Deathclock Backup

The TI BA-35 Solar calculator has an easter egg that makes it count down from a screen full of 8's when all the buttons are pressed. This was discovered by a frustrated student in my physics class. We calculated that the calculator would take about 150 years before it reached 0, so we declared it to be the death clock.

We decided that the calculator should stay going for this duration. We could have hooked it straight up to power, but that would not have been as cool as what we had in mind for it. I decided to sustain it with white LEDs. These proved difficult to mount onto the calculator in a durable way, so I created an injection mold with some scrap plastic, positioned three white LEDs with some clay, and injected a stick of hot glue. After a brief employment as a bedside light, this contruction was taped to the calculator with clear packaging tape. Initially, the power came directly from a 3V wall wart supply. This worked for a time, and then the first power outage came. At that point I decided that more resiliency was needed. I made a box that holds two C cells and a relay. The relay's coil is powered by the wall wart, which holds the relay in a position which feeds the wall wart power to the LEDs, but when the power fails, the relay's arm is released and the battery circuit is connected. This solution is ideal since the batteries are physically isolated and so have no leakage. Plus, the relay automatically switches back when the power comes back on. This solution worked for a time, and then a week-long power outage came. At this point I realized that a fourth LED would be required for the C cells to last for around a month, because the light from 3 was slightly below threshold.