I have a couple of 2x20 LCD displays that I bought long before I was aware of Arduinos. They require a -1.5v supply to make the display visible and +5v for the regular electronics. I can get it to work by using an AA battery to provide the -1.5v, but that is not a long term solution.
I'm thinking of making a project with a standalone 328 and one of these displays so I will need +5v (which is easy with a 7805 or LM317) and -1.5v from my nominally 12v supply - which actually varies between 11v and 15v.
Does anyone have any simple idea how to achieve this dual voltage supply?
Adjustable negative voltage regulator.
Regulate down to a constant 9V (as your 12V fluctuates a lot), bias the ground to give -1.5V and regulate the 7.5V down to 5V
Thanks @Riva, but it's probably the bit about biasing the ground that has me flumoxed. Do you have a link to a suitable circuit. I looked at the LM137 datasheet but the example circuit is no help (for me, at least).
The bias circuit probably draws very little current, and it is easy to get about -3V at a few mA from a port pin on a 5V Arduino, or -1.5V on a 3 V Arduino.
You simply use tone() or output PWM to a pin and use the attached diode/capacitor inverter circuit.
An LDO regulator, Zener diode, 2-3 forward diodes in series, or properly chosen voltage divider can produce the required ~ -1.5 V.
Finally got around to trying this out and it works very well. By trial and error I discovered that a 0.1uf capacitor connected to the Arduino pin gave the desired voltage for the display. A 1uf cap resulted in a too high voltage.
I found this thread researching the "power" issue and didn't want to start a new thread. By chance, is there a wall wart out there that has multiple voltage/current outputs? I have a Mega with Ethernet shield, Data Logging/RTC shield and a servo shield that I am wanting to get into use. The servo shield (adafruit) requires a 5 volt power supply to be fed into it's 2 port terminal block to power the servos. I have read that the Mega requires just a bit more power than that and realistically, down the line I will be wanting to add a few more shields into my project and I expect that power needs will be different from shield to shield.
I am also using a passive POE injector to send the 12v supply down the cat5 to the shield stack. The 12v feeds into the barrel connector on the Mega and I assume distributes power to each shield as they need? With the exception of the servo shield I guess as it needs more (other source) power than what is offered in the 5v pin?
So (yes or no) is there such a wall wart that can feed different power supplies to suit the needs of the entire stack? Otherwise, is my other option to to build jremington's above circuit in accordance with each different power demand?
First, @jremington's circuit is for a very specific situation where I needed a small amount of power at a voltage below zero. It has nothing to do with powering shields or motors. My application is probably drawing a milliamp from that circuit and all the power is coming from an Arduino I/O pin. The absolute max from an IO pin is 40mA. My total system (a barebones Atmega 328, the LCD screen and a temperature sensor) draws about 12 mA from the battery.
I am not aware of wall-warts that have multiple outputs at different voltages.
The principal reason you can't power motors from an Arduino (Mega, Uno or any other) is because they cannot supply enough current for a motor or a servo.
There are AC/DC power packs that supply both 12 V and 5V -- the old IOMega external floppy and hard drives used these, and you may be able to find one at a thrift store or computer recycling shop.
