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Found a nice guide on how to make a led turning signal for bikes based the 555 timer which contained this schematic: enter image description here

But no matter how I do it, I cannot make it work.The leds won't blink.... What's going on here?


I'm using normal 12V-rated LEDs (used for car license plates)

The LEDs are not flashing at all, just are turned on...

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closed as unclear what you're asking by Elliot Alderson, Oleg Mazurov, Mitu Raj, RoyC, StainlessSteelRat Aug 27 at 23:03

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    \$\begingroup\$ Please provide information what LED's you used. And please show the real implementation. \$\endgroup\$ – Huisman Aug 22 at 8:20
  • \$\begingroup\$ Does it flash if you add +9V directly onto pin 8 of the 555? \$\endgroup\$ – HandyHowie Aug 22 at 9:31
  • \$\begingroup\$ @ Huisman. I'm using normal 12V-rated LEDs (used for car license plates) @ HandyHowie. The LEDs are not flashing at all, just are turned on... \$\endgroup\$ – el_zoido Aug 22 at 9:52
  • \$\begingroup\$ Did you try adding power directly to pin 8 ? \$\endgroup\$ – HandyHowie Aug 22 at 9:58
  • \$\begingroup\$ I'm not familiar with those LEDs, if there are only LEDs inside or more electronics. In the latter case it is doubtful 12V rated LED will work on 9V. And in shown circuit the voltage across the LED will be quite lower than 9V. \$\endgroup\$ – Huisman Aug 22 at 10:02
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Compare your circuit to this one:

enter image description here

This circuit has two LEDs, only one will be on at the same time.

You can remove one of the LEDs (and then also remove the resistor in series with it) and you'll have the most standard 555 LED blinker circuit possible.

Note how pins 4 (not Reset) and 8 (VCC) of the 555 connect to the + terminal of the battery. The fact that your circuit does not have this is a serious issue.

Your circuit is quite "unconventional". When starting out with circuits, just copy existing (and working) circuits. There are simply too many things you will miss (and get wrong) if you try and figure it out on your own. Later when you get more experienced (that might take years) you will be able to design circuits "from scratch". So just copy what works, it is the quickest way to learn.

Maybe the "designer" of the circuit you found got lucky and the circuit works for him. However it is then very likely that it needs the LEDs to be of a certain type. If yours are different the circuit might stop working.

My guess is that your circuit only works when using "power" LEDs like 1 or 2 Watt. If you use small LEDs rated for 20 mA then they will not light up. The circuit in my answer works with both types although the series resistor and supply voltage need different values for the "power" LEDs.

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  • \$\begingroup\$ "When starting out with circuits, just copy existing (and working) circuits" Isn't that exactly what OP did? Ok, OP can't know ap priori if (and working) applies... \$\endgroup\$ – Huisman Aug 22 at 8:23
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    \$\begingroup\$ @Huisman Uhm, yeah I guess you're right. However to me the design OP started with seems to be from someone who got lucky that it worked. It doesn't look like a robust design to me. \$\endgroup\$ – Bimpelrekkie Aug 22 at 8:26
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    \$\begingroup\$ I agree with you on that. Feeding the 555 using a diode construction is not a robust design. Probably the designer only had access to nodes L, R and GND. Then, it's a clever design as long as the duty cycle isn't too big. \$\endgroup\$ – Huisman Aug 22 at 9:56
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I'm using normal 12V-rated LEDs (used for car license plates)

That's your problem. Such LEDs have internal series resistors, which will prevent the circuit from working properly.

You need to use bare LEDs with parallel resistors, as shown in the schematic.

Also, turn signal LEDs need to be red (rear) or amber (front or rear), not white.

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  • \$\begingroup\$ But still, using bare LEDs, you may have to recalculate (or: re-try and re-error) the values for 240 and the 100 ohm resistors. \$\endgroup\$ – Huisman Aug 22 at 10:20
  • \$\begingroup\$ So what's the purpose of the 100 ohm resistor in this circuit? \$\endgroup\$ – el_zoido Aug 22 at 18:44
  • \$\begingroup\$ The 100 ohm resistor clearly does nothing when the output (pin 3) is high. It serves to limit the current when the output is low and the LED is lit. Assuming 12V vehicle power and 2V forward drop on the LED, 100 ohms limits the current to about 100 mA. With 9V power, it will be less. \$\endgroup\$ – Dave Tweed Aug 22 at 18:51

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