Wired sensors for Industrial IoT

Is the door open or closed? How hot is the pump room? How much fuel is left in the farmer’s diesel tank out in the woods? The applications for sensors linked to IoT-devices are endless. Here is a guide on how to use external sensors and actuators with your IoT-devices.

Sensors and actuators for Industrial IoT

Both alarm transmitters and IoT sensors can have different inputs to receive signals from wired sensors. The inputs can be digital, analogue, and temperature sensor inputs, to name a few. Furthermore, if you agree with my reasoning that the massive IoT market is based on three different types of devices, then you know that in parallel with sensors are actuators. These actuators can have both relay outputs, open collector outputs and galvanically isolated outputs. So what do the different terms mean, and when to use what? Inputs for sensors and outputs for actuators are collectively referred to as I/Os. Actuators and sensors are found together in some IoT devices; you will only see one type in some. Nevertheless, here is an overview of the different I/Os available.

Digital inputs

Digital inputs are used when you have two levels: Off/On (Binary 0/Binary 1) or High/Low level. For example, a digital input can thus convey the status of whether a door is open or not, but not how much it is open.

Digital inputs can be either NO or NC, Normally Open or Normally Closed, where the low level of the input can be a normally open circuit or a normally closed circuit. It is possible to program the type of input level to be used on some products. On others, there may be several types of different inputs.

Choose the right one from the start

It is worth considering what type of input you have as it can make a difference to what the alarm transmitter supports. Choosing a transmitter with NO when you need NC may, in the worst case, mean that you have to put alternating relays between your alarm point and the transmitter’s alarm input.

You may also need a relay to switch an NC to NO signal and vice versa. Such simple relays can be ordered for quite a small amount of money.

Analogue inputs

Analogue signals have multiple states, and their accuracy is defined by how many bits of resolution they have. And the inputs are used to show temperature, level or flow rate.

With an analogue input, it is possible to show the level of a liquid in a particular tank. Not just whether it is empty or full, as with the digital signal.

Analogue – not for battery operation

Analog I/O in the industry is usually of the 0-10 V or 4-20 mA current loop type. Analogue inputs need a constant voltage supply and are, therefore, unsuitable for battery operation. There are also simpler types of analogue I/O that measure a voltage, for example, but this is more for hobby use or simple testing.

Another factor that affects battery life when working with analogue sensors and wanting to voltage feed them via a battery-powered solution is that many analogue sensors need a stabilisation time where the sensor needs to be powered and stabilised. With analogue sensors, it is not the sensor itself but the input of the alarm transmitter that defines the accuracy; 10 and 12-bit resolution is standard. Choosing one with too low a resolution can sometimes cause problems.
If you have a deep pond where you are measuring a depth, too low an accuracy can result in several metres between the measurement steps presented. After all, a difference of two metres means significant differences in a power plant pond.

Pulse counter/counter inputs

Pulse sensors are actuators that deliver a short pulse, a pulse in Hz, i.e. oscillations per second. The device that will process the information must have a counter input, pulse input or counter input.

There are slightly different names depending on what you want to achieve. If the pulses are not very frequent or have a low oscillation per second, it is sometimes possible to use the digital inputs, although many devices have unique pulse inputs which often have better performance than digital inputs.

Some devices can mirror pulses to another receiving device which can then send out the corresponding pulse via a pulse output. Other devices may instead count the number of pulses and send a message when a certain number of pulses

Temperature inputs and sensors

Temperature sensors and temperature alarms come in a variety of designs, and I will return to these in a separate guide. The advantage of these sensors over the analogue ones is that they draw considerably less current and are better suited to situations with insufficient access to a fixed voltage supply.

Digital outputs

These are similar to digital inputs in that they support two logic levels or states and are used, for example, to start or stop a device or process. Digital outputs can be:
1. Open Collector output
2. Relay output
3. Relay output with galvanic isolation

Open Collector output

Open collector outputs are characterised by the fact that they can only be loaded with low current and often a low voltage. Open collector outputs are usually combined with relays to control larger loads. The output is a transistor with the collector “open”.

When a signal (voltage) is applied to the base of the transistor, the transistor switches to the “on” position and allows current to flow through the transistor. The load it can control is limited to what the transistor can handle, often around 100 mA.

Relay outputs

Relay outputs can control larger loads and accept higher voltage and current than open collector outputs. Often built-in relays on actuators are sufficient to hold reasonably large loads, but sometimes even built-in outputs may need to be supplemented with separate relays.

Relay output with galvanic isolation
Relay outputs with galvanically separated outputs mean that the output does not share any electrical level with the device if the output is not galvanically isolated; for example, ground or voltage level is shared with the transmitter itself.

If you have a 24 VDC alarm panel and want to control 230 VAC, then you need a relay output that is galvanically isolated (or a separate relay).

Analogue outputs

Analogue outputs are variable and can control a valve to the desired position. Analogue outputs are available in 0-20 mA current loop and 0-10 VDC.

Pulse output

Pulse outputs are used to generate a pulse in Hz. These are similar to the digital outputs in that they alternate between 0 and 1, oscillating significantly faster than a digital output.

The right choice?

Finally, how do you choose a suitable wired sensor? There are a few rules of thumb.

1. Whenever possible, use digital I/O, as it is cheap; actuators and transmitters have lower prices.
2. If you need to interpret more modes than just off and on, analogue I/O is an option.
3. If you need to monitor temperatures, use a special temperature sensor for Dallas One-Wire or PT1000.