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:
- probe in still air whose temperature is captured with an accurate thermometer; and
- 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.
- 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.
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
A recent purchase of an inexpensive ($6) speaker polarity tester prompted a need for a stand alone driver for speakers.
Above, the tester has a microphone that senses the polarity of the pressure wave and indicates with one of two LEDs.
The tester comes with a CD containing a file that can be used to provide the test signal on a complete system with CD player, but there is a need for a stand alone driver for testing bare speakers or speaker units. Continue reading Speaker tick generator (for polarity testing)
Two bare dimmer modules sold on eBay with identical specification and similar price are compared.
Both claim to have zero hysteresis.
Zero hints a lie!
Hysteresis is caused in simple phase control dimmer circuits at low settings because in each half cycle the trigger capacitor starts at a different voltage depending on whether the diac fired on the previous half cycle.
A serious issue with this snap-on effect is that if power is turned off at low power setting and re-applied, the controller may not switch on.
Above is type 1, a very triac basic phase control circuit. The red capacitor and resistor to its left are snubber components, the yellow capacitor, 4.7kΩ resistor to its left and the 500k pot are the phase delay circuit, the diac is just visible above the red capacitor. Continue reading A comparo of two bare light dimmer modules
This article describes a setup for derusting small steel components, mainly machine tool accessories, using a Molasses solution.
A 10% Molasses solution can be an effective way to derust steel. Feed grade Molasses costs about $2/kg at the local rural store.
The process is bacterial and activity depends on temperature. Experimentation suggests that optimal temperature is 30-35°, and derusting can be achieved in a few days at that temperature (subject to the degree of rust). At lower temperatures, the process may take many weeks. The nice thing compared to electrolytic derusting is that work is unlikely to be damaged by the process.
Above, the rust treatment system comprises:
- Bain marie stainless 1/3 module 200mm deep with lid;
- 1000W electric cooker;
- 230VAC thermostat with thermistor probe immersed in the process liquid;
- 230VAC dimmer to reduce the power of the cooker element.
Above is an internal view of the thermostat made from a Chinese 230VAC thermostat, a 3m extension cord and ABS plastic box. Continue reading Molasses derusting of steel
The WBT-4000W is a triac dimmer selling for upwards of $25 on eBay.
AC 0V-220V continuously adjustable, zero hysteresis, zero latency, superior heat dissipation.
The question is, does it deliver these things? Continue reading WBT-4000W 230V AC dimmer / motor speed controller
This article expands on A flexible test panel for microcontroller based power control projects with some enhancements and accessories.
A LED power meter that I had ordered finally arrived (slow boat from China syndrome).
Above, the upper rail contains a RCD, the power meter which displays Volts, Amps, and kW, or pf, hours, and kWh, a DIN mount terminal block for mains, and a 40A SSR on a heatsink. A clip on CT can be used for oscilloscope observation of mains current. Continue reading A flexible test panel for microcontroller based power control projects – #2
I do a lot of experiments with microcontrollers switching mains powered equipment, and the test beds have always been improvised. It has always been my intention to formalise something for convenience but mainly for better safety.
The article describes a test panel to fill that need.
The panel is constructed on a piece of 3mm aluminium sheet, drilled and tapped to take two sections of 35mm DIN rail for flexible mounting of accessories.
Above is a pic of the test panel in use to test the generic heating / cooling controller (hcctl), a flexible bang-bang controller based on an ATTiny25. Continue reading A flexible test panel for microcontroller based power control projects
Review: magnetic stirrer with heating plate and digital display XB 85-2 documented problems that prevented the device being very useful.
Attempts to tune the supplied PID controller above were frustrated by a lack of meaningful documentation supplied or found in searches on the ‘net, and the fact that the display is sometimes faked to appear that the temperature has stabilised. With any non-zero I term, it behaved badly and some observations suggest that it suffers from integral windup. It is truly a piece of Chinese junk and unusable.
Above is an independent logger capture of the temperature from switch on. There is a large overshoot, and then, no matter what the settings, it oscillates and the lowest amplitude obtained was 1°pp (above). The overshoot is almost as much as observed in manual warm up when power is cut at 40°. Continue reading Fixes #1: magnetic stirrer with heating plate and digital display XB 85-2
I recently had need to attach four wires to a set of pads on a device for programming its microcontroller. The pads for these sort of things are often on difficult to solve pitch, this one is 2mm which is not too bad.
Above is the target and solution.
The target is the four vias right next to the LEDs on the daughter module. Continue reading A little programming adapter for 4 x 2mm pitch pads