This blog article is about the design process of designing an outdoor wireless Intercom.
Background To Project
An existing industrial manufacturing client emailed me to ask if I could ‘programme up’ a couple walkie talkies.
The customer needed to start locking their store room when unattended, due to workers helping themselves to supplies.
They wanted to mount a couple walkie-talkie radios on the wall, so that workers could call for the store to be unlocked.
The client wanted one radio to be mounted directly outside the storeroom, and the second outside the building.
They thought that perhaps the radios could be mounted in some sort of external case, to protect them.
This is especially important for the radio mounted outside the building, due to rain and snow.
The potential problem with mounting expensive handheld two-way radios outside, is also theft.
The clients site is on a secluded industrial estate, and the entrance to the car park, and hence the exterior of the building is open.
After clarifying with the client as to exactly them wanted, I sent them the rough idea for a radio linked Intercom.
The photo above shows the rough initial idea for a wireless outdoor intercom.
Luckily it was exactly what the customer was looking for.
So now I knew what they client wanted, all I needed to do now is figure out how to make it work.
RF Electronics Options
The design brief from the client, requires the intercom to be able to call the portable digital two-way radios that the factory managers have.
The purpose is so that they can come and unlock the storeroom, or unlock the outside door (both of which are now locked).
After a personal brainstorming session, I came up with the following options.
- Bluetooth link, with audio capabilities.
- DECT communication technology, like cordless phone.
- Licenced PMR Digital Radio.
- Unlicenced PMR446 Radio (analogue or digital)
- Audio over Wifi
Once I had come up with the initial list of possible ways to link the intercom with the existing two-way radios, it was time to evaluate.
Firstly I considered Bluetooth.
Bluetooth was introduced in 1994, and is currently up to version 5.
In addition to what is now known as Bluetooth ‘Classic’, there is now also ‘Low Energy’, and ‘Mesh’.
As the names suggest, ‘Classic’ is an updated version of the original Bluetooth.
‘Low Energy’ is designed to use less current from its power supply.
This makes it suitable for the Internet Of Things, as enabled sensors can last for years on same battery.
Product like Smart Watches use Bluetooth Low Energy, or BLE as it is commonly known.
Bluetooth Mesh allows data to ‘flow’ through multiple ‘nodes’ en route to their destination.
This enables data to travel longer distances than would otherwise be possible using ordinary Bluetooth.
Mesh technology is great for controlling projects like Smart Lighting, but is not needed for our simple intercom design requirements.
As you hopefully have now appreciated, there are different types of Bluetooth for different purposes.
Bluetooth was originally designed as a technology to wirelessly replace RS232 type Serial communications cables.
It has also developed into a technology capable of transmitting audio.
Bluetooth modules capable of audio, have a Digital Signal Processor (DSP) included in their design.
Positives of using Bluetooth for the Intercom
The intercom has the following design requirements:
- To allow instant wireless voice communication at the push of a call button.
- Be capable of being powered by battery, with long battery life.
Bluetooth audio could provide the communication link between the intercom and the two-way radio.
It uses fairly low power consumption.
Could also be made to work with app on mobile phone, as all smartphones now have Bluetooth built in.
Disadvantages
Relatively short range, which might be an issue, if the receiving Bluetooth module of the two-way radio, is too distant.
Time delay to establish the connection, unless left connected (which has power consumption implications).
At the time of writing (27th November 2019) , I am still researching Bluetooth technology in more detail, so I might still use it for the design.
DECT RF Technology
The next RF (radio frequency) technology that I considered for the intercom design, was DECT.
DECT is short for Digital Enhanced Cordless Telecommunications.
Sometimes you may also see it called Digital European Cordless Telecommunications, as the technology originated in Europe.
DECT has been adopted worldwide, and is most commonly used in cordless phones.
However I considered using DECT to provide the wireless communications link between the intercom and the two-way radios.
Advantages
DECT provides clear two-way audio communication.
DECT operates at 1900 Mhz (1.9Ghz) which has the advantage over Bluetooth & Wifi, which operate at 2.4Ghz (2400 Mhz).
1900 Mhz is an advantage because it is a less crowded frequency, and therefore less subject to potential interference from other users.
Disadvantages
There is less choice in DECT modules available, compared with technologies such as Bluetooth.
The modules for DECT enablement of the wireless outdoor intercom also seem to be more expensive than Bluetooth.
Licenced PMR Digital Radio
PMR stands for Private Mobile Radio.
…..more information on the design project coming soon. Come back regularly.
Craig