Raspberry Pi A User Manual page 276

260
P A R T I V
H A R D W A R E H A C K I N G
Figure 16-9:
Using the
Gertboard
motor controller
For sensing or feedback projects, the Gertboard's ADC and DAC components are handy addi-
tions. Unlike the GPIO pins on the Pi itself, which can only receive and send digital signals, the
Gertboard includes two ADC and two DAC connections. The ADC pins, located on the upper-
left of the Gertboard, allow analogue components to be converted into digital signals compati-
ble with the Pi. As an example, Figure 16-10 shows how you can use the ADC pins to read the
status of a potentiometer—a component which varies its resistance according to the position of
a slider or knob. This circuit could be used to control the Pi's volume in a media centre applica-
tion, or to alter the speed of an attached motor. The DAC pins provide the opposite functional-
ity, taking a digital signal from the Pi and converting it to analogue. This could drive a speaker
to create audio, or alter the speed of a motor or the brightness of an LED.
For more complex projects, the Gertboard provides an open collector driver, which uses tran-
sistors to switch on and off devices that have different power requirements to the 3.3 V used
by the Gertboard or that draw a large amount of current and require an external power sup-
ply to operate. The open collector driver can control the power supplies of up to six devices
simultaneously without requiring any hardware. This gives the Gertboard a large amount of
flexibility—although transistors or relays can be used with passive add-on boards like the
Prototyping Pi Plate to achieve the same goal.