Tahitian Lime Marmalade

We had a small excess of Tahitian Limes ripen, so I tried a test batch of Tahitian Lime Marmalade.


The marmalade is light coloured, sweet and tangy.

The downside is that the fruit is full of seeds, not just in the equatorial plane but distributed right through it, so scooping seeds out is quite a task.

Screenshot - 26_07_16 , 10_37_42


It is the first time I have made jam from these, and essentially I used the 40:60 mix from FAO’s Generic Jam Recipe, though being limes no acid was needed, in fact about 6g of Sodium Bicarbonate per kg of fruit to achieve the ideal pH of 3.2-3.3. The limes were cooked to release some Pectin, but a little Pectin 10g/kg) was added as the cooked fruit gave a slightly weak reaction in Methylated Spirits.

Endpoint was assessed by measuring Brix using a refractometer, confirmed by setting a drop of jam on a cold plate.

InsertionVSWR of Grebenkemper’s Tandem Match

The findings at InsertionVSWR of Revex W560 on HF and the suggestion that the low frequency problem is characteristic of poorly designed Sontheimer couplers (Sontheimer, C & Frederick 1966).

These couplers were popularised by (Grebenkemper 1987)  in his Tandem Match – An Accurate Directional Wattmeter and have appeared in ARRL handbooks over the decades, and may have inspired the many commercial implementations of the coupler.

Grebenkemper claims his meter is ‘good’ down to 1.8MHz, but does not clearly claim any particular InsertionVSWR. There is limited value in an instrument that can measure down to 1.05 when it causes significantly higher VSWR itself.

Screenshot - 24_07_16 , 15_31_41

Lets drill down on Gebenkember’s article, specifically the coupler design.
Continue reading InsertionVSWR of Grebenkemper’s Tandem Match

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.


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

KG-UV920P repair / modification for driver FET failure

The KG-UV920P is infamous for failure of the driver FET, they run excessively hot and clearly outside of safe operating limits.

I repaired one for a friend some years ago, and the dealings with Wouxun were enlightening. If I had little confidence in them before that experience, after it I would not give consideration to purchase of any Wouxun radio.

My repair / modification notes have been copied literally from the old VK1OD.net webside, and may contain stale links etc, but if it is of use to hams with a broken radio, see KG-UV920P – a repair / support story.

I have seen lots of articles on this problem over the years since, including ones that try to add a heatsink on top of the driver FET. The driver FET is meant to lose its heat through the bottom metal pad, and heatsinking the plastic encapsulation will not be very effective. Bottom line is to reduce the operating voltage on the driver (as per the factory advice), and keep the radio cool.

Don’t operate the radio sitting on the desk or the like, the bottom is the heatsink.

Wouxun are not alone in manufacturing radios that run red hot, see my notes on supplementary cooling for an IC-220H: https://owenduffy.net/blog/?s=IC2200H+cooling.


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.


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

Thinking about vibration forces from a bent prop adapter

A recent crash trying to tame the update from CF v1.10 to v1.12 did some damage to two prop adapters, bending them just above the lower flange causing axial runout. The weak point in these adapters is at the transition from the hollow tapered lower section to the solid threaded upper section.

PropAdapterRunout01Above is measurement of the runout on the lower part of the spinner with a propeller hub mounted. Total Indicated Runout (TIR) was measured at almost 0.5mm, lets call it 0.5mm, and runout from centre is therefore 0.25mm. Continue reading Thinking about vibration forces from a bent prop adapter

Cleanflight and PPM

The first widely used ‘digital’ communications protocol used with Remote Control (RC) models was PWM.

Though PWM is a generic term, the accepted timing was that a pulse width of 1000µs to 2000µs conveyed 0-100% of the proportional control. The the on-off nature of the PWM signal was discrete (digital), the duration of the pulse was stepless (ie not discrete), and since there was no shared time reference, the receiver’s interpretation of the signal may have error.

So, for a multichannel system, there would be one ‘signal’ wire for each channel, which gets to be a nuisance for lots of channels.

PPM (aka PPMSUM, CPPM) was an early protocol to multiplex multiple channels on one signal wire. The initial protocol description was of a frame comprising 500µs SPACE (S) followed by 500µs – 1500µs MARK (M) carrying the channel information as in the RC PWM protocol, then the same for the next channel and so on. These frames were repeated at a fixed frame repetition rate (FRR) of around 50Hz. Early implementations ‘defined’ this total frame duration variably, eg 18ms, 22.5ms, 28ms were popular. Continue reading Cleanflight and PPM

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.


Above, the rating label is clear and informational, but it does not give the sensor parameters.  It is rated at 10A for resistive loads (AC-1), but it is doubtful that AC-3 motor loads of more than 20-30% of that should be used. Continue reading Review of inexpensive Chinese thermostat – STC-1000