I put together this little bit of electronics for an upcoming art show. My goal was to create a device that would demonstrate the Northern European need for personal space. It sounds an alarm when someone gets too close.
I will put a video of the device in action here.
The prototype was built on a breadboard and powered by an Arduino Uno. Once I had everything wired up and the programming more-or-less finished, I built the enclosure from a plastic box that was lying around. I cut holes for the various components, making sure they would fit together on the inside.
Those components include ultrasonic distance sensor, which I got from Radio Shack. It works like a charm.
The heart of the device is an ATMega 328, which is a great chip. Since I ran out of capacitors, I had to figure out how to get the chip to run with the inaccurate internal timer. To do this, I used two Arduinos: the first acted as a serial programmer to install an internal-timer bootloader on the ATMega 328 in the second. With the correct bootloader, everything proceeds as expected (recalling, of course, to select the internal timer bootloader when uploading from the Arduino IDE).
When the device measures a distance less than X to a person, it does three things. First, it displays a message on an LCD screen, depending on X. Next, it flashes either two or four red LEDSs. Lastly, it makes one of two pitches of sound from a piezoelectric cell. Here are those components, wired up.
The device is powered by a 9V battery, so I use a regulator to reduce to 5V. There are also a couple capacitors and a power switch — all good practice but maybe unnecessary.
Our Design Technology teacher asked for some technical help with a project involving LEDs. We tracked down a shop that sells LED strips, and I picked up a meter of high-intensity bulbs. As a physicist, I was curious about the stopping voltage, currents limits, and heat dissipitation, but it turns out these strips are designed with three LEDs and accompanying resistors on each 5 cm length, so the strip is simply supplied with a 12 V supply. The wall wart supplies have power ratings, and power is used to calculate the appropriate supply. As 0.12 W per LED, with a total draw of 7.2 W for the strip of 60 LEDs, I was able to use a power supply rated to 8 W, or 0.67 A at 12 V to my mind.
A smart persin suggested that LEDs in a bottle would look nice, so after soldering the power supply to the strip, I dumped it into a glass pitcher. A clear bottle would probably look better.
Up next: students investigating aspects of lighting. I cannot wait to see what they come up with!
This week I was delighted by the arrival of my first-ever Printed Circuit Boards [PCBs]. I will be using these with my IB physics class. The recent revision of the curriculum requires that students understand the functioning of a diode rectifier. I think it’s a great opportunity to teach some soldering skills, so I designed a little circuit board the students could assemble, and then use for a bit of lab work.
The circuit board is at the bottom of the above picture. As you can see, the layout is spacious so that sloppy soldering won’t ruin everything. I also included through-holes so that a capacitor and a resistor can be added, in case we want to use a power supply with a bit of bite. However, I think I’ll stick with the hand-cranked generator, in the picture above, which delivers AC up (and down) to about 10 Volts.
I am using a Vernier voltage probe to monitor the electric potential. You can see the voltage plotted against time (for 30 seconds) on the computer screen above. Below is the same set-up, but using the diode rectifier. Yahoo, it works!
I spent some time last week helping a third grade class with science projects. One group was trying to power a small electric fan with a cell made from an aluminium can and a copper sheet in salt water. Needless to say, it wasn’t working very well. They were able to produce about one tenth the minimal current needed to turn the motor.
To alleviate the suffering, I built a new motor. This one has a battery inside. The transistor turns the motor (powered by the battery) on when a potential difference is placed between the two leads. It’s cheating — and I’m not sure whether it was a good idea. But at least the team were able to present their work at the science fair.