## An interesting study in the effect of fixture on impedance measurement

A chap posted a pic and some mini VNA measurement results of a resistor which he reported has a DC resistance of 80Ω.

Above is part of the pic, focusing on the ‘fixture’. The chap reports that the VNA was OSL calibrated, and we might assume that was at the SMA(M) connector (it is difficult to explain the results if the reference plane was at the VNA jack). Continue reading An interesting study in the effect of fixture on impedance measurement

## A common scheme for narrow band match of an end fed high Z antenna – surely it is a 1:9 transformer?

A reader of A common scheme for narrow band match of an end fed high Z antenna commented:

…if the coil is tapped at 1/3, surely then the coil is a 1:3^2 or 1:9 transformer and the capacitor simply ‘tunes out’ the coil reactance, what is the input impedance when it has a 450+j0Ω load?

That is very easy to calculate in the existing Simsmith model.

Above, with load of 450+j0Ω, the input impedance at 50MHz is 8.78+j34.36Ω (VSWR(50)=8.4), nothing like 50+j0Ω. Continue reading A common scheme for narrow band match of an end fed high Z antenna – surely it is a 1:9 transformer?

## A common scheme for narrow band match of an end fed high Z antenna

This article discusses the kind of matching network in the following figure.

A common variant shows no capacitor… but for most loads, the capacitance is essential to its operation, even if it is incidental to the inductor or as often the case, supplied by the mounting arrangement of a vertical radiator tube to the mast. Continue reading A common scheme for narrow band match of an end fed high Z antenna

## Replacing the crystal in a Seiko V158-0AD0 watch

I have a Seiko V158-0AD0 which works very well, and I recently noticed the crystal was scratched more than the case… so a strong hint that it is an ordinary mineral glass crystal.

Searching the net, I could find that the Seiko part number for the original crystal is 310P03HN0 which indicates a diameter of 31.0mm but it does not have the thickness encoded simply. I could not find any articles giving the thickness of the part… so measurement was needed.

Above, the original crystal measures 1.6mm. A ‘standard’ 1.5mm sapphire crystal should be just fine. Continue reading Replacing the crystal in a Seiko V158-0AD0 watch

## IoT water tank telemetry project – ultrasonic sensor – HC-SR04

This is a new project derived from IoT water tank telemetry project – part 1 , but using an inexpensive ultrasonic ranging sensor to determine the height of water in a tank or dam, and so water depth or volume.

## IoT water tank telemetry project – part 4

After a long hiatus… the project activity resumes.

Boxing is underway, and the code has been revised to use a BME280 sensor replacing the DHT sensor. The BME280 is a better sensor (less jitter), and is also capable of barometric pressure if that is of interest.

Experience has been that 18650 cells do not last when exposed to extreme temperatures in solar powered modules like this, so a single cell pouch 2000mAh LiPo will be used (as in the battery trials).

The code has been refactored to make measurements before starting the WiFi as WiFi activity injects noise into the ESP8266 ADC.

The nodemcu core was updated as part of this process.

## Interpolation

The system is easy to calibrate in terms of stored water volume for containers with vertical sides where the change in volume with level is uniform… but there are lots of applications where that is not true, natural dams for example.

Above is a plot of volume versus depth of water for a container like a spherical bowl. The interpolation line is a cubic spline interpolation based on the six input data points, it is of course a very good fit to those points but would appear to be a good estimator of the regions between.

The code now contains a facility to perform a cubic spline interpolation of a small number of data points over the expected range of depths.

Entry{0.000,0.000}
Entry{1.000,313.112}
Entry{2.100,1375.744}
Entry{3.000,2799.159}
Entry{4.000,4959.528}
Entry{5.000,7723.082}


Above, a set of six points (as in the chart) in the format required of the code in a file named dam.xy.

The code calculates and stores an intermediate data file <name>.cstable if it does not exist for more rapid calculation. If the .xy file is updated, the corresponding .cstable file needs to be deleted.

The init.lua file is augmented to call a transform function from the httpreq function which customises the HTTP request.

function transform()
dofile("csinterp.lua")
if level<0 then level=0 end
volume=csinterpolate("dam",level)/1000
volume=string.format(meas_fmt,volume)
print("Volume: ",volume)
end

function httpreq()
--  req=rest_url.."?api_key="..apikey.."&field1="..level.."&field2="..temperature.."&field3="..humidity
transform()
req=rest_url.."?api_key="..apikey.."&field1="..volume.."&field2="..temperature.."&field3="..humidity
body=""
print("req:"..req.."\nbody:"..body)
return req,body
end


Above, the transform function fetches the function definitions for the cubic spline interpolation, and calculates an interpolated volume from the measured level (or depth) value.

Above is a trial run using a calibration generator for the level input, and its conversion to current volume of the storage.

## Hardware

One of the possible paths this project might follow was to use LoraWan… but experience with LoraWan has not been positive, particularly issues with the Laird gateway.

Above is the enclosure, an electrical box with transparent lid. The PV array is mounted inside the lid, the cheap Chinese ones tend to degrade too quickly when exposed to the elements. There is room in the box for a 2000mAh 1S Lipo battery. The switching circuit for the 24V DC-DC converter is mostly on the Veroboard under the (red) DC-DC converter (powers the 4-20mA loop). The module at lower left is a 1S battery management / protection board.

The resistor diagonally across the Wemos module is the 4-20mA sense resistor wired from A0 to ground. Experience is that locating the resistor on-board reduces the errors due to variable contact resistance through the module header pins.

If temperature / humidity monitoring is desired, a BME280 module plugs into the header near the antenna, but it needs to be remote from the box which gets quite hot in the direct sun.

The implementation supports easy development and update, if the battery and solar cell are disconnected, plugging a computer into the USB port on the Wemos board allows update and normal operation / testing.

## Reassembling a certain common chainsaw clutch

I managed to drop a chainsaw clutch and it separated into its component parts.

Thinking there would be a simple trick to this, I consulted Youtube. It turns out that reassembling these has proved a challenge for many people, and their online posts informed as to what approaches don’t work well, describing the many hours spent trying to reassemble. One online expert advised buy another, they are so cheap.

Above is a clutch of the type I will discuss, mine is from a 45-58cc Chinese chainsaw. There are variants, and the technique may be adaptable to them. Continue reading Reassembling a certain common chainsaw clutch