Page 65 - Hacking Roomba

46 Part I — Interfacing

up so its output voltage sags. The capacitor gives the voltage regulator a more steady power
supply to work from, filtering out noise and smoothing out small dropouts. It smooths out
dropouts by acting like a little charge reservoir for them, and it filters out noise by averaging
out small variations in the voltage. Capacitor values are measured in Farads (symbol: F), which
is a measure of how large their charge reservoir is.

Noise Dropouts

5V
Voltage

Time
FIGURE 3-4: Noise and dropouts on an otherwise stable DC voltage

The output capacitor C2 performs a similar role for the users of the +5 VDC power it creates.
Since this output voltage is used as the positive supply voltage to an IC (specifically the RS-232
transceiver IC), it’s historically called Vcc, Vdd, or V+. Vcc will be used here.

Capacitor Voltage Ratings
Another parameter of capacitors is their voltage rating. This is often 16V, 50V, or 100V. There’s
no great trick to choosing this value: the general rule-of-thumb is to pick a voltage rating
about twice the maximum voltage the capacitor will see. In this circuit, the maximum voltage
is around 16V. Double that is 32V, so the 50V capacitors will work fine.

Understanding LEDs

The next sub-circuit is the status LED, shown in Figure 3-5. Its simple purpose is to shine
when there is power present. This sub-circuit is not strictly necessary to make the serial tether
function, but it does provide some visual indication as to whether there is a current running
through the circuit. Also, always follow the general rule: if an LED can be added to a circuit
without otherwise affecting its functionality, add it! It’s fun, and it helps you troubleshoot
whether there is any power in your circuit.

In order to light an LED, you must pass current through it. The amount of current determines
the brightness of the LED, up to some maximum current. Beyond that maximum, the LED
blows up. This is entertaining once or twice but doesn’t really solve the problem of letting you
know when your circuit is functioning.