Friday 17 June 2011

That's just swell

A few otherwise idle moments were used fitting the swell / expression pedal from the old Hammond Organ Donor into my virtual organ project (Blogs passim).

Here's the swell pedal, complete with a nice patina of rust...

I put out a cri-de-coeur a few weeks back in this blog and on the Organ Forum, asking for any information on Hammond Swell pedals, but attracted a null response. Fortunately a new Texan friend gave me some advice and put me on the right track.

The pedal has a light source (an ordinary incandescent bulb) and a light-dependant resistor. There is a tapered shutter (implemented on a piece of fibreglass PCB) which moves with the pedal and stops down the light reaching the LDR. The key components are visible here...

I tried to measure the resistance of the unit as a function of pedal angle - with mixed results. Here's a graph, which reveals an approximately logarithmic taper (as you might expect, given that the pedal originally implemented an audio volume control function). Unfortunately, the resistance went out of range of the meter I used as I got close to the 0 degrees (i.e. fully depressed) position. Still, the graph has taught us its lesson...

I originally had planned to connect the pedal to one of the analog inputs implemented on the main controller - but it occurred to me that I could interface to the separate pedal controller instead.

This had the advantage of physical proximity to the swell pedal, making a completely stand-alone pedal unit. The disadvantage was the fact that the pedal controller had, to date, no existing analog interface. What's more, the only available I/O line on the PIC 16F676 was not one of those that could be set up as an analog input. Accordingly, I had to make a complete re-work of the pedal controller. This gave me chance to add a stabilized power supply for the swell pedal's bulb (which draws about 180 mA at the 5V operating point I've chosen).

Here's a schematic of the revised pedal controller...

and here's the finished system (with the bulb regulator sporting an old TO3 heatsink from the junkbox)...

You can see the additional connectors to provide power to the bulb and to sense the pedal resistance.

It works very well - but the logarithmic taper makes the expression control (too) non-linear. I need to implement a look-up table in the software to undo the log taper - but that will have to wait until the next otherwise idle moment!

...-.- de m0xpd

Sunday 5 June 2011

SSDRA, another Dipper, and Red Roses

EUREKA - I have found a copy of Solid State Design for the Radio Amateur - for just a pound!

I spent an hour at West Manchester's Annual "Red Rose" QRP Rally today - which is where I made my purchase of the classic by Hayward and DeMaw. It is a paperback edition and it is a little tatty - but complete. A genuine bargain!

On the same stall, I spotted another Dipper, which I snapped up (this time for four pounds), thinking I might use the coils with the Heathkit unit I won in a WARC raffle a few weeks back.

However, on testing the device, I found it very sensitive, beating both my g3wpo and my GD-1u raffle win, so I shalln't be stealing away the coils.

Here's the new dipper...

It is powered by a PP3, has one active device (a four-pin metal can carrying the (partial) number 2089 - or is it 7089) which I don't recognise and is either an excellent piece of homebrew or a nicely executed kit. There are no identifying marks of any kind - so if you can tell me anything about it, I'd love to know!

I replenished stocks of some boring items from Will at Bowood, stopped by at the Kanga, RSGB and (of course) G-QRP stalls and took my leave of this excellent rally. I will certainly return next year and encourage all within striking distance to do the same.

...-.- de m0xpd

Saturday 4 June 2011


Just back from a(nother) sailing trip on the Westerly Discus CC sloop "Beatrice Rose", with owner Neil and fellow crew member Paul...

This voyage took us from Beatrice Rose's home port of Southsea Marina to Lymington, Poole, Portland, Yarmouth, Newtown Creek, Folly Reach (R. Medina) and back to Southsea. We logged 195 miles, 95% of which was sailing close-hauled to windward.

The opening weekend saw foul weather (in which an 11m yacht was lost), some of which we sat out for a day in Lymington after becoming bored beating into a Force 7 through the Solent. During that enforced rest, I took out the FT817 in the hope of making some CW QSOs as m0xpd/mm, but discovered that Berthon, Lymington (in common with most other marinas) is QRM hell (there are lots of noisy inverters etc on boats) and 40m was s9 +10dB of solid, ugly noise.

VERY unusually, I had also packed the FT817's microphone, so I switched to 2 metres and answered a shout from Rob, g6bdv. Rob was on a cliff-top above the Needles (the conspicuous rocks at the western tip of the Isle of Wight) and reported 50 miles-an-hour wind gusts at that exposed location. Here's a photo of Rob's location which I took a few days later as we passed on our (engine assisted) run east from Portland - in rather calmer conditions!

We shared Poole Quay with a pretty brace of tall ships: the Sea Cadets' Training Ship "Royalist"...

and another brig, "Øilleleje"...

Later, after a night in the new marina associated with next year's Olympic sailing events at Portland and a visit to Yarmouth, which was appropriately dressed for the visit of the Old Gaffers this weekend...

we spent a night on a buoy in the idyllic Newtown Creek (here seen with shoal draft boats moored close to the "Conspicuous Black Boathouse")...

At last, I had escaped the buzz and drone of the boaties' QRM and could enjoy some real (i.e. CW) QSOs - most notably with Fred, dl4zby, nr Frankfurt.

A terrific trip - which landed me back ashore in time for tomorrow's Red Rose QRP Festival. Thanks, Neil!

...-.- de m0xpd(/mm)