Your schematic has some wrong component values. You need to use the values from the original circuit at http://interface.khm.de/index.php/lab/experiments/theremin-as-a-capacitive-sensing-device/. It shows there that the IC is a 74HC00N.
Your schematic has some wrong component values. You need to use the values from the original circuit at http://interface.khm.de/index.php/lab/experiments/theremin-as-a-capacitive-sensing-device/. It shows there that the IC is a 74HC00N.