Bart Hiddink of Ideetron was asked by FHI to write an article with more technical depth about the components on the Proximo, the WoTS gadget of 2018. How did the engineers of Ideetron come up with the components / hardware engineering and what were the determining factors in choices? Here is his description.
“Let me start with the food. The choice was made to use the Proximo (schematic representation (PDF)) to run on two large 3V button cells. The advantage of these batteries is that they are reasonably available, they are cheap and have a high energy density. The disadvantage of this type of battery is that they have a fairly high internal resistance, which means that the terminal voltage of 3V is not guaranteed under a varying load. Non-rechargeable batteries were chosen because of the costs.”
“The power supply also consists of a number of diodes that are used as switches. This makes it possible to power the Proximo externally without having to remove the batteries. When using it continuously on an external power supply, we recommend not to insert the batteries, to prevent leakage. The rest of the power supply circuit consists of a DC-DC boost converter around the TPS61025 from Ti (U1). This converter is able to produce a stable 3.3V DC voltage from a battery with a voltage between ~1.0V and 3.6V. By switching on the regulator, a stable voltage is created on the Proximo, allowing all electronics to work reliably, even if the batteries are almost empty.”
“The boost converter can also be switched off and then the Proximo runs directly on the batteries. Of course, this is only possible if the voltage of the batteries is still more than 2V. The boost converter is switched off as soon as the Proximo goes 'into sleep', so that the current drawn from the batteries is only a few micro amperes. In this mode, the batteries last for years. The smart power supply meets all the requirements for the Proximo and consists of a concept that has been tried and tested by us.”
“Finally, a transistor is also drawn on the power sheet in the schematic. This transistor can be used to switch a relay.”
Beating heart
“The Nordic sheet contains the Nordic reference design around the BLE 5.0, nRF52832 chip (U2). The chip has a cortex M4 processor and 512kByte Flash on board. The firmware for the operation of the Proximo runs in the chip. The chip is therefore the beating heart, so to speak. All connections are made to the nRF52832 with the rest of the components on the Proximo. A striking part in the diagram is the antenna. This is from Proant and is approximately adjusted to 2450 MHz, the frequency at which Bluetooth works. Because the antenna is not precisely tuned, the impedance will deviate slightly from the ideal value of 50 Ohm. To compensate for this, three components have been placed, which together form a pi filter; L4, C17 and C18.”
“By optimally matching the chip and the antenna, the best RF performance is guaranteed. This chip was chosen because it was the best choice for the design and we are familiar with this chip from Ideetron bv.”
“The SK6812 (3535) from OPSCO was chosen for the LEDs. This LED is also sold under the name 'Neopixel'. It is a complete circuit consisting of 3 LEDs and a driver IC. The LED can display the colours green, red and blue and with an intensity between 0 and 255. This makes it possible to create almost all the colours of the rainbow. The efficiency of the LEDs is very high and during normal use the LEDs are set very economically. The last trick of the LED is the 1-wire serial connection. With the help of smart timing it is indicated which LED and which colour in this LED is controlled. This makes it possible to create very long strings of LEDs with many nice possibilities. Just Google 'neopixel' on YouTube and you will see what I mean. This LED costs about 10 cents so the choice was quickly made.”
Buzzer
“Finally, there are a number of sensors and a signal generator on the Proximo. The buzzer is at the bottom left of the diagram. This is used for acoustic feedback for pressing the buttons and for giving an alarm. The buttons are at the top right of the diagram. Below the push buttons is the TH06 sensor from HopeRF. In fact, this is a Si7021 from Silabs on a small PCB. The chip contains a temperature and humidity sensor. Nice for building an environmental sensor.”
“The motion sensor is located between the buzzer and the TH06. This sensor can be used to detect movements of the Proximo. This makes it possible to create smart bicycle lighting, for example. The sensor detects movements and the LEDs provide light. We have been using the sensor for years in all kinds of projects where we have to 'wake up' the circuit on movement and where power consumption really has to be minimal. For example, an accelerometer or gyroscope still uses more than 10 microamps of standby power for this function. This sensor does it for less than 1 microamp. The light sensor is located at the top left of the diagram. This is also useful for smart bicycle lighting.”
“Together, that forms the Proximo. I hope I have given you some more insight into the Proximo and the motivation to use these components. I wish you much pleasure with your own Proximo.”
Bart Hiddink, CEO, Ideetron bv, Doorn, August 2018.
Nederlands