The scenario
A TTP223 capacitive touch sensor needs to switch a mains-powered AC appliance safely through a relay, with transistor drive and flyback protection.
What is in this drawing
Read the decisions behind it.
01
Use an NPN transistor to sink relay coil current from the TTP223 output
02
Keep AC and DC grounds isolated by using a separate neutral net
03
Include flyback diode across relay coil and fuse on live
Reuse for any low-voltage touch or control signal switching an AC load via relay.
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