Hardware Reference
In-Depth Information
Photo 9.9
Various types of relay
3-15 V) and can be interfaced with the aid of a buffer, open-collector logic gate,
or transistor. Figures 9.35(a)-(c) show typical interface circuits which produce
an audible output when the port output line is at logic 1.
Where a pulsed rather than continuous audible alarm is required, a circuit
of the type shown in Figure 9.36 can be employed. This circuit is based on a
standard 555 timer operating in astable mode and operates at approximately
1 Hz. A logic 1 from the port output enables the 555 and activates the pulsed
audio output.
Finally, the circuit shown in Figure 9.37 can be used where a conventional
moving-coil loudspeaker is to be used in preference to a piezo-electric trans-
ducer. This circuit is again based on the 555 timer and provides a continuous
output at approximately 1 kHz whenever the port output is at logic 1.
DC motors
Circuit arrangements used for driving DC motors generally follow the same lines
as those described earlier for use with relays. As an example, the circuit shown in
Figure 9.38 uses a power MOSFET to drive a low-voltage DC motor. This circuit
is suitable for use with DC motors rated at up to 12 V with stalled currents of
up to 3 A. In both cases, a logic 1 from the output port will operate the motor.
Output drivers
Where a number of output loads are to be driven from the same port, it is often
expedient to make use of a dedicated octal driver chip rather than use eight
individual driver circuits based on discrete components. Fortunately, a number
of octal drivers are available and these invariably have TTL-compatible inputs
which makes them suitable for direct connection to an output port.
