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sensor:pcf857x [2026/05/14 16:11] – vamsansensor:pcf857x [2026/05/14 16:58] (current) – vamsan
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 The current limits of the PCF857x are heavily lopsided because of its quasi-bidirectional architecture. It handles current entirely differently depending on whether you are sinking current (outputting 0/LOW) or sourcing it (outputting 1/HIGH). The current limits of the PCF857x are heavily lopsided because of its quasi-bidirectional architecture. It handles current entirely differently depending on whether you are sinking current (outputting 0/LOW) or sourcing it (outputting 1/HIGH).
  
 +The [[arduino:display_lcd#i_c_lcd_adapter|I²C LCD adapter]] utilizes the PCF8574.  
  
 **Main Control & Power Header** **Main Control & Power Header**
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 {{ :sensor:pcf857x_wiring_1.png |PCF8575 I/O How to Correctly Wire Components}} {{ :sensor:pcf857x_wiring_1.png |PCF8575 I/O How to Correctly Wire Components}}
 +
 +==== Wiring with relay modules ====
 +Connecting optocoupled relay modules to the PCF8575 is a common practice, but it requires strict electrical care. Because of the chip's asymmetrical quasi-bidirectional ports, a mistake in how you trigger the relay will prevent it from working entirely.
 +
 +**The Triggering Rule: Active LOW Only**
 +
 +  * **Why Active HIGH fails:** Standard optocoupled multi-channel relay modules require 2-5 mA of logic current on their IN pins to light the internal infrared LED and engage the circuit. Because the PCF8575 can source only 0.1 mA when outputting HIGH, it cannot trigger an Active HIGH configuration.
 +  * **Why Active LOW works:** When configured as Active LOW, the PCF8575 acts as a ground connection (0V), cleanly sinking the current from the relay module. The PCF8575 can safely sink up to 20 mA per pin, easily meeting the relay's current requirements.
 +
 +For example, the HL-54 (4-channel 3.3V optocoupled relay module) can be directly connected to the PCF8575. Because the HL-54 is hardwired as an Active LOW module, it is perfectly suited to the PCF8575's current limits.
  
 ===== I²C topics on lamaPLC ===== ===== I²C topics on lamaPLC =====