Light Vehicle:

So I set myself the task of making a battery-operated wheeled vehicle that would sit on the foor and monitor the ambient light. . When that darkened., it would move randomly on the floor until it found a spot where sufficient light fell on it. It would then stop and resume its monitoring..
This task would require a means of controlled locomotion and sensing an environmental variable. For a means of locomotion, I employed slot car techniques. They were the rage at the time and there was a wide selection of motors, gears wheels and other parts. I adapted a device used in toys to cause them to change directions when they bump into obstacles called a universal differential. This solved the navigation problem: let it be random. For sensing light level, I found a design for a simple circuit employimg two transistors and a photo cell that would switch a relay to control the drive motor. I etched the circuit onto a copper clad board using some materials I had left over from college printmaking classes. Since I was doing everything for the first time it took me several months to asemble the vehicle but,when I did, there was a problem.

Sometimes it worked fine, sitting there on the floor until a shadow passed over it. then it would lurch to life and move until it was clearof the shadow, then stop. But sometimes, as the light level dropped, it would simply sit and buzz. Or sometimes when it was moving to find a brighhter place it would stop and buzz. I tried to find the cause of this aberrant behavior. I replaced components.. I redesigned the circuit and etched another board. I checked and rechecked everything. To no avail.

Finally, exasperated, I called Al Worth, an aquaintance who was an engineer.
He said "Can you make it buzz right now?'
I said, " Yes, I think so".
He said, "Put it on the phone."
"What?", I said.
He said " Make it buzz and put the phone's mouth piece down by what's buzzing."
I set the vehicle on the workbench in my studio and adjusted the circuit to the sensing point and the relay made a buzzing hum.
He listened for a moment, then he said " "Is your studio lit by flourescent lights?"
"Yes" I said.
Al said,"There's your problem. Flourescent lights turn on and off sixty times a second. Half the time your studio is in total darkness. The other half, it's in light twice as bright as it looks. Nothing is wrong with your circuit. On the contrary, it is seeing that dark/ light condition and dutifully turning your vehicle on and off sixty times a second. That's the buzzing you hear. I recognized the frequency over the phone."
Obviously, I was in over my head.


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