I am currently building a 24 V high side switch controlled by a 5 V Arduino NANO I/O.

Source: PLC 24 V out, 2 A (Max) Transistors Available: BC725 PNP BC337 NPN To drive an 1.7 V LED and to get 24 V signal back to the PLC. below is the circuit that I tried, but comes with an observation that the LED was turned on already with I was testing it with power supply 24 V/100 mA without any 5 V input.

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Any idea will be great, or did I do something stupid?


  • \$\begingroup\$ The circuit looks ok although R11 and R13 seem rather small values, should be ok with 10k/22k. Could something be feeding current in via R22? \$\endgroup\$
    – Frog
    Oct 13 '21 at 5:01
  • \$\begingroup\$ R22 is the feedback that i want to send back to PLC I/O 1k Res is for limiting the current \$\endgroup\$ Oct 13 '21 at 5:09
  • \$\begingroup\$ Measure the voltage on R17 and base of the pnp transistor \$\endgroup\$ Oct 13 '21 at 5:22
  • 2
    \$\begingroup\$ The base of Q5 should be at 24V in the off state, if it is then perhaps disconnect R22 and see if the PLC is pulling the LED anode high \$\endgroup\$
    – Frog
    Oct 13 '21 at 5:46
  • 1
    \$\begingroup\$ Sorry, i think i misunderstood, it's BC327-25 PNP transistor. \$\endgroup\$ Oct 13 '21 at 7:23

R13 when pulled low draws 10 mA with 23V across it or 1/4W at burning hot temps . Change R13 to 10x R11.

R18 turns off LED until R19 is pulled high.


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