Mechanical TV Project

Forum for discussion of narrow-bandwidth mechanical television

Moderators: Steve Anderson, Dave Moll, Andrew Davie

Re: Mechanical TV Project

Postby FlyMario » Mon Jul 16, 2018 10:55 pm

@Dave Moll

Yeah possibly. He said PDF and I saw DOC when I opened it. I am more of a digital electronics person trying to do some analog for this project. I had seen this MOSFET circuit before so wasn't sure if we were talking about the same thing.

In my ignorance, I would have never thought a MOSFET would be able to vary the current based upon the gate voltage. I always had thought of MOSFET as on/off. From this though, I see that I am wrong and that is very cool.
FlyMario
 

Re: Mechanical TV Project

Postby Klaas Robers » Tue Jul 17, 2018 6:39 am

Number 5, yes that it is. You may combine it with the 3-transistor inverting/noninverting pre amplifier of number 6.

What you described is exactly what a MOS-FET does. It varies the drain CURRENT as a function of the gate VOLTAGE. So the CURRENT through the LEDs is controlled. The function of the LEDs in this mode is linear, 3x as much current (mA's) gives 3x as much light. The function of the FET is more or less quadratic. Overall that is what you want. This includes the so called "gamma correction". If you don't know / understand what it is, don't bother, it works as you like it. A MOS-FET is more or less born to be a video driver transistor for LEDs behind a Nipkow Disc.
User avatar
Klaas Robers
Frankenstein was my uncle.
 
Posts: 1431
Joined: Wed Jan 24, 2007 8:42 pm
Location: Valkenswaard, the Netherlands

Re: Mechanical TV Project

Postby Steve Anderson » Tue Jul 17, 2018 2:13 pm

MOSFETs do have far more applications with a switching function (on/off) rather than linear. Switched-mode power supplies being perhaps the most common use for them. I'm generally referring to power devices here, not small-signal or logic.

But they are used often in the output stages of audio power amplifiers. Their advantage is they don't suffer from the 'thermal runaway' of standard bipolar transistors. From "All about circuits"...

"...The problem with increasing temperature causing increasing collector current is that more current increase the power dissipated by the transistor which, in turn, increases its temperature. This self-reinforcing cycle is known as thermal run away, which may destroy the transistor. Again, the solution is a bias scheme with some form of negative feedback to stabilize the bias point."

In a MOSFET the current decreases with increasing junction temperature - a form of 'self negative feedback'. But it is still wise to not rely on this only.

Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 3800
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Mechanical TV Project

Postby FlyMario » Thu Jul 19, 2018 10:59 am

So here is the current status.


I have been doing a lot of part design in SolidWorks. I am pretty happy with the IR Transistor and LightBox area. Though I have modified it in Solidworks to separate the lightbox and sensor mounts off the base to make it more adjustable.

Not too satisfied with my disk (record). Spinning it up it highlights some gaps between the holes. I should have glued the hole pattern to the disk instead of using clips. Nothing I can't redo later.

In hindsight, I should have put the sync holes 180 degrees off so that I could have a smaller arm to mount the IR LEDs. I don't have a line sync hole on the 32nd Line per my interpretation of the NBTV specs. Where the Hole is missing on line 32 I do have a frame sync hole (20mm offset). I am kind of torn because I am starting to like the long arm.

Still apprehensive about the electronics. I am over thinking the whole thing I am sure.
Attachments
Sensor Arm.jpg
Light Box.jpg
Gaps.jpg
FlyMario
 

Re: Mechanical TV Project

Postby FlyMario » Tue Jul 24, 2018 10:22 am

So, I have been working on some Circuits to drive my Mechanical TV.

I have an Arduino Nano that I will hopefully find enough to drive all that I need. There is Mosfet that drives the motor. It is doing that very well from PWM pulses from the Arduino.

You probably notice a buck converter. I am using this to get a stable 5v from the 12v power supply. It was so cheap I couldn't really see a reason to use it instead of making one.

On the right, I have connectors for two IR Receiver Diodes and Two Transmitter Diodes. This is a very basic circuit found all around the internet.

It was really fun to play with the OP Amp and Mosfet. I can see a lot of uses for the Mosfet for my Halloween projects as I normally used power transistors.

Anyways,
Attachments
MechTvCircuit.jpg
FlyMario
 

Re: Mechanical TV Project

Postby smeezekitty » Tue Jul 24, 2018 2:32 pm

Generally, it is a good idea to use a transistor to drive a power MOSFET gate because they are quite capacitive. It is hard on the MCU pin and limits how fast the MOSFET can turn on and off (making it get hotter).

Those defects on the disk look like pretty big gaps. although maybe the camera makes it look like it is worse than it is.
smeezekitty
Evil Genius
 
Posts: 160
Joined: Fri Jan 29, 2010 11:42 am
Location: USA

Re: Mechanical TV Project

Postby Harry Dalek » Tue Jul 24, 2018 5:18 pm

OH a hint on the IR thing , remember natural light has IR dam that SUN ! so you will find it will effect your sensor well it does on my reflective type
A reflective type you have a print encoder reflect off to make the pulse as i tend to use .
Might be missing something but i do recall many a time more or less natural light causing speed control problems.
Another thing your disk is shiny another problem it may be black but it reflects light like a black mirror ...i used to spray paint them with matt Black paint .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
User avatar
Harry Dalek
"Fester! Don't do that to 'Thing'"
 
Posts: 4045
Joined: Fri Sep 26, 2008 4:58 pm
Location: Australia

Re: Mechanical TV Project

Postby FlyMario » Tue Jul 24, 2018 8:13 pm

smeezekitty wrote:Generally, it is a good idea to use a transistor to drive a power MOSFET gate because they are quite capacitive. It is hard on the MCU pin and limits how fast the MOSFET can turn on and off (making it get hotter).

Those defects on the disk look like pretty big gaps. although maybe the camera makes it look like it is worse than it is.


Thanks for your advice! I will add a transistor.

Yeah the gaps are bad. I need to go ahead and re-make the disk soon.
FlyMario
 

Re: Mechanical TV Project

Postby FlyMario » Tue Jul 24, 2018 8:24 pm

Harry Dalek wrote:OH a hint on the IR thing , remember natural light has IR dam that SUN ! so you will find it will effect your sensor well it does on my reflective type
A reflective type you have a print encoder reflect off to make the pulse as i tend to use .
Might be missing something but i do recall many a time more or less natural light causing speed control problems.
Another thing your disk is shiny another problem it may be black but it reflects light like a black mirror ...i used to spray paint them with matt Black paint .


Yeah I notice that I need to adjust the sensitivity more than seems reasonable.
FlyMario
 

Re: Mechanical TV Project

Postby FlyMario » Wed Jul 25, 2018 1:00 am

smeezekitty wrote:Generally, it is a good idea to use a transistor to drive a power MOSFET gate because they are quite capacitive. It is hard on the MCU pin and limits how fast the MOSFET can turn on and off (making it get hotter).


Since this is a logic level Mosfet do you really think a transistor would be that necessary? I keep trying to find a good example of this and finding such mixed information.

The Mosfet is RFP30N06LE. It is interesting that I see a lot of people putting a 10k resistor from the gate to ground.

I do know that I need a diode between the motor coils.

I have played with electronics since I was a kid. It never being my primary focus has left me a bit ignorant. I know that now I am in my mid 50's I won't be as skilled at it as you guys.
FlyMario
 

Re: Mechanical TV Project

Postby smeezekitty » Wed Jul 25, 2018 4:51 am

FlyMario wrote:
smeezekitty wrote:Generally, it is a good idea to use a transistor to drive a power MOSFET gate because they are quite capacitive. It is hard on the MCU pin and limits how fast the MOSFET can turn on and off (making it get hotter).


Since this is a logic level Mosfet do you really think a transistor would be that necessary? I keep trying to find a good example of this and finding such mixed information.

guys.

It will most likely work okay without one. I didn't use a logic level FET so in my case it was required in order to get enough voltage on the gate to turn it on completely. Generally it is good practice to use a strong gate driver though.

I also needed to put a large capacitor (4700uF) across the motor to stabilize the loop and get rid of loud PWM whine. YMMV
smeezekitty
Evil Genius
 
Posts: 160
Joined: Fri Jan 29, 2010 11:42 am
Location: USA

Re: Mechanical TV Project

Postby FlyMario » Wed Jul 25, 2018 10:52 am

So from advice that Harry provided, I modified my IR tachometer portion to be reflective instead of showing light through sync holes. I like this alteration. The Encoder was created so that it could be calibrated by loosening the hub screws and rotating.

I also created a new disk. It is much nicer now.

Placed a diode on the motor terminals to hopefully drain the induction from pulsing the power.
Attachments
20180724_170810.jpg
20180724_191628.jpg
20180724_191916.jpg
FlyMario
 

Re: Mechanical TV Project

Postby Robonz » Wed Jul 25, 2018 5:14 pm

Hi FlyMario

Nice job, you are making great progress. Let me clarify a couple of things from someone that has done the mechanical TV using Arduino

1) You can drive the mosfet directly with no issues. All you have to consider is, will you blow up the Arduino by pulling to much current from the PWM pin. A resistor between the gate and PWM pin is good. Also make sure you do not use a mosfet that has a massive gate capacitance as its hard to switch the gate fast.

2) Through beam versus reflective speed sensing is generally always superior in performance as the contrast between off and on is fully defined. If you have sunlight issues then you make the resistor in series with the photo transistor smaller or shield the light. How you had it was very good except for the white plastic.

3) The parts you have made are white. For maximum contrast and shielding whether it is viewing or opto-sensing. Black plastic is your friend.

If you want to see my build which is similar, it is here
viewforum.php?f=35

Good luck
Keith
User avatar
Robonz
Evil Genius
 
Posts: 147
Joined: Sat Aug 12, 2017 7:15 pm
Location: New Zealand

Re: Mechanical TV Project

Postby FlyMario » Fri Jul 27, 2018 11:52 pm

So I modeled a new Motor Mount / Drive assembly. I got tired of the motor becoming strained based upon parts that I would tighten or move even though I am using a flexible coupler. It became a ridiculous game.
On a test print, the motor and bearings were beautifully aligned.


It is being printed in white... so those who comment about white being the devil can suck it. :) (I do have some black coming in today to print the IR sensor mount.)

I have a nice reference to where the center of this disk will be for all future work. I am a bit too anal at times about dimensions.

Tomorrow I will hopefully have the disk spinning under a controlled 750 RPM from the Arduino. I am debating having a frame interrupt or looping monitoring the port signals. I can see the benefits of doing it either way. I have yet to build any circuit to deal with audio signals. So I will have no other excuses to delve deep into analog electronics.
Attachments
Moter Drive.jpg
Disk Drive Center.jpg
Disk Drive Center.jpg (26.15 KiB) Viewed 4471 times
FlyMario
 

Re: Mechanical TV Project

Postby FlyMario » Sat Jul 28, 2018 1:29 am

Ok here is what I am thinking.
An Arduino has a 16mhz clock. At 12.5 fps 1,280,000 clocks should pass per frame. At 32 Lines, 40000 clocks should happen per line. If I were to sample the ports at 32 times per line I could use 1250 (~1240 realistically) clocks. I do have an adjustable oscillator chip (ICL8038) that I could use to pulse the interrupt line at 12800 times per second. Anyways I have a direction to try. If you guys have any suggestions please let me know. Maybe this is old hat and had a lot of issues.
FlyMario
 

PreviousNext

Return to Mechanical NBTV

Who is online

Users browsing this forum: No registered users and 7 guests

cron