While we are on the subject of calculators, this one has been ever so slightly -- what is the word? -- doctored for use as an animated name plate. The perfect desk toy for a nutty scientist.
When I found out that the supermarket was selling calculators for £0.50 I got a couple for the kids to use as play phones and a third for my own sinister purposes. I hacked it up for use as a name plate, inserting a 50-pin 6 digit LCD display and soldering some electronic trickery on the back. (I also dropped the keys and put them back all jumbly, but I like it that way.)
The circuit is attributable to BEAM robotics innovator Wilf Rigter. It is based around a single 74HC240 inverting buffer IC and runs happily off a 3 V coin cell.
How it works
The rule for displaying any two messages is as follows:
1. segments which are "on" for both words are connected to the ON line
2. segments which are "off" for both words are connected to the OFF line
3. segments "on" in the first word and "off" in the second are connected to ON'
4. segments "off" in the first word and "on" in the second are connected to OFF'
5. the backplane terminal is connected to the OFF line.
One pair of inverters generate the 30 Hz square wave that the LCD display requires between the backplane and any segments that are "on". Another pair of inverters act as a slow oscillator that switches certain segments in or out of phase with the backplane signal, alternately turning them on or off. Each message is displayed for about a second.
Putting it together
A bit of experimentation will be required if you don't already know the pin out of the LCD. (It is important that the display is the non-multiplexed kind — one pin per segment — otherwise the project is doomed.) The easiest way to make the circuit is not using perfboard as I did for the calculator name tag. Instead, I recommend gluing the 74HC240 chip to the back of the display solder bare wires directly to the component pins. This method of soldering is called "dead bug style" because the components rest on their backs with their legs in the air. I am much more satisfied with this later effort of mine:
Easy but time consuming. It will take a while to work out which segments are always on, always off, or on for only one or other of the messages to be displayed. It is then a fiddly operation to route the ON, OFF, ON' and OFF' wires to the pins that map to those segments. The calculator thing was a bit of an experiment but once I got the hang of it I didn't find the process all that difficult. I made the dead bug style tag pictured above in about 5 or 6 hours.