Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 35)

Section 1. Installation and Maintenance
1-11
1.9.1 Use of Digital I/O Ports for Switching Relays
Each of the eight digital I/O ports can be configured as an output port and set
low or high (0 V low, 5 V high) using the PortSet or WriteIO instructions. A
digital output port is normally used to operate an external relay driver circuit
because the port itself has a limited drive capability (2.0 mA minimum at 3.5
V).
Figure 1.9-1 shows a typical relay driver circuit in conjunction with a coil
driven relay which may be used to switch external power to some device. In
this example, when the control port is set high, 12 V from the datalogger passes
through the relay coil, closing the relay which completes the power circuit to a
fan, turning the fan on.
In other applications it may be desirable to simply switch power to a device
without going through a relay. Figure 1.9-2 illustrates a circuit for switching
external power to a device without going through a relay. If the peripheral to be
powered draws in excess of 75 mA at room temperature (limit of the 2N2907A
medium power transistor), the use of a relay (Figure 1.9-1) would be required.
Other control port activated circuits are possible for applications with greater
current/voltage demands than shown in Figures 1.9-1 and 2. For more
information contact a Campbell Scientific applications engineer.
FIGURE 1.9-1. Relay Driver Circuit with Relay