1) IC1D is a CMOS Schmitt trigger oscillator at about 2KHz. It starts and continues to oscillate with a supply down to 1.24V (the lowest output voltage of my LM317 variable power supply) or less.
2) IC1A is an inverter.
3) IC1B is a Schmitt trigger NAND gate. Its output is low only when both inputs are at, or higher than the upper Schmitt trigger threshold voltage.
Plants Watering Watcher – [Link]