5.9. Errata

The term errata in the electronics industry refers not only to printing errors in documentation, but also errors in the design or manufacture of parts which lead to deviations from the specification.

This page documents several kinds of “errata”: errors in the exercises which have not yet been corrected in the text, specific problems we have discovered with lab equipment, and common mistakes we have seen students make.

General information on techniques can also be found under Tips and Tricks and Essential Skills List.

5.9.1. Common Mistakes

1. The power supplies have current limit settings which govern the maximum current it will output from each channel. If the draw is too high (e.g. load resistance too low), it will drop the voltage to meet this limit. Some power supplies had this knob set at zero and were properly refusing to output any current.
2. The power supplies have series, parallel, and independent modes: we will almost always use independent mode. The other two modes gang the two halves together: series can offer higher voltages, parallel can offer high currents. If in doubt, check the manual.
3. The relay schematic is a logical illustration, unlike the pinout illustration which shows the physical connection layout; the two only correspond via the pin naming. This is common in electronics diagrams since schematics are intended to convey the logic of an electronics design.

5.9.2. Errors in Exercises

FIXME: have these been fixed in the exercises?

1. The digital input using a switch needs a pullup resistor to function properly, otherwise the input pin is switched between an ambiguous ‘floating’ state and ground.
2. The thermistors we have in stock are Vishay parts from SparkFun, who hosts a data sheet.

5.9.3. Lab Electronics

1. Apparently our RGB LEDs are common-anode (determined empirically). The longest lead is the common. When in doubt, test with the DMM diode function. (The anode is the positive terminal, the cathode is the negative terminal.)
2. Some of the modular illuminance sensors for the transducers exercise are brand-new and will need the three-pin connector soldered on.
3. The pins should be soldered on the back side of the A4988 such that the driver chip is visible. This does mean that the pad silkscreen text is on the bottom and out of sight, but allows the chip to dissipate heat. If your module is not soldered in this orientation, please be especially careful when wiring the circuit.

5.9.4. Lab Equipment

1. Not all of the DMMs had the probes with the long tips; the long tips make the AC wall measurement much easier.
2. The DMMs have a diode measurement mode for measuring the forward voltage drop, but it doesn’t work for the purple LEDs, most likely because the forward voltage drop is too high because of the particular chemistry, which is also related to the color. The lab meters don’t even work for the red LEDs, although they will measure the 0.6 volt drop of a 1N914 diode. I used a better meter (a Fluke 179) to test one of our red LEDs and it showed a voltage drop of 1.79V. However, even the better meter was only able to light the purple LED but not register its voltage drop.