PIC18F4550 LCD shows garbage when switching 12V solenoid valves via relay

I have made a project which interfaces PIC18F4550 with 16x2 LCD,Presure sensor, two motors(5V) and two 12vdc solenoid valves. The chip is programmed to run the motors and calculate pressure built up in pneumatic line and display on LCD . My problem is that when i do not operate valves the display works fine. But when i swith it on and off through relay that is, my LCD shows garbage text, Please help me out with a solution

Hi Prerna

Some things to check:
1. Do you have back-emd diodes across the relay coils & solenoids?
2. Enough decoupling (also on the LCD power pins)?
3. If you stop updating the LCD in software and then switch the solenoids, is the LCD
still corrupted?
4. Is the LCD data and control lines routed far from the solenoids?

Good luck
Pieter.

Hi Pieter,

Thanks for replying..
1) i have put diodes across each relay.but not on solenoid
2)Not done decoupling on LCD power pins how to do that please guide.
3) First time when i run the system its fine the moment i press a push button which restarts the equipment and it carries out the prev test the LCD shows garbage,
4) LCD data lines arent shielded is that a problem? and all this is inside a small box
80 x 385mm

Hi Prerna

For the LCD decoupling caps, start with a 100nF capacitor and connect this between the Vcc and GND pins of the LCD, as close to the LCD module as possible, i.e. directly on the module pins. (the 0.1 inch header.)

It is possible that an LCD operation is not setting the register select (RS) line when it should. Is the LCD initialized again when the push button is pressed? It sounds like something is happening when the push button is pressed and that is triggering the corruption, i.e. what is different between normal operation and when the push button is pressed? That's the key.

I doubt unshielded LCD lines is the problem. Do some measurements on the LCD lines with an oscilloscope (when the push button is pressed). Usually with LCDs it is timing that is the issue, i.e. not observing the bus times when setting data and enable lines etc. (I assume the cable to the LCD is also relatively short, i.e. only a few inches.)

Good luck.

Hi,

The push button is to start and restart the whole program. shiuld i put LCD_Init() in the while "push button is pressed " loop? Also i observed i disabled the valves one by one, there is one valve which doesnot create this LCD crash prob.But when i disable this and switch on the other the prob re appears. So i dont know what could be wrong?

By the way i tried LCD_Init() after i press push button and it seems to work now so far didnt get the garbage text. thanks will get back if it fails after a couple of tests