Like many people with multiple woodworking tools, [Will Stone] wanted to create a centralized dust collection system. But he quickly found that the devil was in the details, as he struggled to find an economic way to automatically kick on the vacuum when one of the tools started up. His final solution might be one of the most elegant, and surely the cheapest, we’ve ever come across.
As with other DIY systems we’ve seen over the years, [Will] is using a simple inductive current sensor to detect when AC power is being drawn by one of his tools. But where the similarity stops is that there’s nothing so pedestrian as a microcontroller reading the output of the sensor. He realized that when the coils in the sensor were energized they were putting out about 7 volts AC, which should be more than enough to trigger a relay.
So he threw together a rectifier circuit on a piece of perfboard, using four LEDs in true hacker style. With the addition of a capacitor to smooth out the voltage, this little circuit is able to trip the 40 amp solid state relay controlling power to the vacuum using nothing more than the energy harvested from the sensor’s coil.
Using a current sensor is great when the tools are close enough to all be plugged into the same line, but that doesn’t help the folks with cordless tools or supersized shops. In that case, you might need to look into a sound-activated system.
Here is a great introduction to a practical application of electromagnetic theory—the field telephone. It’s a training film from 1961 that covers the sound-powered, local battery, and common battery systems along with the six basic components they use: generators, ringers, transmitters, receivers, induction coils, and capacitors.
Clear illustrations and smart narration are the hallmarks of these Army training films, and this one begins with a great explanation of generator theory. The phone’s ringer uses electromagnetic attraction and repulsion to do the mechanical work of striking the bells. Similarly, the sound waves generated by a caller’s speech move an armature to create an alternating electrical current that is transmitted and converted back to sound waves on the receiving end.
In the local battery system, the battery pushes pulsating DC to carry the voice transmission. An induction coil increases the capabilities of this system, but capacitors are required to filter out the frequencies that would overload the receiver, passing only the higher speech frequencies.
In order for several stations to communicate, the use of a switchboard is required to patch the calls through. There are many advantages of a common battery system with regard to call switching: no local battery is necessary, nor is a generator needed at each station. Calls are easier to place, and communication is much faster.
Continue reading “Retrotechtacular: Basic Telephony In The Field” →