Magic LED Display

326

06:11:04

Works with only one connection!  It's made up of LEDs which magically light up, even without a complete circuit.  

Notice no current is flowing to the common, which is connected to the other side of each LED in the display. Even if there was an internal connection to ground/common we can't see (unlike real life), some current would flow through our external connection as well.

Here's a real-world example of a 7-segment LED display, for comparison: 
https://www.sparkfun.com/products/8546

published 5 years ago

Robert_Kidd

5 years ago

If you look at default values of EC LED’s they are 2V and 20mA. Look at the 7 segment defaults - also 2V and 20mA. It is an led display. Since the display has its own power supply you need only apply a 5V signal to a control line for a segment to light up. Hope this helps.

h_kane

5 years ago

I think you're wrong about that - if you go to a site like Sparkfun the 7-Segment Display is not self-powered. Why would you even build something like that - the end user probably couldn't get to it to change out a tiny battery anyway once it was installed. And how could you build it without a complete circuit? Even if it had its own power supply, it would need to have a connection to the common/ground to work. Do you have a real-world example of a self-powered 7-segment display that would work with a floating common like this?

ViolationMad

5 years ago

@h_kane This is a simulator for working principles of circuits. It does not require a specific component with a partnumber. If you are looking for that, you should use spice on a PC. The 7 segment doesnt have a built in power supply but is referenced to EC's "ground" (0V ref). Most certainly that is for a more stable simulation and calculation.

ViolationMad

5 years ago

*internally referenced

h_kane

5 years ago

If that were true the common line would be connected to common/ground. It's not. I only gave a reference part to show what it's supposed to model (sans power supply). I modified it to show that no current flows from common/ground to the LED common. So where is the current coming from?

Thomas123456

5 years ago

That pin is the decimal point, not a ground

ViolationMad

5 years ago

@h_kane check it again... As Thomas said, the right pin is for the decimal point. The 7 segment is internally (you don't see it) connected to the GND symbol. Since you also have that connected to your power supply you have a closed circuit. Current can flow and the led gets biased proplery... Try to reverse your power supply and it will still behave like a regual diode (no light, no current).

Robert_Kidd

5 years ago

@h_kane, sorry, I have confused you. Ignore the bit about an internal supply but, in any case, I was talking about a simulator model NOT a real life device. What I was getting at, but made a poor job of explaining, is actually related to logic gates on EC where no power pins are provided. This saves real estate, allowing larger circuits to be simulated. Still these are LED displays and sorry again as I kind of went off on a tangent there.

Robert_Kidd

5 years ago

As has already been pointed out the led cathodes are connected together and to ground - but the ground pin of the device is not shown, again, to save space.

add comment in editor