19th February 2012 01:34 AM
Questions on upgrading my Project
I have completed a free project posted from "Talking Electronics website - http://talkingelectronics.com/projec...Display-1.html". I upgraded the code from 16f84 to 16f628a before the site had updated theirs. The project worked great all buttons worked the way they are programmed to. The orientation as you see below in ATTACHMENT#1, the LEDS are in a setup of Anode Columns and Cathode Rows.
Well I have duplicated this project again using male and female pin headers to attach multiple boards and I want to upgrade the led board to three 5x7 LED Display Modules(datasheet is ATTACHMENT#2 below), LTP-2157AKA. I was in a hurry while ordering from DIGIKEY and didn't pay attention to what I was buying. I ordered a module with Anode Rows and Cathode Columns.
My question is: Can I switch something up to be able to use these modules or order a different set? All help would be awesome.
19th February 2012 07:25 AM
It can be done, but I don't have time to work it out in detail at the moment.
However, yoiu may have to make some small changes to the PIC programme.
Are you able to do this?
19th February 2012 08:20 AM
If you were able to change the program, it should be no problem changing the outputs to be inverted, right?
Then you would just need to swap the NPNs for PNPs and swap the +5v and gnd rails going to them. If youre doing this on a breadboard it should be easy to change the rails, on a ready made pcb it might be harder.
19th February 2012 11:58 PM
Yes I can make changes as I have the orginal code. I would take a little longer than most to change it up. as far as the rails being changed is where i get lost. I am making some breakout boards for prototyping this project. I appreciate the responses.
Last edited by Sloetyme; 20th February 2012 at 12:07 AM.
20th February 2012 01:05 AM
I have studied the circuit & I have found it is more complicated than I thought.
The complication is due to the buttons.
I'll need to think about it further to resolve this issue.
20th February 2012 02:56 AM
Len. I'm using tactile momentary push-buttons.
I will make what I call some more "breakout boards" tomorrow.
I also will attach the code if that will add more help.
Talk to you later.
20th February 2012 05:41 AM
The type of button is irrelevant to the issue.
The issue is related to the fact that we need to transpose the collectors and emitters of the column driving transistors so they become inverting switches is rather than emitter followers.
However, once we do that, it prevents the button function from operating.
In the attached diagram, I have only altered one of the column driving transistors, one LED and one of the row driving transistors in order to show you the basic idea.
The solution I have thought of is to use MOSFETs (such as the 2N7000) in lieu of the NPN bipolar transistors that are associated with the button function, i.e. those inside the red box on the diagram.
The 7 transistors at the bottom, i.e. the row the drivers, must also be transposed so they become emitter followers rather than inverting switches.
I don't need to see the code since this arrangement means that you will not have to change the programme.
The MOSFETs should be connected such that the sources are connected to ground, the drains are connected to the 82R resistors and the gates to the 1K5 resistors.
You will have to isolate the tracks as shown in the drawing so that the 5 Volt is connected to the collectors of the row driving transistors and ground is connected to the emitters (and the MOSFET sources) of the column drivers.
You should check where the tracks go to ensure that you are not isolating something that should not be
If you have any questions, just ask.
20th February 2012 10:18 PM
Ok I will try this route, I have plenty of 2n7000's to play with. I like the idea you came up with. Gimme a couple of days to proto-type this and i will respond will results. Your thoughts were great.
20th February 2012 10:28 PM
If you have plenty of 2N7000's available, then all of the column drivers can be 2N7000 if you wish.
If so, you can use the 2k2 resistors on their gates in lieu of the 1k5 resistors I indicated.
The resistors are much less critical for the MOSFETs than for the bipolars, since the bipolars need sufficient base current to be reliably saturated. (the base current needs to be at least 10% of the collector current).
I estimated that the collector current will be about 28 mA, ie. there will be 28 mA through the LED that is on.
Later, I'll do an edit and post a simplified drawing of the arrangement as it will be easier to see how it works.
20th February 2012 10:34 PM
This would not work for the row drivers since the 4017 outputs are Active High and this can't be changed unless you used inverters.
Originally Posted by kubeek
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