I attempt to build a blinking circuit using 12V truck light ( like this one: http://www.princessauto.com/pal/en/Tail-Lights/4-in-LED-Stop-Turn-Tail-Light/8126047.p). I have try to build a single LED blinking circuit and doubt that the same circuit would work with this 12V (24-LEDS) indicator light. I'm seeking advices/directions on how to build this circuit. It would be grateful if someone can give me a working circuit for it. Thank you

  • \$\begingroup\$ Awking for the whole circuit is usually too broad and not always appreciated. Besides, you can find several previous questions (and answers) on the same topic, for instance with the keywords LED driver \$\endgroup\$
    – clabacchio
    Jul 11, 2014 at 6:04
  • \$\begingroup\$ Look at this circuit here: 555-timer-circuits.com/images/… , now since the current and voltage requirements of the BIKE LED are high, so simply use a transistor instead of the LED in the circuit, where the connection to the base of the transistor would be the output of the 555(Pin 3). Something like this eecs.tufts.edu/~dsculley/tutorial/transistors/… , but the 5V here will be the output of 555(but dont power the LED through this as a 555 cannot give out much current. \$\endgroup\$
    – Sherby
    Jul 11, 2014 at 7:31
  • \$\begingroup\$ Or you can do it with transistors instead see Astable Multivibrator Replace R1 or R4 with a LED and series resistor for the circuit shown at the link. \$\endgroup\$ Jul 11, 2014 at 12:31

1 Answer 1


This is your garden-variety 555 astable set for about 1/2 second on and 1/2 second off, driving a transistor switch which turns the lamp on and off.

enter image description here


Here are links to the 555 and to the PN2222 data sheets, and the power budget for the PN2222 follows.

I wired up the transistor with a 24 ohm load instead of a lamp, and it's been running at 1 Hz, with a forced beta of 10 for a while now, and its case temp is at about 31C, so data sheet worst case spec's aren't even close.

I'll let it run for a few days just to see what happens, but I don't expect any surprises.

enter image description here


Although about five times more expensive than a PN2222, an IRF7201 takes all of the guesswork about which 2222 is better than which out of the equation, and is a bulletproof substitute in this application:

enter image description here

  • \$\begingroup\$ The maximum saturation voltage between collector and emitter of the 2n2222 is about 1.6 volts at the ~500mA that it will be sinking in this application. The 2n2222 isn't rated for that kind of dissipation. A beefier transistor will need to be used. Additionally, the minimum current gain of the 2n2222 at 500 mA is only 30, so the base resistor would need to be smaller to guarantee saturation. \$\endgroup\$
    – MattyZ
    Jul 12, 2014 at 3:52
  • \$\begingroup\$ A PN2222 works just fine, as evidenced by the fact that I've got one running fully loaded and its case temp is sitting at about 31C and has been for some time. Please see my edited (downvoted???) answer for the gory details. ;) \$\endgroup\$
    – EM Fields
    Jul 12, 2014 at 15:56
  • \$\begingroup\$ I guess the specs im looking at for the PN2222 and the ones I have are somewhat different. Mine lists the max collector current as 600 mA and saturation voltage as 1.6 volts; if a transistor is particularly bad and has the latter spec then it will be running really hot in the above circuit. It may run hotter than your calculation because I believe the duty cycle equation is only valid if the period is significantly less than the thermal time constant of the device. I don't know what the thermal time constant of the junction of a 2222 is, but half a second seems long. \$\endgroup\$
    – MattyZ
    Jul 13, 2014 at 9:28
  • \$\begingroup\$ Running the Fairchild PN2222 CW will get the junction temp up to about 145C with Ta = 25C, so any duty cycle less than 100% will make things better. However, since there seems to be a pretty big difference between 2N2222's from different stables, and who knows what the OP will end up with, a beefy little IRF7201 should make all of the potential 2N2222 problems go away. I posted the final (hopefully) edit a few minutes ago showing the swap and substituting an equivalent resistance for the lamp. \$\endgroup\$
    – EM Fields
    Jul 13, 2014 at 10:17
  • 1
    \$\begingroup\$ The IRF7201 is only available in surface-mount, not through-hole like the 2222. For something easier for a hobbyist to solder, look e.g. in digikey.com under Products > Discrete Semi > FETs-Single > Packaging (bag or bulk). \$\endgroup\$ Nov 7, 2014 at 22:40

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