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stuartChilds

October 19, 2015 15:10

Hands-on with Cherry Energy Harvesting wireless switches

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A look at the Evaluation Kit and putting the technology to the test

Energy is often wasted as a by-product of the primary function of a machine or process. For example, a combustion engine wastes lots of energy as heat and sound, whilst television and radio broadcasting generates lots of electromagnetic energy that never gets received.

Energy harvesting allows otherwise wasted energy to be put to use. Familiar examples include solar cells and wind turbines, and the electronics industry is increasingly interested in this field for applications such as wearables, portable device recharging and autonomous sensor networks.

In this post we will take a look at Cherry's Energy Harvesting Evaluation Kit, that contains several interesting components, notably their energy harvesting RF switches and receiver module.

What's in the Box

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Included in the evaluation kit are:

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Let's look at the Energy Harvesting Generator first. This contains a patented mechanism that converts kinetic energy into electrical energy in the form of a voltage pulse. It works both when the lever is pressed and released. When built into a switch unit it harvests some of the energy used to actuate the switch. This can then be used to power an RF transmitter and send a signal to the receiver.

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Image source: Cherry website

It is important to remember that the switches in this range are not just passive components. There is a lot more to them than a simple set of contacts. Cherry provide more detailed information on their website. Registering your Evaluation Kit also provides access to further technical resources.

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Enclosed in reasonably compact IP40-rated housings, each switch includes an Energy Harvesting Generator and a wireless transmitter. Each switch has a unique ID (UID) allowing you to build scalable systems with multiple switches.

The units included in our Evaluation Kit operate at 868MHz, with the antenna protruding from the housing. The switches are also available in 915MHz and 2.4GHz options.

Wireless communication is done via a proprietary protocol, though there is mention in the documentation of the possibility of other protocols; either customer-specific or RF-standards such as KNX-RF, ZigBee and Bluetooth Low Energy.

Despite not being rated for safety-critical applications, the default RF protocol sends multiple messages at a time, with a delay between to help ensure robust communication.

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The receiver module can be powered via the input terminals or simply over USB. It includes a relay switched set of terminals and two status LEDs for quick and simple operation – though a standalone receiver board is also available for those wishing to integrate their own solution.

It can also be connected to a PC running Windows and Cherry's Radio Monitor software. This allows for on-screen identification of switches paired to the receiver module, checking of signal strength and other useful data.

Data is also output via the RJ45 port on the receiver module. The AFZE-1003 model included in our kit uses the RS232 TTL specification, though other models with different output are available: full RS232, RS485 and SPI.

Simple but effective

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Pairing the switches with the receiver unit could not be much simpler. Following the step-by-step instructions we had our two demo switches working within minutes. The receiver was connected to a PC and we had real-time display of switch information and signal strength.

This is a powerful platform for determining the suitability of these switches for your project; it is possible to position the switches and receiver module in different locations and monitor the performance with minimal outlay or build time.

Going the Distance

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Though Cherry provide information regarding maximum RF range for the switch modules, we were keen to see how well they performed in our workshop environment. Just how far could RF signals be sent when powered solely by the press of a switch?

The receiver module was connected to a laptop via USB cable. Both were placed at one end of our workshop, and positions marked at metre intervals up to the farthest we could go – up to 16 metres away with direct line of sight. The laptop display was used for visual feedback that the system was working.

We soon ran out of workshop, and the system worked the whole way. This is well within the specification of 30m indoor transmission, but it is nice to test the real-world performance.

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Each time the switch is pressed, data is logged and displayed within the Radio Monitor software. This can later be exported as TAB separated values in a text file – easy to work with later, in spreadsheet software or similar.

Looking at the data quickly plotted we can see there are no real surprises. Note that this was not a particularly scientific test, rather a demonstration of ease of signal logging with the Radio Monitor.

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Curious to see how far the signals would carry, we decided to take one of the switch modules outside. Since this involved going so far that viewing the laptop display is not viable, a bright LED and power was connected to the relay on board the receiver module.

With this connected and tested, the receiver was placed in a window. I ventured out into the grey Yorkshire drizzle and, watching the LED in the window, went for a walk. I made it to the far end of the car park with the unit still functioning, so I had to leave the industrial estate! Once I had lost visual contact of the LED and receiver module – approximately 100m or so away – I headed back.

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The above graph hows the signal strength in dBm received during the outdoor test, and number of messages received each time. We can see that a normal transmission contains three messages – for robustness as mentioned earlier. We can also see that all 3 messages were consistently received down to -80dBm, and still often at the lower end of signal strength, just less than -90dBm.

A powerful switching solution

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Cherry's Evaluation Kit provides quick access to their innovative range of Energy Harvesting switches and compatible receiver, meaning less time wasted prototyping and determining suitability of this technology for your needs.

Further comprehensive documentation is available to help integrate the receiver module into a wider system, making the transition from evaluation kit to functional system smooth and quick to implement.

With standalone operation, unique switch IDs and reliable wireless communication, this technology will doubtless prove essential for industrial automation in challenging environments. Who knew that simply pressing a switch could be so exciting?

stuartChilds

maker, hacker, doer