Time to get some experience with 32bit microcontrollers – some progress

At Time to get some experience with 32bit microcontrollers I wrote that after almost 50 years working with 8bit microcomputers and microcontrollers, I thought it about time to get my hands dirty on some 32bit microcontrollers.

This article reports some work in the STM stream.

I abondoned the ST Discovery board in favour of a low cost basic ST32F103 development board costing about $4 on eBay. The chip is a 32bit microcontroller clocked at 72MHz.

As a learning vehicle, I decided to implement the functionality contained in Arduino thermistor thermometer – a tutorial.

The development environment is Eclipse with the GNU ARM toolchain, a debugger probe, the basic development board, and a 1602 LCD display with I2C backpack and an inexpensive CP2102 USB RS232-TTL interface.

ST32F103-d01

Above is the working trial. The GDB debugger allows On Chip Debugging (OCD).
Continue reading Time to get some experience with 32bit microcontrollers – some progress

Fan controlled by humidity sensor – design technique

This article gives an outline of the process used in designing the Fan controlled by humidity sensor to use my Generic heating / cooling controller design (hcctl).

Arduino thermistor thermometer – a tutorial gave a method for designing a thermometer based on a formula predicting the behaviour of the sensor. This article explains a different approach where that is not possible.

HDS10-02

Above is a characteristic from the manufacturer’s data.

The curves to no lend themselves to simple curve fits, so a cubic spine interpolation will be made based on key points from the curve.

Four our purposes, the mean curve (green) is sufficient for design.

Screenshot - 24_07_16 , 08_16_07

 

Above, the mean curve was digitised to capture the shape of the curve, 17 points were used.
Continue reading Fan controlled by humidity sensor – design technique

Fan controlled by humidity sensor

I have a problem with machine tools getting condensation on them when conditions in the shed read dew point.

A possible solution being explored is to circulate air with a fan, possible inducting outside air, when humidity approaches condensing conditions.

A quick search reveals the HDS10 resistive humidity sensor for a dollar or so on eBay.

Above is the HDS10 humidity sensor.

Most low cost humidity sensors use a humidity dependent capacitive element, the HDS10 is different in being a humidity dependent capacitive element and is therefore simpler to use with microcontrollers with ADC input.

HDS10-02

The above graph is from the datasheet. It is intended primarily for sensing high humidity (dew point, condensing conditions) which suits this application.
Continue reading Fan controlled by humidity sensor

Review of inexpensive Chinese thermostat – STC-1000

This is a review of an inexpensive STC-1000 Chinese bang-bang  thermostat that was purchased on eBay for around A$12 complete with thermistor sensor and postage.

Above is the front view of the thermostat. There are many thermostats on the market with similar front panels, but they differ in internals and most importantly, performance and quality.

STC-1000-03

Above, the rating label is clear and informational, but it does not give the sensor parameters.  Continue reading Review of inexpensive Chinese thermostat – STC-1000

Can a hand held analyser be used to evaluate Insertion VSWR of a VSWR meter?

Some recent articles here used a two port analyser to evaluate Insertion VSWR of some coax switches, and it raises the question about application of a hand held analyser and Insertion VSWR of a VSWR meter.

(Duffy 2007) listed tests for evaluation of a VSWR meter:

Testing a VSWR meter

The tests here need to be interpreted in the context of whether the device under test (DUT) has only calibrated power scales, or a VSWR Set/Reflected mode of measurement, and whether directional coupler scales are identical for both directions.

  1. Connect a calibrated dummy load of the nominal impedance on the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be 1. (Checks nominal calibration impedance);
  2. Repeat Test 1 at a selection of test frequencies and for each test, without changing transmitter power, reverse the DUT and verify that repeat the forward/set and reflected readings swap, but are of the same amplitude (checks the symmetry / balance of the detectors under matched line conditions).
  3. Connect a s/c to the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be infinite. (Discloses averaging due to excessive sampler length);
  4. Connect an o/c to the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be infinite. (Discloses averaging due to excessive sampler length);
  5. Connect a calibrated wattmeter / dummy load of the nominal impedance on the instrument output and measure calibration accuracy of power / ρ / VSWR scales at a range of power levels in both forward and reflected directions (Checks scale shape and absolute power calibration accuracy).
  6. Repeating Test 1 additionally with a calibrated VSWR meter connected to the input to the DUT, and measure the VSWR caused by the DUT at a range of test frequencies (Checks Insertion VSWR).

It is not unusual for low grade instruments to pass Test 1, but to fail Test 6 (and some others, especially Test 3 and Test 4) towards the higher end of their specified frequency range.

Item 6 in the list was to evaluate the Insertion VSWR. Continue reading Can a hand held analyser be used to evaluate Insertion VSWR of a VSWR meter?

U-BLOX LEA-6T GPS module – for experiments

This article documents a LEA-6T module build for general experiments.

The LEA-6T is an inexpensive GPS module (~$40 at time of purchase, but getting cheaper) that can supply raw pseudo range data.

LEA-6T02

The module above is supplied for use on UAVs of various kinds, and came complete with a plastic radome and cables to suit an APM copter. The module also contains a 3D compass (magnetometer) which is not used here.

LEA-6T03

Above is the internals of the module with a custom cable to pick up just the RS232-TTL signals from the GPS (and supply 5V). The connector is a 8pin Hirose DF13. Continue reading U-BLOX LEA-6T GPS module – for experiments

Coax switches – is shorting unused port necessary for isolation?

At Ratings of coax antenna switches I showed characteristics of a home made switch which has very low InsertionVSWR, but poor isolation.

A couple of correspondents have offered an explanation that the unused port must be shorted to get good isolation.

If that was the case, then we would expect all coax switches that leave the unused port open to have poor isolation.

Let us look at a very good coaxial relay

 

Above is a Dowkey 402 series relay which has good performance to GHz. It does not short the unused port.
Continue reading Coax switches – is shorting unused port necessary for isolation?

Ratings of coax antenna switches

In a recent article I discussed how InsertionLoss implies InsertionVSWR in lossless devices.

This article looks at measurements of a few antenna switches at hand.

Daiwa CS-201G II

It is difficult to find comprehensive data on the very popular Daiwa CS-201 series switches.

CS201spec

Above is the data from the packet of one of these switches, a CS-201G II. The specifications are pretty loose, and one must depend on one’s own measurements.

CS201N01

Above, the CS-201G II, a basic CS-201 series switch with N connectors, advertised as useful to 2000MHz where InsertionLoss is given as 1.2dB (or better?). If there were no TransmissionLoss in the switch, that would imply InsertionVSWR=3.6, but there is probably some significant TransmissionLoss and InsertionVSWR would be somewhat less. Nevertheless, IMHO InsertionLoss=1.2dB indicates it as unsuitable such frequencies. Continue reading Ratings of coax antenna switches

InsertionLoss implies InsertionVSWR in lossless devices

Devices inserted in transmission lines often characterised by one or more of:

  1. Insertion VSWR (the input VSWR when terminated with a matched load);
  2. Return Loss (RL) in dB (20 times the log of the magnitude of the complex reflection coefficient); and
  3. Insertion Loss.

Practitioners often find Insertion VSWR (1) of most use as it indicates whether the device is worse than other system devices, the weak link in the chain if you like. You might see a coax antenna switch specified to have InsertionVSWR<1.2 to 60MHz.

Return Loss (2) is a function of VSWR and vice versa, so it appeals when the designer thinks in terms of Return Loss rather than VSWR (and it is a better metric for VSWR<1.2). You might see a coaxial relay specified to have ReturnLoss>30dB to 500MHz.

Insertion Loss (3) is not so readily compared to the other two which are measures of input reflection with a matched termination. It often yields some numbers that appear very acceptable, but might be deceptively so. You might see a coaxial relay specified to have ReturnLoss>30dB to 500MHz. You might see a coax antenna switch specified to have InsertionLoss<0.2dB to 100MHz. Continue reading InsertionLoss implies InsertionVSWR in lossless devices

Atten APS3005S – a better thermostatic fan control

The Atten APS3005S is a 0-30V 5A linear DC power supply.

Atten-hcctl00

This later model includes a thermostatically controlled fan which at moderately light currents short cycles (10s on 20s off) and is very annoying… especially since it sits above my desk.

This project describes application of the generic heating / cooling controller (hcctl) to control the fan, reducing the short cycling nuisance.
Continue reading Atten APS3005S – a better thermostatic fan control