# Powering an old alarm clock LCD (need to invert DC to 3 V AC)

I'm tinkering with a cheap old alarm clock LCD screen. I think the LCD is multiplexed (there is only one row of connectors along the top of the screen, etched (or something) into the glass, and there doesn't appear to be any common ground or anything from the circuit board the LCD rests against). The clock itself is powered by a single AAA battery. There is no driver circuit board or anything directly attached to the LCD (as many hobbyist purpose-built ones seem to have), so I need to provide the current directly to the LCD. It appears that the screen itself receives about 3 V AC to activate a given region, as measured by a multimeter from the alarm clock circuit board.

So my question: how can I create this AC current from a DC source of around 1.5 V DC? Would it be possible to also do this from 5 V DC (provided by my Arduino)? I would like to avoid (if at all possible) up-converting to 110 V AC first (it seems to be a common recommendation from google searches). I have many basic components (resistors, caps, transistors, pots, diodes, etc) laying around in my 'nerd boxes' which I would prefer to use over specialized components, such as purpose-made inverters or the such, to do this.

As may be evident, I am somewhat new to electronics tinkering and as such may be way off with my expectations. I will respond to comments/answers, and can post pictures once I get home if it would help. I appreciate any help I can get on this :)

You identified the LCD as being multiplexed—which it very likely is—so the AC you're seeing is probably just some segment being sequenced.

A microcontroller toggling pins in succession will indeed produce AC, however calling "x V AC" that is an extreme red herring. It is better described as a multiplexed signal with such a frequency and some voltage (amplitude, not AC voltage as measured by a meter).

Atmel's app note AVR340: Direct Driving of LCD Using General Purpose IO may be of use to you. This portion describes the "AC" that you're seeing and shows why.

To energize a segment, the waveform to that segment must be 180 degrees out of phase with its COM waveform. The voltage difference between the segment and COM signals will therefore be typically 5 Volts AC, which causes the segment to become visible after 300-400 msec of refresh cycles.

To de-energize (turn off) a segment, the COM and segment waveforms should be in phase with each other while that segment’s COM is active, that is, not at 2.5 volts. So, a LCD with all segments OFF will have the segment inputs IN PHASE with each COM input. (COM3 was omitted from Figure 2-2, since only 4 channels were available.)

• That is a useful PDF also, thanks @NickT. Is there any easy way to produce this AC voltage without the AVR340 from DC input? Mar 1, 2011 at 18:40
• @Will, I believe you are interpreting your measurements poorly, I expanded the answer to help I hope. Mar 1, 2011 at 19:00
• That does help, but I am still a little confused. To clarify: you are saying if I have, hypothetically, pin 1 on the LCD to pin 1 on my Arduino, and pin 2 of the LCD to ground on the Arduino and, at a particular frequency, switch on & off pin 1 that it would produce a signal which could drive the LCD (assuming arbitrary pin 1 and 2 on the LCD)? The Arduino Uno (my only microcontroller), when pins set to high, puts out 5V, so I would have to somehow regulate this down, correct (maybe PWM, as recommended by @Madmanguruman)? Mar 1, 2011 at 19:19
• @Will, In order to switch on or off an LCD segment, it must be charged like a capacitor. However, you should not apply a static DC voltage lest the (small) current that flows will damage the LCD. Hence, the suggested method to drive the LCD requires that you apply an in-phase pulse (effectively 0 V) to discharge the segment, or out-of-phase pulses (so the segment sees no DC bias) to charge it. All this info is from the PDF, so read it for more, and look at Figure 2-2 in it. Mar 1, 2011 at 20:29
• I did read that, but it was a little over my head. I'll do a bit of reading on in/out-of-phase pulses so that the PDF will make more sense. I guess I assumed this would be easier than it is :) Thanks for hanging in there and helping. Mar 1, 2011 at 20:38

Incidentally, it's possible to drive a 3-way multiplexed LCD entirely with logic-level signals. To drive the display, run the common wires through all eight combinations of high and low, and drive each segment wire such that at least two of the three segments it controls will be "correct" (a segment will be "clear" if the state of the segment wire matches the state of its common wire, and "dark" if it differs). It turns out that no matter what pattern of light and dark segments one wishes to display, every segment that's supposed to be "dark" will be energized 3/4 of the time, and every segment that's supposed to be "light" will be energized 1/4 of the time.

This is a technique I invented myself in the 1990's, and have used in a couple of product designs. The contrast ratio is the same as what one would get using 1/2-bias (as shown above), but not as good as what would be obtained using 1/3 bias. In my implementation, I had to clock data through shift registers every frame, which used some current; if the circuitry were built into an ASIC I'm not sure how the efficiency would compare with that of a conventional multi-voltage technique.

Note, that Motorola claims a patent on a technique somewhat like this, but they use a six-phase waveform rather than an eight-phase waveform, and they only achieve a 2:1 drive ratio rather than 3:1. Their patent would cover the use of a four-phase waveform to drive a 2-way multiplex LCD, but explicitly specifies the use of 2n phases rather than 2^n.

• +1 because I assume that since much of what you are saying is over my head, it must be correct. Mar 4, 2011 at 16:35

Does your LCD look something like what's in this datasheet?

You may find that using a micro (to generate PWM) is more practical than inverting DC to AC. I doubt that the LCD will draw much current.

• It does look similar to that one! I did try manually powering the LCD using a single 1.5v battery with a 1M pot, but it seems that when I touched my leads to certain regions, a section on the display would appear then dim out, reappearing when I reverse polarity of the leads. Would PWM (which I know the arduino I have can provide) do this? I thought PWM just generated between 0v and full voltage. Mar 1, 2011 at 18:17
• If you AC couple a PWM waveform with 50% duty cycle, you get positive and negative-referenced AC... Mar 2, 2011 at 17:51