Two channel relay causes screen dimming

Projects General Guidance
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Hello, all!

If this has already been brought up, I apologize for the duplicate question. I assure you, I searched as best I could.

I'm building a VERY custom thermostat for my garage, using an RTC module, a temp sensor, a 4-button keypad strip, and a 2-channel relay. Without boring you with the details of how this works, let me just say that the focus here is on the 2-channel relay and the 4x24 LCD screen.

I'm also using an 8-channel I2C hub interface for managing my two I2C devices (the RTC and the LCD screen).

In short, when the conditions are met (either manually or by schedule) for the heat to come on, I have one relay activate to turn on the furnace signal, and 2 minutes later the other channel activates to turn on the heater fan. Under no-load testing conditions, I've found that when both relays are on, the letters on the LCD screen dims significantly, almost to the point of not being able to see it. What's more, it dims even more over time (while the relays are on), but comes right back to full contrast.

Once one or both of the channels of the relay go off, the characters instantly come back to full contrast, and stay there.

I am feeding power in through a power supply module (not through the Arduino itself) so power is going to all of these modules directly from the supply. The system works perfectly otherwise. For example, I have time displayed on the screen with a blinking colon (:slight_smile: between the hours and minutes, and it still functions fine, and displays the correct time, and shuts the relays off when it should. It responds to keypresses instantly. I also have a motion sensor which turns the screen backlight off when nobody is around it. I've found that when the backlight is off, the letters on the screen (while still dim) are somewhat easier to read. Once the light is on, they are almost completely washed out.

I have triple-checked my wiring (it's all fine), and systematically replaced all of the components, including the Arduino (Nano every) itself, and it still does this. Can anyone give me a clue as to what's causing this?

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I am completely stumped by this. Thank you in advance!

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Please bore us with a schematic of your system, including power supply.

1 Like
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Your breadboard power supply voltage regulator is probably over heating and hence shutting down.

Measure the 5v to GND voltage with a DMM then run thru the different scenarios.

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Why?

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Because it's the only way I know of to use 2 I2C devices on one Arduino?

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Larry: As I stated, I replaced EVERY component, including the power supply. I even ran power through to Arduino, it amounts to the same. Multimeter shows everything is fine

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Paul: I'll get it drawn out in Fritzing when I get home and upload it! Thank you

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Are you saying when you measure at the LCD power pins the DMM stays at 5v all the time ?

I2C devices can share Arduino I2C pins when addresses of the devices are different.

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Then you've missed the point of i²c bus. Theoretically you can connect as many devices as you like, as long as they have different addresses to each other, to the same 2 Arduino pins. You only need i²c multiplexer when you need to use more than one device with the same address. In practice, connecting many i²c devices to the same 2 pins will eventually cause problems with the impedance of the SDA and SCL lines, and the multiplexer can be useful there also, but that's rarely a problem.

In your case, your RTC and LCD will have different addresses and you will not need the multiplexer.

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Welcome @darthkoder

your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advice on) your project :wink: See About the Installation & Troubleshooting category.

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