Does anybody know what that "f" looking sign is next to the 5, left-hand bottom corner of the LED labeling?:

It Indicates the diameter of the LED package.

Summary of Conclusion: Don't seek precision unless you have the means to obtain it.

...I'm a young electronics hobbyist, who has the passion to become a robotics engineer at later stages of my maturity.

Lately I have been tinkering with my first circuit designs, simple stuff really. My first circuits were "plug-and-play", based off of rough, highly in-precise calculations, but it would get things lit like the image below:

Simple circuit

Later, I decided to revise my scketch, adding in LED's and using a more scientific method to make more precise calculations about the circuits functionality...

Long story, but I got my circuit working. Thanks.

  • $\begingroup$ See if they are 5mm diameter. $\endgroup$
    – Eric S
    Sep 19, 2017 at 1:34
  • $\begingroup$ @EricShain They are. I can't get my circuit to work with KVL. If I add resistors, it just won't light up. $\endgroup$
    – Iam Pyre
    Sep 19, 2017 at 1:36
  • $\begingroup$ I think the symbol refers to the LEDs diameter. As for the circuit, maybe a EE can help. $\endgroup$
    – Eric S
    Sep 19, 2017 at 1:38
  • $\begingroup$ Have you fixed your multimeter problems in your earlier question? If not, treat your measurements as random numbers until you do! It could be anything - flat battery in the meter, defective power supply, worn-out contacts on the breadboard, etc, etc - or just "user errors". $\endgroup$
    – alephzero
    Sep 19, 2017 at 2:21
  • $\begingroup$ @alephzero A lot to say, but I believe the multimeter problem was more error on my side. First, I didn't understand KVL and it's relationship to voltage drops, I get that now and the readings make sense. I applied KVL to a simple circuit, 9v battery and three resistors. I got the appropriate readings at each point in the circuit. But now, I can't get KVL to work with a series circuit of resistor and led, I can only get my leds to light with resistor if I put the LED's in parallel. A lot to talk about here, will post it with pictures later if I don't solve the problem. $\endgroup$
    – Iam Pyre
    Sep 19, 2017 at 2:27

1 Answer 1


Note the circle thru the middle of the symbol. That symbol is telling you that the following number is the diameter. Somewhere else it probably says that millimeters is the default unit of length.

So, these LEDs are 5 mm in diameter. That size is also known as "T1 ¾".

  • $\begingroup$ Is that a standard CAD symbol ? Sadly (or amusingly), us mathematicians see a line integral :-) $\endgroup$ Sep 19, 2017 at 13:25
  • $\begingroup$ I checked, the ISO symbol is a $\phi$ ... $\endgroup$ Sep 19, 2017 at 13:26
  • $\begingroup$ @CarlWitthoft are you sure the symbol is not Ø (diameter) instead of ϕ (small phi) like n your example. Yes it is a standard cad symbol to allow you to draw side projections of rotationally symmetrical items without needing the end projections. $\endgroup$
    – joojaa
    Sep 21, 2017 at 5:06
  • $\begingroup$ @joojaa you are correct -- I just grabbed the first LaTeX symbol that came close. And thanks for the info about the original symbol $\endgroup$ Sep 21, 2017 at 11:17
  • $\begingroup$ whoever made that package label probably also grabbed a handy symbol in whatever font they had to work with. $\endgroup$
    – agentp
    Sep 22, 2017 at 1:32

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