I’m trying to learn about the opensprinkler design and looking at the schematic I can’t understand why the mcu pin does not get fried.
For example P0 when it’s HIGH (5v / 3.3v) by the MCU or the shift register it turns ON the triac, but it’s aparently connected with a pulldown to the ACRET which I understand it closes the AC part of the circuit to energize the solenoid. How this 24VAC don’t destroy the MCU pin ?
The traic is not that much different from a transistor or MOSFET for switching DC devices. A logical level control signal applied between its gate and MT1 (equivalent to base-emitter in NPN transistor, or gate-source in MOSFET) allows current to flow through MT2 and MT1. Why would you think this will destroy the MCU? This is the basic mechanism of how transistors work.
Thanks for the reply , I understand that the TRIAC is basically a transistor used to switch AC loads, I’m probably missing something very obvious as I’m just learning electronics but why the 24vac is not going back to P0 through the pulldown resistor to 24VAC?
Because ACRET is basically the circuit ground (well, it’s connected to a current sensing resistor which is very small so it’s practically the ground of the circuit). For that reason, ACRET is basically 0V. In a circuit all voltages are measured against the ground, so if that point is ground, then it’s always 0V.