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I am trying to close the normally open (NO) mechanical relay contacts using the circuit below. circuit. The mechanical relay is Panasonic Relay. I notice that when I give the high command for my GPIO I do see the LED turn-on and I hear a very soft click coming from the relay. Much quieter than I am used to but if I test the resistance across the NO relay contacts there is no connection. On the transistor I read approximately 0.78V on pin 1, 0V on pin2, and 0.026V on pin 3. I read 4.95V on A2 of the relay and 3.14V on the GPIO.

Does anyone know what may be the issue? Also is there any polarity in the A1 and A2 contacts? If so I have not considered this..

The transistor I used transistor datasheet

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    \$\begingroup\$ I see no issues with your schematic. Is your relay a 5V coil? \$\endgroup\$
    – DKNguyen
    Commented Jul 17, 2021 at 19:15
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    \$\begingroup\$ I once had a relay that was polarity sensitive, drove me crazy for a while. \$\endgroup\$
    – Brevor
    Commented Jul 17, 2021 at 19:16
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    \$\begingroup\$ A mechanical relay? Or solid state? I don't see any reason why a mechanical relay would be polarity sensitive. \$\endgroup\$
    – DKNguyen
    Commented Jul 17, 2021 at 19:20
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    \$\begingroup\$ @DKNguyen plenty of mech relays are polarity sensitive because they are biased by a permanent magnet. Try looking up many of the offerings from Omron. \$\endgroup\$
    – Andy aka
    Commented Jul 17, 2021 at 19:28
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    \$\begingroup\$ @DKNguyen - Many small relays use permanent magnets to improve their sensitivity and hence reduce coil power requirements. As a side-effect there may be restrictions on how close together you can mount them on a PCB without interference. \$\endgroup\$
    – Kevin White
    Commented Jul 17, 2021 at 19:41

2 Answers 2

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Read the table in the data sheet: -

enter image description here

You are using an 18 volt relay and you are expecting it to work at 5 volts: -

enter image description here

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The ALQ118 relay has an 18 V coil.

You have it powered from a line marked 5VGreen which suggests a 5 V supply. That's not enough to pull in the coil. You need the ALQ305 version.

enter image description here

Figure 1. The maximum pull-in voltage can be as high as 75% which, for an 18 V relay would be 13.5 V.

Convention is A1 to positive supply but this model doesn't seem to be polarity sensitive.

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