Hello, I'm sitting here with some temperature code and a LM60 sensor. What is so interesting - to me anyway - I plug a 20X4 display into the Arduino and I have 78F displayed. With NO changes what so ever to the code or other hardware, I remove the 20X4 LCD display and plug in a 16X2 LCD display, press RESET to get a clean start and 71F will be displayed. I switch back to the 20X4 and 78F is displayed, switch back to 16X2 and 71F is displayed.
What is really neat, I have an alarm point set at 75F and with the 20X4 display showing 78F the alarm trips. Put the 16X2 back in with 71 alarm is ok.
Is this possible? Am I dreaming? What's going on? 71F is close to room temperature.
OK, I just figured it out. The Arduino is powered from USB port - with the 16X2 display "line" voltage is 4.95vdc. With the 20X4 display the voltage is only 4.75 vdc. That drop changes the calculations.
Had me going for a bit there - but is it normal for the USB power to drop that much?
I powered Arduino from external 9VDC battery and the voltage was at 4.98 and within 1 degree.
Thanks for being a sounding board, and offering any ideas/suggestions.
Does the bigger display have a backlight and the smaller one doesn't ?
In theory USB good for 500mA and 5 volts, but I suspect the voltage might drop a bit with current. I get a similar voltage (4.8V) with a backlit 16*2 LCD. I've never tried it without the backlight though........
I'm sure the backlight is the cause of the voltage drop - both are on bright. It's just the 20X4 display has the larger backlight.
It just threw me for a loop when I saw the temperature change 7 degrees F just by changing the display... When the alarm point tripped, I realized there was something internal to the Arduino and not the display itself that was causing the change, that's when I checked voltage.
Thanks for all the help with this learning stuff. I'm closing in on getting the code for the project running. It seems to be working, but we all know once it's transferred to a PCB with actual sensors there always something to go wrong
A solution to the temperature problem would be to use Dallas digital thermometers, voltage differences won't affect them. I'm a big fan. I get them direct from China on Ebay since they tend to be a bit pricey from UK dealers (especially Arduino dealers).
I had looked at those 1-wire sensors, but since I didn't have any idea how to make the Arduino read those, I just used something I understood a little - voltage reading. Then a conversion to temperature.
Perhaps as I go along I'll learn the 1-wire coding and will be able to use that - that would open many more doors... I think anyway?
