Friday, October 16, 2015

Museum Exhibits Preparation

I'm preparing a robotics exhibit for a local museum that is opening up soon. To that end, I learned how to weld just now (I'm not very good yet. Also now I'll smell like welding all day).

The exhibit is simple--drive a small remote controlled car around a map of the city, from a monitor that tracks the car and displays a birds-eye view camera image, auto-rotated to the car.

The biggest hurdles:
  • Light weight and high capacity battery (but low max discharge rate--which is fine, I don't need a high discharge rate)
  • Sleep mode for the wireless radio and microcontroller to conserve battery when nobody is using the exhibit (chose the Wixel here, for reasons I'll cover in another post)
  • Safety against, stall, over-current, under-voltage, and other battery damaging conditions
  • Making sure the parts the user touches (console, perimeter wall) stand up to the forces users will likely put on these

To adjust thresholds for my wake-up circuit, I finally installed drivers for the portable oscilloscope I have:

First impressions: Quick-start was very easy. Installed drivers and software on Windows to get this screenshot, but you don't need to, as the scope has a tiny display.

In the image above I am bringing a flashlight closer and farther from the sensor. The green shows the phototransistor circuit output directly. The red is the output of the comparator circuit (LM358N, CNY70 if you want the parts. Just what I keep in stock, not chosen for any other reason. Don't turn on the CNY70 LED for this application, durr).

I was able to get it to robustly sense when I walked under the fluorescent light fixtures in this building, holding it at chest height.

More on this later.

EDIT: One more fun image.

My fluorescent bulb desk lamp apparently pulses on/off with a cycle taking 7ms or so. The other pattern (where the waveform is getting "pinched" and then expanded) is aliasing from the slow sampling frequency of the oscilloscope (or at least the points drawn in the software) at this large time scale and the 7ms lamp pulse.