There are three ATtiny85 chips used to build this instrument
The code was taken from https://robstave.github.io/acs/
The instrument uses the following three sketches uploaded onto the chips via Arduino:
ACS_85_0003_alternating-freq
ACS_85_0652_Bytebit
ACS_85_0006_squarewave_withPWM
Clever idea. Looks like it would fit on https://hackaday.io/list/4575-atmel-avr-projects .