Switching Things Up

Believe it or not, another year is already coming to an end. For many of us, that means it is time to pick out a new calendar to hang on the wall or set on a desk. But if a paper calendar is a little bit too boring for your tastes, then a recent project created by Jens of FriendlyWire might be more your speed. Jens has created a very unique calendar with more than enough LEDs and switches to please any electronics hobbyist.

The design is very basic; the calendar consists of a flat panel with 30 switches. Each switch is paired with an LED that lights up when the switch is flipped. You can probably see where this is going — you manually flip a switch each day to track which day of the month it is. This works well enough for a display piece (so long as the month doesn’t have 31 days!), but adding up all the lights can get tiresome.

For this reason, Jens also included an analog gauge on the front panel that points to the day of the month. Like the switches, this gauge also comes with a caveat in that it can only count up to 24. That may not matter if this calendar is just something you're toying around with; however, if you do want to support 31 days and actually use this as a calendar, that is possible with a few small modifications.

To make the gauge function, Jens used a classic analog panel meter — the kind normally used to display voltage or current. By running each LED from a 5-volt supply through its own resistor and switch, every illuminated LED draws roughly the same current. When all LEDs are wired in parallel, their currents add up. Accordingly, the more LEDs you turn on, the more current flows, and the farther the panel meter’s needle moves.

But panel meters are delicate instruments. They only handle small currents, often under a milliamp, while the full array of LEDs can draw thirty times that. To safely scale the current, Jens used a shunt resistor — an adjustable 100-ohm, 25-turn potentiometer — wired in parallel with the meter. By tuning this resistor with all LEDs switched on, the current through the meter is limited so the needle precisely reaches the edge of the scale.

Getting this right required determining two characteristics of the panel meter: its internal resistance and its saturation current. After removing the series resistor that made the panel meter function as a 50-volt voltmeter, Jens measured the coil’s resistance, then used a multimeter to find the needle’s full-scale current. With those values in hand, Ohm’s law did the rest, allowing him to calculate the necessary shunt range.

Once wired up and framed inside a modified picture frame, the calendar is an eye-catching piece that mixes retro analog charm with lots of glowing LEDs. It sure beats hanging up another paper calendar filled with nature scenes or funny quotes!