ioBridge modules can be interfaced with a wide range of sensors that have output signals that are proportional to sensor values.
The four channels on the IO-204 or Gamma consist of the following input, output, and power pins as shown below:
The analog input and digital input pins can be connected to various types of sensors.
Analog Sensors
The analog input on each channel uses 10bit (1024 step) analog-to-digital conversion (ADC). As an input voltage varies from 0V to 5V the raw analog values range from 0 – 1023. The equation for this is:
Raw Analog Value = 1023 x (Input Voltage in volts)/5
This relationship is shown in the Analog Voltage Lookup Table below. For example, an analog voltage of 2.6 volts between the Analog In and Ground pin corresponds to a raw value of 532 read by the IO module. The resolution of the analog input is 5/1023 volts or 4.9 mV.
Thermistor Sensor Example
The ioBridge Temperature Probe (FB-2) is a thermistor sensor. A thermistor is a type of resistor whose resistance varies significantly with temperature. In order for this type of sensor to be connected to an ioBridge module, variation in thermistor resistance needs to be converted into a variation in voltage that can be read by the analog input. The following is an example of how this can be done using a simple voltage divider circuit connected to the IO module.
The analog input voltage is now related to the thermistor resistance using the following equation.
Analog Input voltage = 5 volts x R/(R+Rt), or
Raw = R/(R+Rt) = 10000/(10000+Rt)
The thermistor resistance varies with temperature in a non-linear fashion described by the Steinhart-Hart equation.
1/T = a1 + b1*ln(Rt/Rref) + c1*ln(Rt/Rref)^2 + d1*ln(Rt/Rref)^3
where Rref is the resistance of the thermistor at the reference temperature. That is, Rref = 10,000 ohms, and Rt is the resistance (in ohms) of the thermistor when its temperature is equal to T in Kelvin.
The 10000 ohm thermistor used in the Temperature Probe (FB-2) has the following parameters: a1=3.35401643468053E-03, b1=2.5698501802E-04, c1=2.6201306709E-06, d1=6.3830907998E-08. R is a 10000 ohm (0.1% tolerance) resistor.
The equations can be rearranged to show raw analog value versus temperature in Kelvin as follows:
T = 1/(a1+b1*ln((1023-Raw)/Raw)+c1*ln((1023-Raw)/Raw)^2+d1*ln((1023-Raw)/Raw)^3)
The above equation is tabulated in ioBridge Temperature Probe (FB-2) Lookup Table as Raw value versus degrees Fahrenheit or Celsius.
This table is used when the user selects the option in the module set up to display the analog input as Temperature in degrees F or C. Note that the above expressions are only valid for a temperature range between -55 and 150 degrees C.
Honeywell HIH Series Sensor Example
The Honeywell HIH series of sensors can be directly connected to the IO modules as follows or one can purchase the ioBridge Humidity Sensor (FB-8).
The out pin of the HIH sensors provides an output voltage that varies as a function of humidity
The Honeywell HIH sensor linear voltage output is a function of VSUPPLY, %RH and temperature. The output is “ratiometric” i.e. as the supply voltage rises, the output voltage rises in the same proportion. The sensor behavior for temperatures between 0 ºC and 85 ºC is well approximated by a combination of two equations:
1. A “Best Fit Line at 25 ºC [77 ºF]” or a similar sensor specific equation at 25 ºC. The sensor independent “typical” Best Fit Line at 25 ºC
% RH = (VOUT/VSUPPLY – 0.16)/0.0062
A sensor-specific equation may be obtained from an RH sensor printout. The printout equation assumes VSUPPLY = 5 Vdc and is included or available as an option on every sensor.
2. A sensor-independent equation which corrects the %RH reading (from the Best Fit Line equation) for temperature, T:
Or:
True RH = (% RH)/(1.0546 – 0.00216 T): T = ºC True RH = (% RH)/(1.093 – 0.0012 T): T = ºF
The equations above match the Best Fit Line at 25 ºC or the sensor-specific equation at 25 ºC to within the following tolerances:
±1% for T>20 ºC ±2% for 10 ºC< T<20 ºC ±5% for T<10 ºC
The sensor-independent equation at 25 ºC is available to the user when setting up the module. The Honeywell Humidity Sensor Lookup Table is listed below.
The Expression Builder functionality may also be used to implement the sensor specific equation with temperature compensation.
Digital Input Sensors
Simple Switch
The digital input pin can be connected to a simple on-off switch as shown below.
This could be the output of a magnetic security switch or any other type of switch. It is important to note that the digital input has so-called pull-up resistors between the digital inputs and +5V. When nothing is connected to the digital inputs the input voltage is 5V. This is read by the IO module as High or 1.
Pulse Meter
The digital input of the IO modules can be set up to count pulses and to send the pulses to the server every so often. This features is ideally suited to interfacing with pulse output meters such as a power meter that outputs pulse counts proportional to energy used.
