Stepper Motor II experiments:
Rotary encoder directly controls stepper motor (no PIC needed) - 28th April to 1st May
I have been commissioned by Whitgift School to build another physics demonstration and this circuit will be useful to control the postion of mirrors in an optical demo.
This circuit takes the quadrature o/p's from a rotary encoder to drive the (bipolar) stepper motor coils directly (using a ULN2003 buffer).
No PIC or clever switching circuits are needed as the rotary encoder is doing all the 'clever' stuff.
circuit based on that shown here (www.instructables.com)
1/12 wave coaxial transformer: a 75 ohm to 50 ohm converter - 3rd May
I have some very good quality 75 ohm coax cable that I will be using on a 4m dual dipole antenna (more details below soon).
Here I will be phasing up the two crossed dipole to get various polarisations using switchable coax phasing lines.
Now feeding a 50 ohm system with 75 ohm should only create a 1.5:1 VSWR, if there is no reactance present.
But some reactance (and additional resistance) may creep in via the matching / phasing circuits - in which case this circuit will help.
PIC control board for SV1AFN band pass filter unit - 6th May
I am using a PIC16F877 to control a SV1AFN BPF unit (more details soon). It's rather 'over the top' to use this PIC to control such a simple circuit
but I happen to have a batch of them and it also means I can use a very powerful PIC BASIC complier, which makes development really easy.
The other board on the right is a 5V PSU board which also has space for the RS232 unit required for in-circuit bootloader programming.
Experimental 4m band crossed-dipole & phasing unit - 1st - 13th May
I built a set of crossed dipoles (45 degree and 135 degree) for the 10m band but 'plugged' in elements for the 4m band.
These are wired using high quality pair of 75 ohm cables into a phasing switch unit.
By switching in various lengths of 1/4, 1/2 and 3/4 wave sections I can get: 45 degree slope, 135 degree slope, vertical, horizontal and left and right circular polarisations.
Everything is done at 75 ohm (using 50 ohm quarter wave sections to transform the two 'parellel' dipoles back to 75 ohm.)
As its hard to get perfect phasing harness lengths and perfect switching, I kept everything at 75 ohm and then used the 1/12 wave transformer (above) to make 50 ohm.
The bench-test shown here uses two 75 ohm dummy loads (purple) but early tests show the system also works with the dipoles.
(Phasing circuit from VHF-UHF handbook, 2nd Ed. 2013 Ed. Andy Barter, ISBN 9 781905086313 page 158)
3D printed center holder for light weight 4m band crossed-dipole - 16th - 21st May
The crossed-dipole set-up for 4m band shown in the 1st-13 May entry was a modified 10m band anttena and was simply too heavy to go on top of my fibreglass mast.
So I 3D printed a X-piece to take small aluminium rods that cam hold 1m long Al welding rods for the dipole elements.
The result is something much, much lighter than the original. VSWR on the set-up very good, less than 1.5:1 on all polarisations.
Note: the 'old' 10m band baluns (coiled coax) are still in place.
Light weight 3D printed 4m band crossed-dipole with 1:1 baluns - 27th - 28th May
I replaced the 'coiled coax' baluns (that were made for a 10m band version of the antenna) with 1:1 coax baluns based on the I0QM web page.
The VSWR on the output of the phase switch set-up is very good, less than 1.5:1 on all polarisations.
I0QM 1:1 balun web page / PDF
Stepper Motor III experiments:
Bi-polar motor driver - ca. 30th May - 16th June
This version uses the DRV8825 bi-polar stepper motor driver to control a standard bi-polar motor (often used on 3D printers etc.)
You can buy very cheap ready populated pcbs on e-bay and I just made up a 555 pulse circuit to drive it.
At first the unit would not work but once I added the +5V and 0V rail to the board swich inputs everything came alive.
(Note: the board DRV8825 needs at last 12V to reliably drive the stepper motor)
U-Aluminium for experiment 'boxs' 9th June
Its expensive and a bit of a waste to use nice new boxs for my prototype experiments so I decided to buy some
1m lengths of U-channel aluminium extrusions (1m x 4cm x 8cm) from e-bay and cut them into convenient sized 'box's.
3D printed extension knob for FT817 - 10 -11th June
I have been experimenting with push on knobs to go over the small tuning knob on the FT817 transceiver.
This one has an arm that allows for fast tuning. I will add more details of this to my 3D printing page asap
CSC 3D printing web page
3D printed element holders for I0QM 7 element 2m band yagi - 3rd to 17th June
I made up an I0QM 7 element 2m band yagi about 4 years ago and found it worked really well,
but my version used a wooden boom (which I have used many times before for 2m band yagis) but I found the SWR went high when it rained.
So I built an aluminium boom version and used 3D printed parts to hold the elements.
It will be interesting to see if the SWR now changes when wet and also the electrical logevity of the 3D printed parts.
I used a 2m band 1:1 balun to feed the central dipole, as described above (27-28th May).
I0QM 7 ele 2m band yagi PDF
3D printed holder for Proskit magnifier - 24th June
The Proskit magnifier is a very useful tool for electronics (as well as removing splinters etc.). It also has powerful white and UV illumination LEDs.
If you do get splinter it often feels like you need three hands:
one to hold the magnifier, one to use a tweezer to remove the splinter and the other the hand with the splinter to remove !
So I made up a 3D printed device to hold the handle of the magnifier onto a solid metal base (that I already had).
.... More ideas coming soon include: capacitance hat for helical LF antenna (?),
op-amp Analogue Computers, Energy Harvesting unit, switched capacitor laboratory pre-amplifier,
HF 5/8 wave vertical with coax-matching circuit, using a LMC6041 cmos op-amp to measure Circadian Rhythms, ...
click here for my 'X-files' page (other 'interesting' projects / ideas)
THE CREATIVE SCIENCE CENTRE
Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ.
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