Copyright © Parallax Inc.
Digital I/O Board Kit (#27113)
v1.0 2/19/2010 Page 11 of 13
Setting Input Voltage Range
This kit comes with 1.5K resistors on the inputs (R25-R32). These resistors were chosen as the defaults
based on a typical input voltage range of 15-30VDC (24 nominal). If you wish to input a different range
of voltages you will need a different resistor value in place of the default 1.5K unit. To calculate this
value it is necessary to have a few key pieces of information. First and foremost the Digital I/O Board is
configured to require a minimum of 8mA of current across an optocoupler LED for the output transistor to
trigger an output. Ideally you’ll probably set the current range for your input voltage to 10-20mA.
Given this range of current we can calculate the required resistor using the following math:
R = (V
IN
– V
FWD
) / I
LED
(where V
FWD
is the LED forward voltage, ILED is LED current and V
IN
is our input
voltage)
Assuming we want be able to input a 12V signal and accounting for the typical forward voltage for the
4N25 of 1.2V we can now calculate as follows:
R = (12V – 1.2V) / 0.01 = 1080
The closest value is 1K which will mean a current of 10.8mA @ 12V using I = (V
IN
– V
FWD
) / 1000
We can also calculate the maximum voltage we want on the input using a 1K resistor:
V
INMAX
= (1000 * 0.02) + 1.2 = 21.2V
Sensitivity
Because of the way the input LEDs are driven on the output, they will indicate voltage on an input even
when it is not sufficient to trigger and output. Voltage as low as 2V will cause the LED to start to glow,
however the output won’t trigger until the minimum current has been reached. This provides some
safety in the sense that you’ll have an indication of input voltage in spite of the outputs not being
triggered.
Precautions
As with any device capable of switching high voltages, precautions should always be taken to avoid
damage and/or injury. The following precautions are recommended in addition to any precautions by the
manufacturer of any equipment connected to this device.
The Digital I/O Board should be mounted inside a suitable enclosure or electrical panel using
standoffs. The board should not be able to move.
Any electrical connections to the board should be secured so they can not move and any wires
extending outside the enclosure should have strain relief.
Any incoming voltage rails to be switched by the relays should be fused prior to entering the
Digital I/O Board.
Always be sure any incoming voltage rails are powered off prior to connecting them to the Digital
I/O Board.
When connecting the V+ pin to VIN on another board, be sure that board does not have its own
power supply. The target board will now get power from the Digital I/O Board.
