DIY 7 segment display - how to drive it?

MagIO2

My son want's to build a clock which has a 7 segment display out of blue SMD LEDs and a circle of RGB SMD LEDs (12). The blue LEDs have been soldered to a Euro-size (100x160) PCB. Each segment has 4 LEDs in series. So, to drive a segment 12-15V is needed.
Of course the digits are multiplexed, so we need 7 outputs to drive the 7 segments plus 4 outputs to select the digit.

Now here is my question:
I do not have an endless support of all existing parts at home ;o) , so usually I would have to order parts. But I tried to think about a way to solve the problem with the parts I have here. My idea now is to use
2 HCF4066 and 1 ULN2001

The ULN2001 would then be used to select the digit by driving the low-level of the common kathode of the 7 segment digits.
The HCF4066 would be used to switch on the segments power supply. The test cirquit of that showed me that the 3.3V of the propeller is not enough to switch on. So, my idea is to connect the switching input with a 1Mohm resistor to 12V leaving the propeller pin in tri-state-mode. If the segment has to be switched off the propeller pin would be switched to output pulling the 4066 pin to low.

Someone seeing some problems here? Will the propeller complain if it's connected to 12V via 1M ohm?

MagIO2

Hmmm .... maybe I should have made a quizz out of it:

You have 2 4066 and a ULN2001 and of course a propeller. How can you make these drive 4 digits of a 7 segment display which needs 12-15V per segment?

Graham Stabler

In the index sticky is a link to a thread on interfacing with 5v, chip says:

Chip Gracey (Parallax) wrote: »

To input from a 5V system, a series resistor in the 1k-100k range is probably·the best approach.·There are parasitic catch diodes on every pin of the device which will clamp incoming out-of-range voltages to within ~0.4V of the VSS and VDD rails. This will force the resistor to drop 1.3V (5.0V-(3.3+0.4)) for an incoming high signal.

So think yes

Graham

MagIO2

Thank you for the pointer. But it makes the right answer being NO.

I need more than 3.3+0.4V on the input pins of the 4066. So, you are right, that the propeller would not have a problem with this setup, but the 4066 would not switch on ;o)

So, I think I need to add some more transistors/resistors.

tonyp12

This darlington array show a 2v gate inputs
http://www.mouser.com/ProductDetail/ON-Semiconductor/MC1413BPG/?qs=g2rIOKKlpoatr1pKnvCyow%3d%3d

MagIO2

You mean switch on the digits on the kathod side? Yes, but that can be done with the ULN2001.
You mean switch on the 4066? Yes, but that can also be done with a bipolar transistor out of my drawer. Remember, this is what it's all about. I don't want to order parts ;o)
You mean switch on a segment on anode-side? No. That's what I need the 4066 for. My collection of transistor types and FETs is verry small.

Maybe I should use this thread to ask for some useful and "you should definitely have those"-type transistors / FETs ... suggestions please!


PS: Previous post has been edited. Now I understand why a lot of users quote the posts they answer. Now this post is senseless. The original text suggested a N channel FET.
And, tonyp12, the ULN2001 is also a darlington array - it's even pin compatible with the one you suggest now.

tonyp12 said:
This darlington array show a 2v gate inputs
http://www.mouser.com/ProductDetail/...pKnvCyow%3d%3d

tonyp12

NPN and PNP transistors arrays can both source and sink currents.
http://www.electro-tech-online.com/general-electronics-chat/113182-sinking-sourcing-current.html

http://www.falstad.com/circuit/e-npn.html
http://www.falstad.com/circuit/e-pnp.html

MagIO2

NPN and PNP transistors arrays can both source and sink currents.
http://www.electro-tech-online.com/g...g-current.html

http://www.falstad.com/circuit/e-npn.html
http://www.falstad.com/circuit/e-pnp.html

Thank you.
But I think you don't see what the problem is:
I have to source AND sink. This is called a matrix. For example I switch on source for segment b and c and I switch on the sink of digit 1 (common cathode). In this case a 1 appears on the first digit of the display, all other segments are dark because the sources are switched off and all other digits are dark because the other sinks are switched off.
In the next step the segments for the second digit are switched on and the sink of digit 2 is switched on ... this is what's called multiplexing.

Common to all (to propeller and display) is of course GND. The source of the display part is 12-15V. The power supply of the propeller is of course 3.3V. Any transistor that has to switch the source needs a voltage somewhere around the 12-15V.

And by the way I think the problem is solved by adding a biplar transistor (NPN) to the 4066. A resistor pulls the switch input of the 4066 to high and the transistor switched by the propeller can pull it down to low.

tonyp12

Good if you could send a link to this DIY LED PCB,
as complete layout of it is needed to really grasp the problem at hand.

I understand that you have to multiplex
I made this 102 Led's multiplexer
http://forums.parallax.com/showthread.php?129182-New-LED-driver-for-PPDB-comming-soon.

And this 12 led charlieplexer.
http://www.youtube.com/watch?v=8apRaZQbv5w

But that you want to switch 12v and only using parts you have makes it a little harder.
I think using the npn to the 4066 will work.