A new 90-line Mirror Screw

[Panrock]

Thanks Steve A.

Now, on to the laser module (used for setting up the slat angles):

https://www.ebay.co.uk/itm/375760988959

You'll see it requires "3 to 6 volts" for powering. When I set up the slat angles, I'll be feeding this off a decent bench power supply, which has variable voltage and variable current limiting. However, sad past experience with laser modules has taught me they can be very intolerant of switch-on-or-off power rail spikes, even the briefest. They can just 'die' with not even a puff of smoke to mark their departure.

Now I'll be switching this thing on and off a lot. I am therefore asking for advice as to what you think I should connect as protection across the 3 - 6v power input. Zeners and small capacitors come to mind, but will they be quick enough? Would some sort of soft start be advisable?

Steve O

[Steve Anderson]

Unfortunately there's no mention of supply current required...or I simply missed it. I would have thought that a series resistor dropping a couple of volts plus a cap across the lasers' power leads should be adequate. A series/parallel arrangement. Perhaps adding a suitable voltage Zener? It can't harm can it? Do you really need to keep switching it on and off?

Steve A.

The only time I've used a laser-diode device was in a NBTV opto-link thingy some years ago. The diode was PWM modulated at many kHz, I can't recall the frequency, probably well over 20kHz, no failures. Let's hope this device is just as robust!

[Panrock]

It states 40mA or less. I'll use a suitable R to drop 6v to 4v and a Zener and a couple of C's. I also have a 1N5817 Schottky diode lying around, which I'll include for good measure (as a rapid spike shunt). I'm being completely over-the-top about this, (I hope).

The laser unit states it has 'APC line power stability'. I am not conversant with this phrase.

Steve O

[Steve Anderson]

Neither am I, I think APC are a company that make UPSs' for PCs amongst other power-conditioning gear. It's familiar. But for this I wouln't have thought so. APC do make UPSs' and the like, but no direct reference to individual surge suppression components or the like. Google APC.

Also look on the RS, Farnell and like sites for "Transient Voltage Suppressor (TVS) Diode". The following pdf came from a local (Bangkok) supplier. It is in English...

A TVS diode is basically a 'brute force and ignorance' Zener. NOTE, there are both unipolar and bipolar versions. They're not really designed for regulation of suppies. the name is a giveaway as to thier designed function.

1.5KE-ST.pdf

(137.02 KiB) Downloaded 120 times

Years ago, mainly in the analogue telephone world, small Neon-filled spark-gaps were used extensively, I'm not sure if they're still available. A TVS diode would probably do a better job anyway....at lower voltages and faster response where needed, e.g. RS485 etc.. The TVS downside might be their relatively large junction capacitance. Allowance needs to be made for this where data or modulation is involved. Not really an issue here...i.e. DC power.

Steve A.

[Panrock]

Fine business! I was due to order on RS anyway this morning, to get a suitable stand to clamp the laser module to. I'll now look into TVS diodes as well.

I am thinking ahead, to when I set the slat angles, one by one. This could allow a cumulative error to become embedded, so that when I get to (say) slat 90, the setting won't quite meet - or could over-meet - the correct finishing point. This would render as tilted verticals on the picture.

I thought that, if I drop a spot of glue on top of each slat when set, this would both prevent preceding slats moving when I adjust the next slat on top, and also give rise to a 'semi-rigid' assembly of all the slats. In case of a cumulative error (likely) I could then grab the whole screw - like a concertina - and compress or stretch as appropriate.

The basic intention is to give Karen's unit the least work to do.

Steve O

PS. In case you're wondering, this 'cumulative error' problem was taken care of on my earlier 120-line screw by a peg-and-hole system. This was arranged to permit doubling up of the slats to allow a 60-line system if the 120-lines had proved totally impracticable. However the slight 'slop' between the pegs and holes meant precision single slat angle setting was still necessary...

[Panrock]

The laser has arrived. It works well and casts a fine line across the workshop... should do the job. It is operating comfortably from 3v (within the specified 3v to 6v range). It is great to have the peace of mind that comes with my 'over the top' soft powering arrangements.

So, next to set up the slat angles critically as I build up the screw. More to follow...

Steve O

[Panrock]

The <3mW laser, when producing a vertical line, and intersected by one of the 1mm-wide slats, has turned out to be too dim to use!

I am therefore to now set the slats up using a micrometer gauge, which fortunately I have to hand. It's evident that about 50 microns of metal has been lost to the polishing. Taking account of this, and after a little geometrical plotting and trigonometry on the computer, I reckon the stagger between the outer opposing edges of adjacent slats should be set to 22.72 mm. A picture will follow when I have got started. This method might work tolerably well. We can but hope...

Steve O

[Panrock]

The eye serves as a surprisingly critical instrument when comparing the positional settings of the individual slats, making sure they're all the same.

I have also run a check on the dimension to be expected over the ends of the first six slats. According to the simulation (shown), this should be 40.6 mm. I'll repeat similar checks as I go. And for example at 22½ slats, the stack should occupy an exact right angle.

Steve O

[Panrock]

And so to "first quarter". Slight adjustment over this first stack of 22 and 23 slats was needed.. the quadrant needed stretching ever so slightly, but it wasn't far off.

On this picture you can see the two 'globs' of goo I add to the top of each slat before the next one is pressed down on top. This is my way of getting the stack to behave 'as one' when stretched or compressed in a rotary fashion. The goo is actually one component of Araldite: the hardener is not used.

Each slat edge is given a last individual polish before being added.

No, using brass tube with the aluminium slats is not ideal, but it's what came to hand and - stored in the warm and dry - in practice there should be no problem.

Steve O

[Panrock]

Today we reach another landmark - half way - 45 slats. Again, this is another good time to check how the build is going. I'm now doing this 'by eye'. The radial angles were pretty close... just a slight squeeze was needed.

I'm keeping going in the workshop with the help of American elevator music, here relayed on 357 metres AM for the benefit of radios around the house. Giving them a plug: The Elegant Sound.

Steve O

[Steve Anderson]

...here relayed on 357 metres AM for the benefit of radios around the house.
Steve O

Having a look around the house, there's nothing that can recieve AM at all. Even the car is FM only. Certainly no DAB either. But the FM band, like Japan, starts at 77MHz. Maybe just because the car is Japanese, Toyota. But there's nothing transmitted below 88MHz here. or above 108MHz.

Generally worldwide108.00 to 136.975MHz is reserved for air-ground communication and navigation. The military often use frequencies in the 300MHz range.

My old UK aviation 'stomping ground', Blackbushe Airport, (Camberly, Surrey) still has an R/T frequency of 122.30MHz AM some 40 years after I learnt to fly there.

Steve A.

[Panrock]

Continuing briefly on an off-topic ramble, here's an "AM" receiving aerial I made a while ago. It was intended to use with the ill-fated 405-line television transmissions from Welham Green, on 45 MHz vision and 41.5 MHz sound... all AM.

This construction was never really satisfactory, since I thought the (bought-in) dipole centres were too small for an aerial with elements exceeding 10 ft in length.

A refinement was the whip-cord hidden inside the elements, to damp out any aerial 'hum' in the wind. The aerial was destined for the chimney of a detached house in Leatherhead, where our first 'viewer' lived. He was to watch with a 1949 Baird Townsman console in his drawing room.

Steve O

[Steve Anderson]

Hi Steve, just wondering, what was/were the intended transmitter(s) you were hoping to use? I would hope something appropriate for the period, tube/valve based, no semi's. I imply seperate vision and sound transmitters as that was often the case, the RF outputs fed into a combiner then fed 'up the spout'. The combiner also acted as a final harmonic filter. This was often followed by a number of 'splitters' to divide the power between the driven elements in the radiating array (if need be depending on power levels and the radiation pattern desired).

Steve A.

[Panrock]

Well yes, our first transmitter (re)build efforts did indeed concentrate on using 'something appropriate for the period'. See here: https://405-line.tv/cg1-transmitters/ to take a look at our S.T & C CG1 transmitters.

Later on, with a view to increasing the power and better EMC compliance, we built up from scratch a more modern outfit (pictured). Vision and sound were kept entirely separate throughout the chain, up to and including the use of separate vision and sound masts.

Steve O

[Steve Anderson]

Vision and sound were kept entirely separate throughout the chain, up to and including the use of separate vision and sound masts. Steve O

Wow! Can't afford that these days, the feeder costs alone would be prohibitive. But at 405 frequencies a lot less than 470-800MHz feeder cable, also less losses. Add in the power levels involved at 50 ohms and that can be a lot of voltage! Mentioning voltage, not forgetting the possibility (probability) of lightning strikes...a real problem here...also the theft of the copper used for grounding the mast and radiating elements where needed.

Often work up a mast has to be suspended if there's even the remotest hint of thunderstorms in the area, sometimes for weeks. Also if maintenance is required on the antennas, transmitters need to be turned off or run at reduced power due to the radiation risk to the humans working up the mast.

Steve A.

This can apply to Amateur Radio transmitters too, though the vast majority are less than 100W, there's always some clown running a "Californian Kilowatt."