Building of two circuits that flash alternately to two LEDs at a fixed cadence.
These are very basic circuits and they show two ways of building circuits using transistors BJT PNP and NPN.

These are classic circuits that use the characteristics of transistors and capacitors to vary the aperture and closing of them. If you want to vary the frequency of the vibration of the LEDs you can exchange the two resistors of 100K with others of values. The 470 Ohm resistors are for protection of the LEDs.

These circuits use the principle of RC Time Constant Which will influence the time it takes to discharge and recharge the Capacitor.
There’s a page here with a calculator of the values that must be placed for other results.

The transistors used are the most common ones, however any other ones may be used, the NPN 2N2222 can be replaced by 2N4401 or 2N3904. For the PNP circuit, 2N3906 or 2N4403 can be used in addition to 2N2907.



Bill of materials (BOM)

  • For circuit 1:

  • 1x Red LED 5mm

  • 1x Blue LED 5mm
  • 2x NPN Transistor 2N2222
  • 2x 470 Ohms Resistor
  • 2x 100k Ohms Resistor
  • 2x 10uF Electrolytic Capacitor
  • 1x 5 to 9 V Battery

  • For circuit 2:

  • 2x PNP Transistor 2N2907

  • The other components are the same.

Transistors Pin-out


Note: This circuit has been taken from the site.

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