91.548 Lab 1: LogoChip

due February 3

In this lab, we will experiment with current flow (i.e., destructively test LEDs) and learn how to create digitally-controlled devices using the LogoChip.

1. Using experimentation, determine how much energy is required to destroy a cheap T1-3/4 (standard/large) red LED. Does a brief burst of high current or a longer period of lower current make a difference?

   Notes:

   • You have a budget of about 10 LEDs. We have lots, and they cost about 5¢ each, but pretend the supply is not unlimited.

   • Ohm's Lab is V = IR (voltage = current * resistance). You can measure current by running it through a resistor of known resistance and then measuring the voltage drop across the resistor (I = V / R). Or, you can run the current through a digital VOM (assuming it's less than 10A) and get a reading. Or, you can use the instantaneous current display on the lab power supplies.

   • An LED is a non-linear device. This means it does not obey Ohm's Law. In normal operation, it is modeled as a diode, which means it has a fixed voltage drop.

   • Describe your results in Joules, using the 1 J = 1 Watt-second definition. Talk about current flow, voltage drop, and time.

2. Using the Getting Started with the LogoChip document, wire up the LogoChip on a breadboard, get its boot flash, then hook it up to the PC, and get bootflash to run from the software's command center.

   Hook up a beeper, configure its pin to be an output, and write a loop that makes it beep.

3. Complete one of the following five activities/experiments. Alternately, using these, the Predko book, or anything else as inspiration, design your own cool project using the LogoChip. (If you choose to do something other than one of the ideas here, please check it with me first.)
   • Design a stepper motor control circuit using the LogoChip. We have 4-phase unipolar stepper motors in the lab. You may be able to run them directly from LogoChip digital outputs, but you'll probably need driver transistors. Write a program to make the stepper motor go. Implement position control (i.e., commanding the motor to rotate to a specific position) and velocity control (i.e., cause it to run freely at a specified number of clicks per second). The velocity control program should keep track of the absolute position as well.

     For reference, see the Predko book, pp. 289–292. Also, we have motor driver chips, the TI SN754410NE.

   • Implement a programmable display using a 5x7 LED array component. Create a multiplexed row/column scan method so that only 7 LEDs are on at one time. Create a character font so that an arbitrary symbol from the ASCII character set can be displayed.

   • Implement a driver for the DevanTech sonars that are part of the Botball kit. Have the LogoChip indicate distance to target in a standard unit of length.

   • Create the following experiment in the spirit of Harold "Doc" Edgerton's seminal strobe light work. Connect a piezo to the LogoChip, but instead of using it to produce sound, use it as a microphone. (When it receives a loud noise, it should produce a signal in the 50 mV range, which is detectable by a LogoChip analog input.) Use this to implement a clap detector.

     Then, after hearing a clap, light a bank of LEDs for a short interval (10 to 50 milliseconds or so). Now you have a system that when it hears a clap, it produces a short burst of light a moment later. (Transistor drivers may be needed to get adequate light from the LEDs, because they should only be turned on for a very brief period.)

     Finally, take the setup to a darkened area along with a regular latex balloon and a pin. Pop the balloon near the piezo “microphone” and in range of the LED illumination. You should see the open shell of the balloon being popped, before it has collapsed into shreds!

     (This experiment was suggested by Robbie Berg.)

   • Build a simple digital-to-analog circuit (DAC), perhaps using four bits and a resistive network to create a stepped range of output voltages. Load up some sampled or computed audio data (e.g., a sine wave) into the LogoChip, and write a program to spit it out at a rate fast enough to make sound. Listen to the output and look at it on the scope, and describe your results.

For #1 and #3, prepare up a Lab Report to be turned in. This should be a written description and/or circuit diagram and/or code listing (as appropriate) explaining what you did and how you came to believe your interpretation of what was going on was correct.