I have to connect PIR sensor HC-SR01 to Arduino using RS485 such that PIR sensor's TTL signal gets converted to RS485 while transfer and gets converted back to TTL while receiving on Arduino side. Since my circuit is not working as expected I replaced Arduino with a LED for testing. Can any let me know what is wrong with below circuit? my LED gets on for few seconds as soon as I give power and goes off after that indefinitely. Ideally it should be on and off when there is a motion in front of PIR sensor.


simulate this circuit – Schematic created using CircuitLab

the data sheet for pir sensor is https://www.mpja.com/download/31227sc.pdf

my objective is

----> TTL ----> RS485 transmitter ---------> Long Cable-------> RS485 Receiver -----> Arduino


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    \$\begingroup\$ Do you have an LED on your PIR sensor? If so, what does it do? If not, why not? \$\endgroup\$ – brhans Nov 2 '20 at 13:17
  • \$\begingroup\$ Please describe your troubleshooting steps. Does the output of the PIR go high? Etc... \$\endgroup\$ – Mattman944 Nov 2 '20 at 13:25
  • \$\begingroup\$ hi brhans, my pir sensor hc-sr501 has no built-in LED. this is how it is manufactured. not sure why. components101.com/hc-sr501-pir-sensor \$\endgroup\$ – Ravi Nov 2 '20 at 13:36
  • \$\begingroup\$ hi Mattman944, yes the LED goes high only one time as soon as i turn on and then switches off. for troubleshooting I just did some continuity check and PIR sensor testing as a standalone unit. as a newbie in electronic i am not sure how to proceed in such situations might be this forum can help me in that. I am completely blocked here. \$\endgroup\$ – Ravi Nov 2 '20 at 13:41
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    \$\begingroup\$ Can you put two LEDs and check. Add one at PIR output. Check if both LEDs behave same. \$\endgroup\$ – Mitu Raj Nov 2 '20 at 14:53

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