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I need to design light blinker which are mostly used on road side for indication purpose. Their main purpose is to blink with a rate of 500ms or 1sec. These type of circuits contains two PCBs, one is 555 timer circuit and another one is led circuit. Now I have studied about 555 timer and have calculated the values of resistance and capacitance required for 1sec blink. Following is the circuit for timer:

enter image description here

So I have used a R1=1K R2=150K C=10uF. At the output pin I connected a led via 330ohms resistance and led was blinking with 1sec delay. Now I need to design the led circuit for blinker. Total leds will be around 170 and I need to control all the leds with this 555 circuit and blink them with 1sec. Now I am confused how to design this circuit. How to connect all the leds(parallely or in series) to avoid voltage drop. I will be using 5MM led with 330ohm resistor. I guess I need to connect them parallely to avoid voltage drop but I am not sure. Is there any other circuit I need to add to avoid voltage drop. Does anyone has already made such kind of circuits. Any help or reference guide please. Thanks

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  • \$\begingroup\$ 170 leds? Are you trying to blind everyone else? ^^ \$\endgroup\$
    – PlasmaHH
    Aug 29, 2016 at 13:42
  • \$\begingroup\$ What's the power supply you intend on using in order to energize all of those LEDs? \$\endgroup\$
    – jonk
    Aug 29, 2016 at 17:29
  • \$\begingroup\$ @PlasmaHH no I am not trying to blind everyone. But I guess a normal light blinker contains this much amount of leds. \$\endgroup\$
    – S Andrew
    Aug 30, 2016 at 4:01
  • \$\begingroup\$ @jonk I am planning to use 12v dc \$\endgroup\$
    – S Andrew
    Aug 30, 2016 at 4:01
  • \$\begingroup\$ @Andrew: Traffic light LEDs are pretty bright. More than \$20mA\$ each, I think. I can't imagine that you are seriously considering ones with built-in \$330\Omega\$ resistors in them. Do you have a link to the LED, itself? The 12V would fit a lead-acid battery. Is that your intended source? I think you mentioned 1.7A somewhere. How did you calculate that? Was it \$10mA\cdot 170\$? \$\endgroup\$
    – jonk
    Aug 30, 2016 at 5:07

1 Answer 1

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You don't say what the LED voltage is, or (more importantly) the LED current, but definitely you cannot drive 170 LEDs directly.

So you will need something like a single N-channel MOSFET to drive the LEDs. If each LED is 10mA and they are all in parallel, then you need to drive 1.7A. The 555 can drive the MOSFET gate directly, preferably with a small resistor in series. Make sure the MOSFET type (gate drive) is suitable for the 555 output voltage, especially if it is much less than 10V.

Suppose that your supply is 5V and your LED Vf is 1.8V, you could connect them in pairs with 140 ohms per LED pair. (5-2*1.8)/0.01A = 140. Then the current would be only 0.85A (half the total current draw). With higher supply voltage it would be possible to put more LEDs in each small series chain.

Concept:

schematic

simulate this circuit – Schematic created using CircuitLab

There are a lot of MOSFET and LED questions on this site. Please give this a try and hopefully you can figure it out.

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  • \$\begingroup\$ Thanks for giving schematic. I am planning to give 12v to the 555 timer. In the schematic you are giving vled to the leds. I didnt get this. As you said, I need to drive 1.7A but then how will it achieve using mosfet. Can you explain more. \$\endgroup\$
    – S Andrew
    Aug 30, 2016 at 4:58
  • \$\begingroup\$ You didn't disclose what voltage(s) you are using. You can use different voltages or the LEDs and the 555 or both can use 12V, no problem. If you have 12V you can put several LEDs in series (at least 3, even if they are blue or white) and reduce the current draw, and number of resistors, as well as the MOSFET current. Since you have 12V drive, just use any MOSFET that can switch several amperes (even 20A-50A would not be too much) and rated for at least 20-30V. I would suggest a more forgiving TO-220 type. If you pick one that has low Rds(on) with 10V drive you certainly won't need a heatsink \$\endgroup\$
    – Spehro Pefhany
    Aug 30, 2016 at 9:24

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