Category: IoT

Calibration of the 4-20mA input

This article is a tutorial on calibrating the 4-20mA input which is designed for flexibility that is achieved through exploitation of the calibration.

The input device for this tutorial is a Pt100 RTD temperature sensor and inexpensive Chinese Pt100 – 4-20mA converter (loosely) calibrated for -50-150°. The Pt100, the converter, the load resistor, the divider resistors on the MCU board, and the MCu voltage reference all introduce error which is compensated in this end to end calibration procedure.

For this demonstration, two scenarios are measured:

1. probe in still air whose temperature is captured with an accurate thermometer; and
2. probe in boiling water whose temperature is calculated from known altitude and barometric pressure.

Another option would be to use a container of water filled with ice to obtain close to 0° for scenario 1… you don’t need a triple point cell for the end system stability and accuracy.

Temperature of boiling water

Using Calculate cooking time for soft boiled egg :

No, we are not boiling an egg, but the results include the temperature of the boiling water under current altitude and pressure. Continue reading IoT water tank telemetry project – part 2

This is the first in a series of articles describing a simple maker / DIY IoT water tank telemetry system.

Design criteria

• capture water depth, temperature and relative humidity;
• IoT connectivity;
• solar / battery powered;
• wireless connection;
• use existing inexpensive electronic modules.

Design choices made initially:

• 4-20mA water pressure sensor for depth measurement;
• ESP8266 Wemos D1 mini pro for the MCU and wireless elements;
• NodeMCU / Lua software environment;
• external antenna for improved WiFi range;
• 6V 100mA PV array;
• module with TP4056 batter charger and cell protection chip;
• 2500mAh 18650 cell;
• AM2320 temperature and humidity sensor;
• bipolar transistor switch for boost converter;
• Thingspeak RESTful interface for data accumulation and presentation.

Block diagram

Above is a block diagram showing the major system components. Almost all of the electronics is on easily obtained low cost electronic modules source from eBay, assembled on a Veroboard backplane. Continue reading IoT water tank telemetry project – part 1

I have some IoT projects that would benefit from range afforded by a better antenna than the on-board antennas in most ESP8266 modules.

The Wemos ProMini has an on-board IPX socket for an external antenna so it is a candidate. Note that a 0R 0603 resistor needs to be removed and another or a wire link soldered in to route the RF to the IPX socket.

Above the Wemos ProMini with a 7dBi SMA-RP antenna ($1.80) and flylead SMA-R(F) to IPX (M) ($1.00).  Continue reading High gain external antenna for Wemos ProMini

After scouring eBay for a packaged esp8266 with 220V 10A relay, two products were identified:

• Yunshan WiFi relay; and
• LC Technology relay.

As is usually the case, finding a schematic and specifications is very difficult and the sellers were of no help (no surprises).

The LC Technology device was offered with indistinct pics that hinted it had a 8Mb flash chip, ESP8266EX processor, and a STC 15F104 8 bit processor on board for some unidentified purpose.

A schematic was eventually located for the Yunshan board, and from pics it appeared to have a 12E module on it which hinted the flash size.

A Yunshan module was purchased for about $10 posted, and it was indeed a 12E with flash-id 4016, so 4MB of flash memory.

The board does not incorporate a USB-TTL adapter which is a nuisance not just requiring an external adapter for programming, but there is no integration of the RTS and DTR signals as in the NodeMCU devkit. Adding a quality USB adapter (eg CP2102) would not increase the price a lot, you can keep the CH340G etc). Continue reading ESP8266 relay module review – Yunshan WiFi relay