Campbell Scientific CR3000 Micrologger - Manual (Page 333)

Section 8. Operation
333
8.2.4 Control Outputs
Controlling power to an external device is a common function of the CR3000.
On-board control terminals and peripheral devices are available for binary (on /
off) or analog (variable) control. A switched, 12-Vdc channel is also available.
See Switched Unregulated (Nominal 12 Volt)
(p. 91).
8.2.4.1 Digital I/O Ports
Each of eight digital I/O ports (C1 – C8) can be configured as an output port and
set low (0 Vdc) or high (5 Vdc) using the PortSet() or WriteIO() instructions.
Ports C4, C5, and C7 can be configured for pulse width modulation with
maximum periods of 36.4 s, 9.1 s, and 2.27 s, respectively. A digital-I/O port is
normally used to operate an external relay-driver circuit because the port itself has
limited drive capacity. Drive capacity is determined by the 5-Vdc supply and a
330-ohm output resistance. It is expressed as:
V
o
= 4.9 V ‐ (330 Ohms) * I
o
Where V
o
is the drive limit, and I
o
is the current required by the external device.
Figure Control Port Current Sourcing
(p. 333)
plots the relationship.
Figure 108: Control port current sourcing
8.2.4.2 Relays and Relay Drivers
Several relay drivers are manufactured by Campbell Scientific. For more
information, see the appendix Relay Drivers
(p. 565),
contact a Campbell Scientific
applications engineer, or go to www.campbellsci.com.
Compatible, inexpensive, and reliable single-channel relay drivers for a wide
range of loads are available from various electronic vendors such as Crydom,
Newark, Mouser, etc.