i am trying a relay circuit. I saw one diagram online in google images and another one when i actually test. Here are the two diagrams:

enter image description here

Diagram 1

This one i tried but not worked for me. But its a common diagram available on all over the internet.

Although it did worked once for me. But not sure because if its relay or something else.

Diagram 2

This one is working absolutely fine in when i actually make this circuit on bread board. But not sure if this is the correct way to connect to relay. As my emitter is connected to coil pin 1.

Which one should i refer to? In case if i am making any mistake in understanding something then please guide.

Here is my relay: 5V 7A 5Pin PCB Mount

enter image description here

Any suggestions the right way!

Thank you!

  • 2
    \$\begingroup\$ Simple explanation: You got the transistor backwards. 1 and 2 are the same circuit except the transistor is the other way around. 2 is wrong, but if 2 works for you then it means you actually built 1 when you thought you built 2 because you got the transistor legs mixed up. \$\endgroup\$
    – user253751
    Jan 4, 2021 at 11:55
  • 1
    \$\begingroup\$ Diagram 1 is correct. \$\endgroup\$ Jan 4, 2021 at 11:56

1 Answer 1


The first diagram is correct. The second one will "sort-of" work with enough base drive because the transistor is in inverse mode. The low beta will mean that 10K is too high for it to work reliably.

If I was to guess what happened with your tests, perhaps you used the incorrect pinout for the "European" BC547, which is just the reverse of the typical US part:

enter image description here

Japanese parts such as (2S)C8015 have a third standard pinout in TO-92.

  • \$\begingroup\$ That seems to be a very likely case! I should be using US parts then. Instead of EU one. How can i select between the two in EasyEDA? \$\endgroup\$ Jan 4, 2021 at 12:50
  • \$\begingroup\$ @user3201500 I wouldn't worry about which parts you use, as long as you look up the specific pinout of the individual part you have rather than relying on "standards" that are violated more often than not! \$\endgroup\$
    – Hearth
    Jan 4, 2021 at 13:48
  • \$\begingroup\$ Pick a part number that starts in 2N .. such as 2N4401 but there's nothing wrong with BC547 or other parts, and they are very common/inexpensive, just pick the right one for the footprint and symbol mapping. Some parts are even made with more than one pinout and a similar part number so check the exact manufacturer datasheet. \$\endgroup\$ Jan 4, 2021 at 13:59
  • \$\begingroup\$ @Hearth you are right, but sometimes things work and sometimes dont, so its just get confusing. \$\endgroup\$ Jan 4, 2021 at 14:24
  • \$\begingroup\$ @Spehro yes you are right. If i pick something from 2N series it has almost same pin identity. \$\endgroup\$ Jan 4, 2021 at 14:25

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