length of two-conductor jacketed wire through a hole I had previously drilled in the back of the case. I tied a knot to secure the wire and soldered the capacitors to the pre-cut and tinned ends. Two pieces of shrink tubing and a few seconds with a match completed the job. I dropped the smouldering match into the flip-top can Mike held in his hand.

He stared at the can. "It's a good thing that was empty," he grumbled, tossed it toward the wastebasket and ignored it when it bounced off the edge and rolled into a corner.

Meanwhile I replaced the four tiny screws that held the bottom of the intercom in place. I placed the intercoms side my side on the bench, plugged them in and connected the new pigtails to each other with a pair of clip-leads. I pressed the talk button on the left intercom and was rewarded with an ear-slpitting howl of feedback.

"That's all well and good," quipped Mike. "But if it's all the same to you, I'd rather go home, cut grass and listen to the lawn mower!"

I quickly lowered the volume on the intercoms. I picked one up and pressed talk again. I made a rude noise that sounded remotely like a lawn mower, "to you!" I added. The sounds came clearly from the other unit on the bench.

Mike picked it up and made the same noise. It resounded from the intercom I still held by my face.

Satisfied he asked, "So now what do we do? Spend a week running another wire through the woods from here to my house?"

"You know me better than that," I replied, yawning. "I'm much too lazy. I'll connect this intercom to my end of the existing wire like this and put it on the corner of my computer desk so I can reach it from there or from the workbench." I suited my actions to my words. "You take this one," I handed him the other intercom, "and connect it to your end."

"Will it work that far away?"

"I don't see why a mile of used telephone company drop wire with a splice every hundred feet or so and lying on the ground should have too much attenuation for 160 KHz," I temporized, turning the volume back up just in case. "The easiest way to find out is to try it."

"Easy for you," he sneered. "You just have to sit here and wait while I pedal that confounded machine back home!"

"Sounds like a fair division of labor to me."

"And does this mean I can't listen to the Country and Western music you've been sending down the wire any more?" He whined, glancing at the slowly revolving tape reels in the rack across the basement workshop.

"Not at all, "I replied smugly. "If the music goes beyond 10 KHz I would be surprised. The intercoms operate at 144 KHz. The one will have no affect on the other.

"That's OK then. I'll take this gizmo home and hook it right up. Be talkin' to ya." He knotted the two wires from the intercom together and hung it around his neck like a pendant; then ran out the door, righted his bicycle and took off at breakneck speed down the driveway.

I closed the door after him and began cleaning up the workbench. Twelve minutes latter Mike's voice came blaring through the intercom next to my computer. "Breaker, breaker, wired wireless channel!" I cringed and turned down the volume.

I jabbed down talk. "If you don't promise to cut that out right now, I am going to take a large pair of pruning shears and cut that entire mile wire into tiny pieces. What will that do to your music?"

"10-4, I promise." came the sheepish reply.

Tom Hunter (N3CRK), 06 DEC 2000