Schematik Project Blueprint Smart Power Pedestal

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1. 01

   Keep the first test away from AC power

   For this desk test, leave the relay contact terminals marked COM and NO completely unused. Do not connect the PZEM, an outlet, a boat, or any 120/240 V wiring. You are only testing the 5 V relay coils and the safety timer.

   • Tip: Put a small label over the relay contact terminals so they cannot be mistaken for the coil terminals.

   • ⚠ Connecting mains power during this first test can cause electric shock, fire, or damage; the contacts are not part of the desk-demo wiring.
2. 02

   Make a common low-voltage ground

   Connect the negative or GND output of the 5 V USB bench supply to a GND pin on the T3-S3. This shared ground gives the two MOSFET drivers the same electrical reference as GPIO10 and GPIO21.

   • Tip: Use black wire for every ground connection to make the circuit easier to inspect.

   • ⚠ Without the shared ground, the relay drivers may not turn on even if their indicator LEDs light.
3. 03

   Wire the SET coil driver

   Connect relay SET+ to the bench supply +5 V. Connect relay SET- to the Drain (D) of mosfet_set_1. Connect its Source (S) to GND. Connect GPIO10 through gate_resistor_set_1 (100 Ω) to its Gate (G), then connect gate_pulldown_set_1 (10 kΩ) from that same gate point to GND.

   • Tip: On the AO3400A SOT-23 package, verify the pin positions from the seller’s datasheet before soldering; the package is very small.

   • ⚠ Do not swap the MOSFET Drain and Source; the coil may fail to switch or behave unpredictably.
4. 04

   Add the green SET pulse light and diode

   Connect the long leg of the green set_pulse_led_1 to set_indicator_resistor_1 (1 kΩ), then connect the other resistor end to the SET MOSFET Gate (G). Connect the LED short leg to GND. Put flyback_set_1 directly across the SET coil: its striped cathode end goes to SET+ and its unstriped anode end goes to SET-.

   • Tip: The green LED should flash only during a SET pulse. The stripe on the diode is the cathode mark.

   • ⚠ A reversed flyback diode makes a direct short across the 5 V supply when the coil is pulsed.
5. 05

   Wire the RESET coil driver

   Connect relay RESET+ to the bench supply +5 V. Connect relay RESET- to the Drain (D) of mosfet_reset_1. Connect its Source (S) to GND. Connect GPIO21 through gate_resistor_reset_1 (100 Ω) to its Gate (G), then connect gate_pulldown_reset_1 (10 kΩ) from that gate point to GND.

   • Tip: Use a different wire color for the RESET coil route so it cannot be mixed up with the SET coil.

   • ⚠ Do not join SET- and RESET- together; each coil needs its own MOSFET driver.
6. 06

   Add the red RESET pulse light and diode

   Connect the long leg of reset_pulse_led_1 to reset_indicator_resistor_1 (1 kΩ), then connect the other resistor end to the RESET MOSFET Gate (G). Connect its short leg to GND. Put flyback_reset_1 across the RESET coil: striped cathode to RESET+ and unstriped anode to RESET-.

   • Tip: The red LED flashes at startup and when you send LOCK or let the watchdog expire.

   • ⚠ Make sure the LED long and short legs are not swapped — a reversed LED will not show the pulse.
7. 07

   Power and test the desk prototype

   Power the T3-S3 from its USB-C port and power the relay coils from the separate regulated 5 V bench supply. Open Schematik’s serial console after Deploy and send UNLOCK followed by Enter. The green LED should flash once. Send HEARTBEAT every 10–20 seconds to keep it enabled, send LOCK to flash the red LED, or stop heartbeats and wait 60 seconds for the red watchdog pulse.

   • Tip: Send STATUS at any time to see whether the firmware believes the relay is enabled.

   • ⚠ Do not use an unregulated supply or a supply above 5 V for the relay coils; excessive coil voltage can damage the relay.

#include <Arduino.h>

// LILYGO T3-S3 desk prototype: low-voltage relay-driver and watchdog test.
// GPIO10 closes (SET) a 5 V latching-relay coil through its MOSFET driver.
// GPIO21 opens (RESET) the other coil through its MOSFET driver.

// Forward declarations
void pulsePin(uint8_t pin, const char *label);
void openRelay();
void closeRelay();
void handleCommand(String command);

constexpr uint8_t RELAY_SET_PIN = 10;
constexpr uint8_t RELAY_RESET_PIN = 21;
constexpr unsigned long PULSE_DURATION_MS = 100;
constexpr unsigned long HEARTBEAT_TIMEOUT_MS = 60000;

bool powerEnabled = false;
unsigned long lastHeartbeatMs = 0;

void pulsePin(uint8_t pin, const char *label) {
  Serial.printf("[RELAY] %s pulse for %lu ms\n", label, PULSE_DURATION_MS);
  digitalWrite(pin, HIGH);
  delay(PULSE_DURATION_MS);
  digitalWrite(pin, LOW);
}

void openRelay() {
  if (powerEnabled) {
    pulsePin(RELAY_RESET_PIN, "OPEN / RESET");
    powerEnabled = false;
  }
}

void closeRelay() {
  if (!powerEnabled) {
    pulsePin(RELAY_SET_PIN, "CLOSE / SET");
    powerEnabled = true;
  }
}

void handleCommand(String command) {
  command.trim();
  command.toUpperCase();

  if (command == "HEARTBEAT") {
    lastHeartbeatMs = millis();
    Serial.println("[WATCHDOG] Heartbeat accepted.");
  } else if (command == "UNLOCK") {
    lastHeartbeatMs = millis();
    closeRelay();
    Serial.println("[STATE] Power enabled; send HEARTBEAT within 60 seconds.");
  } else if (command == "LOCK") {
    openRelay();
    Serial.println("[STATE] Power disabled.");
  } else if (command == "STATUS") {
    const unsigned long age = millis() - lastHeartbeatMs;
    Serial.printf("[STATUS] power=%s, heartbeat_age_ms=%lu\n", powerEnabled ? "ON" : "OFF", age);
  } else if (command.length() > 0) {
    Serial.println("[HELP] Commands: UNLOCK, LOCK, HEARTBEAT, STATUS");
  }
}

void setup() {
  pinMode(RELAY_SET_PIN, OUTPUT);
  pinMode(RELAY_RESET_PIN, OUTPUT);
  digitalWrite(RELAY_SET_PIN, LOW);
  digitalWrite(RELAY_RESET_PIN, LOW);

  Serial.begin(115200);
  const unsigned long serialWaitStarted = millis();
  while (!Serial && (millis() - serialWaitStarted < 1500)) {
    delay(10);
  }

  // Deliberately pulse RESET once at startup: the desk prototype begins open.
  pulsePin(RELAY_RESET_PIN, "STARTUP OPEN / RESET");
  powerEnabled = false;
  lastHeartbeatMs = millis();
  Serial.println("[SYSTEM] T3-S3 relay watchdog desk test ready.");
  Serial.println("[HELP] Commands: UNLOCK, LOCK, HEARTBEAT, STATUS");
}

void loop() {
  if (Serial.available() > 0) {
    handleCommand(Serial.readStringUntil('\n'));
  }

  if (powerEnabled && (millis() - lastHeartbeatMs >= HEARTBEAT_TIMEOUT_MS)) {
    Serial.println("[SAFETY] Heartbeat timed out; opening relay.");
    openRelay();
    lastHeartbeatMs = millis();
  }
}

“Deploy to device” opens this project in Schematik, where you can flash it to your board over USB.