Nuts and Volts - January 2015

that repeats it 136 times to display the complete message two times. The scrolling does work, but sometimes it starts at the beginning of the message and other times it starts at the 41st character. This action seems to be random.

As I stated, I have tried inserting several different commands just before the scrolling routine but none of them have worked. These commands include repositioning the cursor to position 128, the start of the first line, homing the cursor ( 2), and adding a cursor right and a cursor left command after that. None of these measures seems to have any effect.

Does anybody know of a reliable way to have this scrolling message always start at its beginning? If possible, I need a solution that works on all HD44780-based displays. I know this is a tall task and may need to use different approaches for different displays.

#1153 Paul Alciatore

Beaumont, TX

[#12142 - December 2014]

Unregulated Power Supply

I picked up a 9V switching power supply wart from one of the online suppliers that advertise in Nuts & Volts.

Unfortunately, it melted two Arduinos before I realized what was happening. Apparently, the no-load voltage on a switching supply can be up to twice the rated voltage — in my case, 18V. Should I add a load resistor to the internal circuit of the wart so there's always a load? Any other suggestions?

All you need is an appropriate three-terminal voltage regulator to protect your Arduino boards. I don't know what voltage your board needs, so I suggest integrating an LM317T programmable regulator between the board's power jack and the board. The regulator will take the wall wart's high voltage and reduce it to what the board requires. To program the regulator, connect it between the wall wart and your board's DC input using the Figure 2 diagram as a guide.

Wiring isn't too critical ( point-to-point), but keep lead lengths as short as practical. Be sure to include the two capacitors as they'll help "keep things sta-ble" and prevent unwanted high frequency oscillations. NOTE: The voltage rating of C2 should be above 24V as it's typically an electrolytic. Variable resistor R2 will let you dial in the output to what your board requires. BE AWARE: Keep the maximum current draw BELOW one amp; use a heatsink on the regulator (a TO-220 type clamp-on will be sufficient) and keep the regulator's input voltage below 35 VDC!

If you don't want to bother with a variable resistor for R2, visit www. reuk.co.uk/LM317-Voltage-Calcul ator.htm as it has a nice table of standard resistor values to use for R1 and R2 for programming various output voltages (and other good info on the regulator).