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Why can't we use a 5mm LED as the flasher in this circuit? What's the difference between a 3 mm and 5 mm led? How do you identify one?

enter image description here

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  • \$\begingroup\$ 5-3=2mm. Diam. Resulting in lower intensity \$\endgroup\$ May 29 '19 at 15:57
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Nothing in that says it needs to be 3mm. However it does say an ORDINARY LED will not work (You need a flashing LED).

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  • \$\begingroup\$ How to identify a 3mm led by the way \$\endgroup\$
    – user29463
    May 29 '19 at 14:32
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    \$\begingroup\$ @user29463 A ruler tends to be helpful. \$\endgroup\$
    – Hearth
    May 29 '19 at 14:32
  • \$\begingroup\$ @Hearth 3mm is the length of what? \$\endgroup\$
    – user29463
    May 29 '19 at 14:33
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    \$\begingroup\$ @user29463 It's the diameter of the LED. 3mm and 5mm are just two standard sizes. But as others have said here, the size doesn't matter for this as long as it has an integrated flasher circuit. \$\endgroup\$
    – Hearth
    May 29 '19 at 14:35
  • \$\begingroup\$ Not length but diameter of the LED body, i.e. the size of the hole you need to drill to fit the LED in. \$\endgroup\$
    – Justme
    May 29 '19 at 14:35
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Any size LED will work - the warning is that it needs to be a flasher type LED, since it relies on the current drawn by the flashing LED to drive the transistor switching the other one.

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  • \$\begingroup\$ What is a flasher LED? How do you identify it? \$\endgroup\$
    – user29463
    May 29 '19 at 14:29
  • \$\begingroup\$ You will know. It will be advertised. solarbotics.com/product/fled \$\endgroup\$
    – DKNguyen
    May 29 '19 at 14:33
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Note that the paragraph that accompanies the circuit specifies the LED type as a flashing LED. It says that a normal LED will not work. The paragraph does state that the LED in the picture is 3mm, so I can understand the confusion. However, a flashing LED has a small circuit built in to make it self-flashing.

This circuit needs that internal circuitry of the flashing LED to drive the NPN transistor.

In summary, if you can find a 5mm flashing LED, you can go ahead and use it! Just make sure it is a flashing LED though! A quick Google of flashing LEDs will give you numerous results with different sizes.

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