Electronics question

[Bribba]

Hello everyone. I apologize that this is not for a replica but I hope you guys can help or even point me in the right direction. I have looked for a general prop making forum with no luck so I hoping you guys can help.

I am making a prop for an indie short film. This prop, without going into too many boring details, is made to look like a broke and slightly crazy physicist slapped together whatever cheap parts he could afford. I want to include a display from an old VCR that I scavenged. There are 10 pins that illuminate different cells. I want this display to just display random things, not even numbers. It will be purposefully not recognizable to the audience, it is meant simply to convey that there is activity in the device that is understood by the character.

Is there any easy way to accomplish this? I have a basic understanding of electronics but have a lot more to learn so I will not be offended if you explain things in very simple terms. Any help would be greatly appreciated. Thanks.

Here's a picture of the device I am talking about:

 

[13doctorwho]

The simplest way would be a 555 timer with a 4017 decade counter. You can do a wide range of flash patterns that way. They still come in leaded parts so you can wire it on a piece of perfboard. If you want it small there are surface mount parts you can use on a cheap printed circuit board, but you (or someone) would have to layout the board. It wouldn't be random, but it would look random unless someone went frame by frame to figure out the pattern.

If you Google "555 timer Sequential Flasher" and go to images you'll find a ton of circuits you could use. It really depends on you're comfort level. Here is a circuit I just got off Google.



PM me if you have more questions.

 

[Bribba]

Thank you so much for that info. I have never built a circuit before so this should be interesting. This sounds perfect. The appearance of randomness will be just fine. It won't be on screen for that long. Thank you so much. I'll get started on this right away.

 

[Abelugo]

For the amount of time your going to invest and the amount of time it will be seen in your video, maybe you can just fake it. I faked some led letters by having stencils cut on vinyl, applying those to some clear plastic and lightly sanding the back, then i just lit the back of it with the leds of choice. If you are proficient in electronics and programming by all means, wire this up. But i would consider how important it is within the story. Does the display change?, just make multiples and cutaway and back.

i had a project that just needed the letter to be lit with a persons name, but opted for putting in the effort into the speedo that actually counted up.

 

[Bribba]

I can definitely appreciate your point. There are also way I can cheat this in post production as well. And I would do that or one of the methods that you describe but for the fact that I have some time and have never done this before. That's the exact reason as to why I want to learn it. This will give me more experience when it comes to building bigger and more complicated props. I do appreciate your suggestions though.

 

[propmaster2000]

Good evening Bribba,

The Display Driver 4017 uses a "positive output" signal to illuminate the 7 segment LEDs.
In this case you have 3 segments, each to indicate 0 - 9 and one to indicate a 1.
You will need to find out if your display is common anode or common cathode.
Depending on the voltage you use, you may need resistors as well in series with
each LED. The 4017 is a decade counter (only 10 outputs). If you want more segments
lit randomly or individually you will need at least 2-4017's being clocked by the 555 (timer).
As shown in the above schematic, all of the leds are shown to have a resistor in series and
then tied to ground. Your display would need to be a common cathode type and the resistors
would go from the output of the 4017 to a single segment of the display (series).
Looking at your display, it may be a matrix (EDIT: Multiplex) type since there does not seem to be enough driver pins
to light each segment ( [a], , [c], [d], [e], [f], [g] x 3 ) individually.

.

 

[Bribba]

I believe I have what you are referencing by the "maxtrix type" ( I do not know this term in this context). I have played around with it and a 9 volt battery and wires to each pin just for exploratory fun. I did notice that providing power to each pin individually lit up some of the leds but not whole numbers. I thought at the time that a specific combination of pins will produce certain numbers.

As an additional side note, I want the output to be completely random segments, not merely random numbers. This will be indicating that the output of the device is sort of alien (not extraterrestrial) to the viewer.

 

[propmaster2000]

Wow, be very careful with a 9 volt battery with no resistors. Each LED may need only 20ma (or a bit less) at 3 volts.....

 

[Bribba]

How do I determine if my display is common anode or common cathode?

 

[Scotophor]

You say you've already connected a battery to some of the pins and gotten segments to light. Before proceeding further, I recommend you not use a 9 volt battery again, because most LEDs can't handle that voltage in reverse and will burn out instantly, even if you use the correct resistor for (forward) current limiting. You should use a 2-cell AA battery supply and a 56 ohm resistor (or thereabouts) in series.

Connect this supply to different pins until you get a segment of one of the three full digits to light, then leave one of the wires where it is and move the other wire to different pins. You should then see either: A) different segments of the same digit light, or B) in some positions, the same segment of one of the other full digits light, but in other positions, something else will light (the "1" at the left, one of the colon dots, or one of the various symbols/abbreviations at the far left and right sides). In either case, there will be several pins that will not light anything.

If you get situation "B", put the wire that you moved where it will light a segment of one of the full digits, then try moving the other wire around to different pins. You should now have situation "A". When you have it set so that moving one wire around lights different segments of the same digit, the wire that you're NOT moving determines whether the display is common anode or common cathode. If the wire you're not moving is from the positive side of the battery, the display is common anode. If the wire you're not moving is negative, the display is common cathode.

Once you have determined common cathode or common anode, you can then map out the pins as to which pins are common for each digit and group of symbols, and which segment/symbol is lit by each of the other pins. Since you've already applied excessive voltage to the display, there's a fair chance that some segments or symbols will never light, or will be much dimmer than the others.

 

[propmaster2000]

Good morning Bribba,
Scotophor is correct, you may have over driven a few of the segments already....

(EDIT above: I meant to say MULTIPLEX type instead of matrix)
Multiplexing lowers the number of connections between the driver and individual segments.

(Added explanation)
With Multiplexing, all of the [a] seg. are connected together internally, as well as , [c], [d], [e],
[f], and [g]. Then each of the 7 segment displays are turned on and off very rapidly by there common
connection and the controller IC which are eyes can not perceive (maybe a slight flicker).

You will be able to use 2 - AA batteries and a low value resistor (47 ohm) to test the segments
of the display safely.
If you have not over driven the ones that did light with the 9v or burned
any out completely you should still have something to work with.....

Your only concern now, is if you have a common cathode display (this pin to the negative supply).
The other pin of the single LED segment will go to a resistor of the proper value, then to a pin-out
of the 4017.

If you are using a 9 volt battery to drive the whole circuit, you will need an appropriate
value resistor in series with each segment (between 330 ohm and 470 ohm).
If you do not have a common cathode display then you will have to get one if you want a 4017
to drive it directly. You can still use a common anode type, but it will require more components
to drive the LED seg.
NOTE: It becomes way more involved at this point.

You can change the output of each leg of the 4017 with an NPN transistor (TO-92 package).
The 4017 pins will each drive a 10k resistor, then to the base of the NPN.
All the emitters of the NPN's will go to negative. While the collector of all the NPN's will go to the 470 ohm res.
then to the individual LED seg.
A fair amount of work......
If you can get a common cathode display, that would make things a ton easier.

Side note: At this point, to really make things easy, you may want to consider a micro controller....
But, if you can get a common cathode display and not a Multiplexing type, then you won't have to take these extra steps

.

 

[Abelugo]

Bribba
here is the project i worked on. We ran out of time to get the persons name done in letter segments properly. The sounds are coming from within the unit. The speedo numbers are done the way your looking to do them.

 

[propmaster2000]

Good morning Bribba,

Have you decided on which route to take on your project?

 

[skyit]

you may control it with a code and arduino