CHAPTER 27 ■ ENCOREVoice recognition is difficult enough for high-powered computers, much less a robot. However,
recording and playing back brief messages can be achieved with commercially available modules. For
recognition of simple tones, a microphone and inexpensive chip will do.
Relays
One concern with transistors is the limited amount of power they can switch. Relays provide a switching
function similar to transistors, but allow much higher voltages, greater current, and even different voltage
types (such as DC controlling AC). Most importantly, mechanical relays allow one circuit to control an
adjacent circuit that has a completely separate power source.
A typical mechanical relay (see Figure 27-18) has an electromagnet that physically moves metal plates
together to connect. This physical switching explains why some relays make a distinctive clicking sound
during operation. When power is disconnected, a spring, or the bent metal plates themselves, forces the
metal plates back into their original position.
Figure 27-18. Miniature relay (left) and solid-state relay (right)
Because a bit of current (10 mA and up) is needed for the relay itself, a transistor often switches the
power to the relay. So relays supplement switching—they don’t replace transistors. In fact, because relays
can be large, noisy, heavy, and relatively slow, they’re usually employed only for heavy-duty jobs.
Some robots use relays for controlling large motors. That’s understandable. But, for mid-sized and
lunchbox-size robots, current-efficient transistors (called MOSFETs) are superior choices under most
circumstances.
Adding Gears
Gearhead motors contain tiny, precisely manufactured, precisely placed gears for optimal motor
performance. However, that doesn’t prevent you from adding additional gears (see Figure 27-19) to the
motor shaft in order to alter the final output speed or deliver the rotation to a different location.