What's the purpose of a diode to Vcc at an LM339 output?
When power supplies turn off, the diode discharges the capacitor, so it's empty when power supplies turn on again the next time.
I am guessing that this is a power on delay timer, because it reads &quot;time&quot; on the potentiometer, and because this is turning on a relay, so each time supplies are connected, it should have an identical delay.