OLED 20x4 Vs OLED 16x2 Issue

Hi all,

Any thoughts on the following?

I have built an RFID reader to be used in cross country races. The device saves RFID tag numbers with a timestamp into a CSV file on a VDrive2.

I developed it using the serial OLED 20x4 screen available on the picaxe store and it worked fine. It was powered by 3xAA batteries and the RFID range was about 10cm.

I then swapped the screen to a 16x2 OLED screen, again from the picaxe store, and the RFID stopped working. When I unplug the screen it worked fine, when I went back to the 20x4 screen it worked fine, but when the 16x2 screen was replaced it stopped.

I plugged the board into a mains power supply and the RFID reader worked with the 16x2 but the range was 0cm and only if I orientated it multiple times.

I feel it must be a power supply issue; however, I would have thought that the 20x4 OLED would be more power hungry than that the 16x2.

Any other ideas what it may be? I much prefer the 16x2 screen for the project, but it currently makes the project stop working..

Thanks for any advice and thoughts in advance.

Best wishes

Jamie

Does the 16x2 OLED work correctly when it is connected to a test circuit?
Have you checked the interface between the PICAXE and the OLED unit?
When you say that the RFID stopped working, how are you checking this - is it that it is not reporting on the 16x2 screen or do you have another way of checking the RFID operation?
The idea that it could be a power supply issue is plausible - I would check the wiring up of the 16x2 unit very carefully.
Just a starter for ten...

I'd suggest two areas to investigate.

1. Isolate the OLED's +ve supply pin and attach a temporary battery pack to it. This will indicate if the OLED is taking more than a reasonable share of the available power.
2. It is possible that the OLED module is radiating a large amount of RF noise, swamping or distorting the RFID reader's field. If this is the case, it may be necessary to shield the OLED module or use longer leads. Alternatively, a largish capacitor across the OLED's power pins may help.

The current drawn by the 16x2 and 20x4 should be comparable when using OLED. The electronics is pretty much the same and the current for the display itself should be proportional to the number of pixels lit.

It could be an issue of RF interference but I would not have expected the 16x2 to be worse than the 20x4 though the different display format may require different refresh rates which could produce some difference.

The big question is exactly how it "stopped working"; whether that is the RFID not responding as expected when using the 16x2 and it all follows from that, or if there is something more to that.

Some testing may be in order to determine exactly what is going on. I would remove the VDrive2 so there is less to test, separate the RFID reader from the PICAXE and display by longer wires, add additional decoupling capacitors.

To check if the issue is RFI stemming from the 2x16 OLED, I would suggest that you (temporarily) wrap the display several times in cling wrap (to insulate it) and then wrap it in tin foil (the kind used for cooking). Make sure the foil doesn't cause a short. Make sure the entire display is covered in the foil and work it around the power/data leads. Then retest your RFID and see if it works.

Another option may be to have it working with the 20x4 display, then separately power-up the 16x2.

It might be enough just to power the 16x2 though one might need to connect it to a separate PICAXE to get it initialised and producing output.

If it works with the 16x2 off but not with the 16x2 on then it would seem to point to the 16x2 being the culprit.

Wow! Thank you for so many useful replies.

To answer the questions raised;

- I have two different 16x2 displays that both exhibit the same behaviour and cause the RFID reader to stop. The screens themselves work and display as expected;
- RFID stops working means that it doesn't 'beep' to acknowledge it has been touched, although all the lights are on;
- removing the serial line between the screen doesn't rectify the problem. When removing the + the screen remains on (suspicious?) but is unresponsive, removing the ground turns the screen off and the RFID starts to read tags again.

I will try:

- extending the leads to have the screen further away;
- powering the screen off a separate power supply;
- measuring the current draw to see if there is a big difference;
- I have already tried placing caps in the obvious places; however, this made no difference.

I will let you know what I find.

Jamie