红灯
hóng dēng · HSK 7
red light
🚦 红灯 = 红 (red) + 灯 (lamp/light)
灯 is the same character used in 台灯 (desk lamp) and 灯笼 (lantern); pair it with 红 (red) and you get the red light that halts traffic.
红 — silk radical 糹/纟 + work 工 (phonetic)
红 is a semantic-phonetic compound. The silk radical 糹 (simplified 纟) signals the meaning, since the character originally referred to a kind of pink or light-red dyed silk. The right side 工 supplies the sound. Over time the meaning broadened from the dye color to 'red' in general.
灯 — fire radical 火 + ding 丁 (phonetic)
灯 is a semantic-phonetic compound: 火 'fire' gives the meaning (lamps once burned oil or flame), while 丁 hints at the sound. It is the simplified form of the traditional 燈, which used 登 as the phonetic. Today it means 'lamp' or 'light'.