The time fountain works by using flashes from UV LEDs to illuminate falling drops of fluorescent dye. If the drops fall at a consistent rate, the UV LEDs can be flashed at the same frequency as the drops are falling so that the drops appear to be suspended in mid-air. Or, if there is a small frequency differential, it appers that the drops are rising upward.
I used a 555-timer configured for low-duty-cycle astable operation, with a 100 uF timing capacitor, 100 ohm resistor on the charging path, and 10k ohm pot on the discharging path. This corresponded to roughly 15 ms on-time and 0.5 Hz to 20 Hz frequency. An n-channel FET switches the LEDs.
While it would be nice to detect drops and reset the cycle so phase errors don’t accumulate, the effect is still pretty good with a simple timer as long as you have consistent drops.
I bought some fluorescein dye from Ebay but it turns out that highlighter dye works just fine and doesn’t precipitate out of solution like the fluorescein. I diluted the dye from one highlighter into about 100 mL of water.
To make the drops I started by punching a hole in a food container with a needle, but wasn’t able to achieve the consistency I wanted. The best solution I found was to punch a hole in the bottom of the container and use a fish tank aeration valve, sealing with super glue. This was able to produce consistent drops, with an easily adjustable drop rate.
It’s a cool effect to see in person, but difficult to capture on camera because of the lighting conditions. Here are a few videos that do a better job of showing off the effect: