In the words of JHS Pedals:

Pink Floyds Breathe, Jimi Hendrixs Star Spangled Banner live at
Woodstock 1969,” and Robin Trowers Bridge of Sighs cemented the sound
of a photocell-based vibe effect as an electric guitar standard. The
JHS Pedals Unicorn is our version of this heralded effect, continuing
the inspiring modulation/phasing/rotary sounds that never fail to push
musical boundaries. And just like the originals, we included a toggle
for choosing between a Uni-Vibe* effect and a traditional vibrato
setting. What sets Unicorn apart from all other vibe-style circuits is
that it is the first-ever fully analog, photocell-based vibe with tap
tempo as well as ratio, external tap control, and external rate control
via expression pedal (optional). And the Unicorn does it all with only
four knobs and a mini-toggle switch. In a day and age when everything
has been seen and done, we bring you the first-ever, 100% analog vibe
photocell modulator with onboard tap tempo. Youre welcome.