Electronic Circuits and Projects Forum

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?

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.

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.

Tom,
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
isolated.

If you have any questions, just ask.

Tom,
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.

Originally Posted by kubeek

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.

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.