Extron electronics HC 400 Series User Manual page 37

To determine the value of the current limiting resistor in the circuit shown on the
previous page, you need to know the values of three variables:
i = LED forward current in amps (found in the data sheet for the LED)
= LED forward voltage drop in volts (found in the data sheet for the LED)
V
f
V = supply voltage of the external voltage source
s
Insert those values into the following equation to determine the resistor value:
V - V
s f
R =
i
Example calculation:
}
i = 5 mA (0.005 A)
V = 2 V
f
= 5 V
V
s
NOTE: If the value calculated for the current limiting resistor is not a
standard resistor value, you can round up the number to the next highest
common resistance value.
The connected LED is off when the port and switch 1 are open.
The connected LED is on when the port and switch 1 are closed.
Digital I/O digital output with pull-up enabled
•
When the port is configured for pull-up, switch 2 is closed, activating the
•
+5.0 VDC pull-up resistor.
When switch 1 closes, the port is on.
•
When switch 1 opens, the port is off.
•
Example application, digital output with pull-up: controlling another device
•
via its contact closure input port
Connect the digital I/O port to the contact input port of another device, such
as an Extron HCT transmitter. When activated, the digital I/O digital output port
momentarily shorts pin 1 to ground (pulsed contact for 0.5 seconds), closes
switch 1, which selects the input on the connected device.
+5.0 V
1k ohms
SW 2
CTL
SW 1
Figure 18. Digital I/O Digital Output Application With Pull-up:
Contact Closure Input Selection on a Connected Device
V - V
5 V - 2 V
s f
R = =
i 0.005 A
Digital
I/O
Voltage
Protection
GND
HC 400 Series • Hardware Features and Installation
= 600 ohms
HCT 102 D
RESET
29