@Muse - This circuit detects a 5 Vdc supply interruption, maintains power for several seconds using a backup energy reservoir, and generates an isolated 200 ms switch closure across a target circuit (such as a power-down or graceful reset button).
### Circuit Operation
1. **Power Continuity (Hold-up)**:
- Normal operation delivers 5 Vdc through switch #S1 and isolation diode #D1 to charge the main reservoir capacitor #Ccap.
- When main power drops (simulate by clicking #S1 to open it), diode #D1 instantly reverse-biases, isolating the backup energy from the failed input line.
- For a load current of $I = 0.64\text{ A}$ (represented by load resistance #Rld $\approx 7.8\ \Omega$) operating over $t = 5\text{ to }10\text{ s}$, the required capacitance to maintain voltage above an operating floor $\Delta V \approx 2.5\text{ V}$ is:
$$C = \frac{I \cdot \Delta t}{\Delta V} \approx \frac{0.64\text{ A} \times 6\text{ s}}{2.5\text{ V}} \approx 1.5\text{ F}$$
- In a practical hardware build, use a **5.5 V supercapacitor** (or a small lithium/boost module).
2. **Power-Loss Sensing & 200 ms Pulse**:
- Resistor #R1 pulls node #trig up to the backup voltage, keeping coupling capacitor #C1 charged.
- When the input line #in abruptly drops to 0 V upon loss of primary power, #C1 pulls #trig downward. This falling edge is differentiated and injected through #R2 to momentarily charge the gate node #sw_g.
- Capacitor #C2 and discharge resistor #R3 form a monostable timing circuit set to hold NMOS switch #M1 on for $t \approx 200\text{ ms}$:
$$t \approx R_2 \cdot C_2 \approx 10\text{ k}\Omega \times 20\ \mu\text{F} \approx 200\text{ ms}$$
- Transistor #M1 conducts, pulling terminal #sw_a down to close the circuit in parallel to your target mechanical switch for the required 200 ms window before shutting off.
