59. Optocoupler for MCU Signal Isolation

The MCU runs on 3.3V; the relay driver runs on 24V. If these two sections share a direct electrical connection, noise, ground loops, and voltage spikes from the relay side can damage the MCU.

An optocoupler provides galvanic isolation — the signal transfers across an internal light barrier (LED → phototransistor) with no electrical connection. Even if the relay side has a catastrophic fault, the MCU side is completely protected.

Required: Transistor Output · 1-Ch · ≥3750Vrms isolation · CTR ≥50% · Vout ≥30V · DIP-4 · Through Hole

Go to DigiKey.com → Isolators → Optoisolators → Transistor, Photovoltaic Output

Apply these filters:

• Output Type → Transistor
• Number of Channels → 1
• Voltage - Isolation → 3750Vrms or higher
• Current Transfer Ratio (Min) → 50% or higher
• Voltage - Output (Max) → 30V or higher
• Package / Case → 4-DIP
• Part Status → Active

Tip: CTR (Current Transfer Ratio) = output current / input current × 100%. A CTR of 50% means 5mA input produces at least 2.5mA output. Higher CTR lets you drive the input LED at lower current.

The filtered list will show matching parts. Before you pick one, check: Is it Active? Is stock available? Is the manufacturer reputable? Is a datasheet available?

Full checklist: How to Select a Safe, Production-Ready Component — EWskills Guide

➡ View Matching Parts — Transistor, 1-Ch, ≥3750Vrms, CTR≥50%, DIP-4