Ben Heck’s Mechanical Television Part 2 Episode -- Episode 233

This project on Hackster.io may be of interest to those following this show episode.

Nipkow Disk Based Digital Display Device

https://www.hackster.io/christophe-fieldman/nipkow-disk-based-digital-display-device-2cc98a

Nipkow disk display device using Arduino

https://www.youtube.com/watch?v=gTvPC_hkeGs&feature=youtu.be

I vote you revisit this project.The lessons learned are invaluable because understanding the dynamics of the issues you encountered will help designers of servo-mechanisms appreciate the problems they will be faced with. It is also a good demonstration of several failure modes in modern machines. My response provides purely mechanical solutions as that seems to be the intent of building a "mechanical TV". As I see it there are three issues with the design. 1) servo control of the two discs, 2) rigidity of the base and 3) propagation delay from the "camera" to the "projector".

You are using a toothed belt which is a good idea because it eliminates slip but there is a lot of flex in the belt and with the holes in the record being so small that flexure will add a lot of speed distortion - enough that the strobed light in the projector may not be turned on at the right time. I work for a company that makes vibration spectrum analyzers and loose belts add a lot of vibration to machines. Think of a guitar that has the strings attached to fixed locations on the guitar. Loosening and tightening a string is equivalent to the belt flexing. Note the difference in pitch of the string as it is loosened and tightened. If the pitch frequency is halved by loosening then the string is moving twice the distance. Even with a toothed belt that flexing considerably changes the speed of the load. When the belt is at maximum tension (arced towards the center) the load is slowed down and when the belt is at minimum tension (arced away from the center) the speed is increased. In the old audio tape days this was called wow and flutter. The conventional solution in tape and record systems was to add a capstan which keeps constant tension on the tape using springs as the servo-mechanism.

The capstan may not be good enough because recording and playback occur at different times on a tape player but in this experiment you are capturing and transmitting light pulses synchronously in real time. Another approach would be to have a larger gear attached to the motor which is mounted between the two discs. This gear has to have the same gear pitch as the gears attached to the records. It will slow the turning speed slightly but the motor gear is acting as a differential and both sides are affected equally and the gear reduction should also add a little more stability as slight changes in motor speed will be reduced. By using the differential gear, the tensioned portion of the belt is the same distance to both records and there will be less distortion because the tensioning sections of the belt (behind the direction the belt is traveling at the motor's gear) always have fairly proportional tension. The driving end of the belt (being pushed by the motor) just flexes and no longer affects the system due to the differential. With this approach you may have to add a slight friction braking to both records and adjust one of the brakes until you have good synchronization thereby maintaining constant velocity on both records. Imbalance and warping in the records will also cause distortion as imbalance cause velocity changes in the discs. The differential and brake methods should resolve most of the problems caused by imbalance and warping as long as the base is reliable. No matter what you do, the two disks have to be held rigidly in place and the tension on the belt must be constant. In the video it looked like foam board was being used and that alone could have caused the system to fail.

The last thing to consider is delay through the detection and light driving sources. I would measure the delay and then add a little lag in the playback record by either changing the angle of the mounting holes or elongating the holes on the playback side. I would opt for the slotted holes because the imperfections in the driving system are probably more a factor in distortion than the elliptical dashes caused in the playback. The holes on the outer diameter of the record are traveling at much higher velocity than the inner holes and consequently are going to be more affected by delays. You might have to do both, add a slight phase shift by adjusting the mounting holes on the discs and elongating the holes.

As a side note, another approach to solving the MOSFETs burning up would be to use PWM to drive the DC motors. The motors windings, being inductive, will have linear currents but the power dissipated by the MOSFETS would be more dependant on the transition time. I would put good Schottky diodes across the MOSFETS and not rely on the internal diodes to shunt the flyback voltage when the transistor turns off. The power supply will need low inductance paths and good bypassing to keep the flyback current from driving the supply regulator out of regulation by reverse biasing it. Even with PWM you should look at understand the power dissipated by the MOSFET. Otherwise it is best to use a proven driver circuit because a MOSFET failure can be catastrophic to your system by introducing high voltage, high energy pulses into your low voltage circuits! I've seen boards destroyed beyond repair with nearly every semiconductor destroyed and heavy traces (0.125 inches wide) burned to carbon. Your decision to go to the transistors used in the drill was a good one. Since you are using them in a linear application I would suggest putting 10K 1/4 W resistors in series with the gates and put protection diodes on the output of the opamp driving the gate. The 10K resistor limits the current to safe levels the delay caused by gate capacitance won't be significant with the frequencies the DC motor uses. If you use PWM then the resistor should be reduced to 100 Ohms or the switching time of the MOSFET can cause excessive heating,especially in an H Bridge implementation. The 100 Ohm resistor minimizes the effect of capacitive loading due to the gate capacitance of the MOSFET and will act like a fuse during catastrophic failure of the MOSFET but the damage to the rest of the system should be minimal.

In the Name of Purism

The approaches I've presented are mechanical solutions that would have been available in the 1920s and don't add electronics to the project as presented. A slightly more purist approach would be to use incandescent lights instead of LED which would have been around in the 20s. You would probably have to bias the lights all the time to keep them warm so their response is regular and therefore predictable. By using incandescent solutions the lag induced issues become quite apparent. A purely mechanical approach to this project would use a hand crank instead of the electric motors and and would use candles instead of lights altogether. Then the challenge is how do you modulate the light? Perhaps a slotted disc turning at slightly lower speed than the record to introduce a slight delay in the light which causes the horizontal scans. But there are enough challenges in the project just as it is.

I did a similar project back in the 80's for a high school science fair project... and again a few years later during a slow semester at college.

Anyway, a few notes on how to salvage something cool from failure:

- The disk most synchronized with your display disk is... the display disk itself.  Those holes go all the way around so use the bottom of the record for image capture and the top for display.  One record, one motor, you can spin it by hand if you like. (yes, the image is flipped and mirrored)

- For lots of fun you really only need to build the display unit, and it can have a constant light source always on.  You can make an image by putting another disk between the light source and the spinning record.  That intermediate disk has slots cut in it so it turns the light source on and off.  This intermediate disk doesn't have to be the same radius... it just has to have slots cut in it so it chops up the light.  I made a whole set if disks, each was cut to show a different image (a stick man, a triangle, letters, etc).  Synchronization of the slot disk was done manually by the person viewing the image by using a controller from a slot car race track.  And the slot disk also doesn't have to be intermediate between the light and the spinning hole disk (it wasn't in my science fair project).  You can place it in front of the record and look through it.  For a quick approximation just wag your fingers back and forth in front of the record while it is spinning and there is a constant light source behind it.

- My college version used an array of LEDs for the light source (the brightest red ones that Radio Shack had in 1993).  They were controlled by a parallel port hooked up to a  486 system running a version of linux most of you might not recognize today.  Relatively simple software turned the led array on and off  (as a whole) to produce an image.  Synchronization was manual, but the computer could keep pretty good time (even from a user mode application) and the motor would spin at a fairly regular rate.  The software had a couple keys you could press to speed up or slow down the timing.  Animation was just a few lines of code more... The ladies really swooned when they saw that red pixelated stick man do jumping jacks.  Had I not gotten a job and started life I might have eventually gotten Doom to show up on the 'display'.

-Lastly, if you really insist on the camera portion (don't) it's better to put the image onto the surface of the scanning disk via a lens rather than projecting a flying dot and sit in the dark.  So, light your subject as brightly as possible (the sun makes a nice subject), put a cracker jack plastic lens in front of the scanning disk and focus it on the disk.  Put another lens behind the disk then the photo diode behind that. The holes slice across the image from the lens and the whole thing works... sort of.  But... just stick with the viewer and make a mechanical computer monitor or a mechanical video disk player.

Have you tried a checkboard of holes and mirrors instead of black and white?  The belt and pulleys was a nice trick. You should check with a high speed camera if dots are synched. Perhaps larger pulleys would give you more resolution.  Try synching a single dot at a time, hiding the others on the sensor and projector discs. I think that would help debugging the issues

I watched episodes out of release order (Mechanical TV 1+2 then luggage detective) so I don't remember which episode had Felix start going over how he uses Linux, but I want to encourage you to show more of how Felix uses Linux tools in future episodes.  One option would be to go into more of the software used when you do start doing Raspberry Pi Zero projects.

Revisit. The future of this project is with the 2 stroke petrol engine. They work fine on their side and there'll be zero problems with getting enough power. Could spin a 6 foot disc! Slip? No problem. Speed? Two of the same motors would be near identical in speed, especially if you used the same fuel. Ben and the team have hardly ever used the grand power of the combustion engine, it'd be an excellent opportunity to show off some of the benefits! You could even rig up an auto starter with those left over drill motors. Noise wouldn't be a problem as this TV form doesn't have any audio!

Would two AC motors stay in-sync provided there is no slip?  How did the original inventor do it?  Did the replicas of this invention in science museums do it the same way?

I vote to not devote another whole episode on reinventing this technology.  However, you might want to do a short segment that discusses viewer suggestions, commenting which ones seem to be most viable, so that interested viewers can build and test the fixes themselves as a homework project.  They can then upload their videos on what worked and didn't.

I have two ideas on how to make it work.

DIRECT DRIVE MECHANISM (Sony Walkman Disk Drive)

1. Use a single motor with a 3D printed gear to drive smaller gears attached to the Nipkow Disk shafts.

2. The gears ensure that the Nipkow disks remain in sync and at constant speed as they rotate.

or...

MANUAL TUNING W/ ANALOG MAGIC EYE

1. Use two motors to run each Nipkow Disk independently.

2. Add an LED/Photodiode to the bottom of each Nipkow Disk in order to control a RED or GRN LED.

3. As the outermost hole passes the LED/diode, it will cause its respective RED or GRN LED to flicker.

4. The LEDs flicker frequency will illuminate a specific portion of a color gradient printed on a transparency above it.

5. This will create a needle effect, allowing the speed of both disks be adjusted with pots.

6. The motors can also be disabled momentarily, with a button press, to allow one disk to fall behind and hopefully sync.

7. Then it will only be a matter of bringing the needles together and to the center of he indicator.

For sync:

Add one photoswitch on each disk. Make sure the switches have 50% duty cycle over the full turn.

Combine both photoswitches with a XOR or XNOR gate. Use the output to drive the MOSFET of the receiver.

That's it, really.

Please revisit!!

1. Analog brightness signal !!!! The source brightness should directly control the playback light brightness. Simple analog stuff. Get rid of the comparators here.

2. Sync records with some sort of analog encoder, (potentiometer, ideally) so you can get high speed, and absolute positioning.

3. Servo the playback motor so it is in sync with record motor. Simple servo circuit here: If the playback motor is ahead of the record motor, the comparator goes low. If the playback motor is behind the record motor, the comparator goes high.

4. Low pass filter the comparator output, (capacitor in the feedback loop of the comparator) so it becomes a slow moving voltage. Put a high current buffer (power amp...) on the filtered comparator output that directly drives the playback motor. Boom, synchronization!

5. Transmit the record brightness signal and the record encoder position over two AM radio signals. It is TV after all!

OTHER IDEA:

Instead of a spinning record, use lasers and a XY mirror control, like a barcode scanner. Use the mirrors from a laser printer or barcode scanner? Way better resolution. LASER TV!!!