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16 X 2 LCD Noise Issue

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Suraj143

Active Member
In my design I use 16X2 LCD in 4bit mode.

From controller to LCD it uses a 1Feet ribbon cable.

My LCD code works nicely but the only problem is when relays & contactors operating the LCD shows unseen characters & sometimes the LCD got blanked.

I have placed 1N4007 across relays.

How do I overcome this problem?
 
Panic mode the circuit is normal & its working in the workbench & in simulators.

This is a common problem in the industry that noise adding to the LCD.Specially from relays & contactors.

I need some ideas to prevent noise coming to the circuit.
 
i am not very good at understanding "circuit is normal", i prefer to see some circuit. simulators have their place but they can never replace real thing. for example how did you specify in the simulator length of the wires or their proximity to relays?

btw. i am using LCD without issues on a project with 96 relays operating on 24VDC in a shoe-box sized enclosure and LCD cable is about a foot long.
 
btw. i am using LCD without issues on a project with 96 relays operating on 24VDC in a shoe-box sized enclosure and LCD cable is about a foot long.

Nice to hear such a thing.

I have a PIC controller card.It is driving 2 relays from two D313.From relay outputs its driving 2 contactors.I have added free wheel diodes on 2 relays.
PIC also driving a 16X2LCD but the LCD board is separate from controller board.It uses a 1Feet ribbon cable to LCD board.

The relays & contactors are placed near the controller card.The design working fine.Sometimes when contactors are operating the LCD will blank out or some unseen characters are displaying.Note that the PIC never get resets & underneeth the code is working fine.But without the LCD display characters very hard to identify the process.
 
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there could be different issues:

- are you keeping LCD enable and write active all the time?
- are you tristating any of the LCD signals?
- how is LCD power derived/isolated from rest of the circuit? do you at least have capacitor near LCD?
 
there could be different issues:

- are you keeping LCD enable and write active all the time?
- are you tristating any of the LCD signals?
- how is LCD power derived/isolated from rest of the circuit? do you at least have capacitor near LCD?

Very clever points.

*I tied R/W pin GND all the time.So it is write activate all the time.

*I don't Tristate any of the LCD lines.

*LCD is powering from the same 5V power which is also powering PIC.

*I use a 0.1uF cap on LCD board.

Note that I uses 4bit mode on LCD, Pins D0 to D3 I left open do I need to GND those?
 
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You don't have an LCD noise issue; you have a relay noise issue.

Are the relays also powered from the 5V? Where are the ground side of the relay drivers tied to the PIC ground?

Most people avoid these kind of problems by using opto-isolated relay drivers with no common connection between the relay power supply and the PIC power supply.

If you posted a schematic of the whole set-up we could help you more instead of just playing "twenty questions"!
 
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Are the relays also powered from the 5V? Where are the ground side of the relay drivers tied to the PIC ground?

Most people avoid these kind of problems by using opto-isolated relay drivers with no common connection between the relay power supply and the PIC power supply.

If you posted a schematic of the whole set-up we could help you more instead of just playing "twenty questions"!

I'm sorry I don't have a schematic otherwise I'll sure upload it.

My supply is 24V.I use 24V relays.24V taking to relays from input side of 7805.Yes relay GND & PIC GND is same.
 
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My supply is 24V.I use 24V relays.24V taking to relays from input side of 7805.Yes relay GND & PIC GND is same.

So there is only one 24V supply. What circuit is driving the 24V relays? Is the "Com" pin of the 7805 tied with a very short wire to the Vss pin on the PIC? Where is the ground side of the relay drivers tied? To the 7805 Com pin, or a separate wire to the negative terminal of the 24V supply?

A hand-drawn block diagram would be nice. You can use pencil and paper, and use your camera to convert it to something you can attach here.
 
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Here is the circuit.It has only one ground.

The red area is the LCD board with 1Feet ribbon cable.
 
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Look at the attached figure. Connect R all the way back to the filter capacitor at the OUTPUT of the 24V supply. Connect G all the way back to the filter capacitor at the OUTPUT of the 24V supply. Make it so NONE of the relay currents flow though the PIC board.
 
I would add a second electrolytic to the 5V side of the 7805. Another thing to try is a 1k resistor from the Enable line to ground to ground out any spikes in that line.

Mike.
 
Look at the attached figure. Connect R all the way back to the filter capacitor at the OUTPUT of the 24V supply. Connect G all the way back to the filter capacitor at the OUTPUT of the 24V supply. Make it so NONE of the relay currents flow though the PIC board.

I'll give a try.Thanks
 
Do you have snubbers across the contacts of the relays and contactors?
 
diodes are fine, transistors and relays are fine. common ground is fine. if you have scope, check power rails. you may have loose connections or transients causing brownouts. PIC may be more tolerant to this condition (wider supply range) or simply recovers faster. if you don't have it already add larger cap across 5V and GND. 47-100uF should do.
 
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