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# Building a 12v LED Stroboscope Circuit

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#### olof102

##### Newbie level 5
led strobe circuit

Hello,
I'm considering building a LED stroboscope circuit with as many 3v 20mA LED:s as possible. I've set this circuit up in Multisim and it seems to work fine.

Do you guys see any problems or is this circuit ready to go?

And how many LED:s can i parallel attach do you think?

Is this a good circuit that you guys think will work or do you have any other tip on how to build a flashing LED circuit (atleast 15 LED:s)

strobe led circuit

Change Q1 to a power darlington transistor.

For each two LEDs like you have at the moment, add a resistor (R4 ?? difficult to read) and you should be able to use hundreds of LED strings in parallel.

Brian.

### olof102

Points: 2
Re: LED Strobe Circuit

Do you know any specific darlington transistor? And why would a "power darlington" transistor be better in this circuit?

Sorry for my noobish questions!

Re: LED Strobe Circuit

The transistor type depends upon the amount of current the LEDs will be carrying in total. Although the original message says 20mA it also asks how to add more LEDs which means more current. For example, if there were two chains of LEDs the current would be 40mA, if there were 10 it would be 200mA and so on.

I use BD682 in similar applications, they are rated at 4A absolute maximum so in theory they could be used to switch 200 LED chains, each at 20mA.

The reason for using a darlington transistor is that the base current in Q1 is calculated by dividing the collector current by its gain (hfe) and the values of R2 and R3 used on the schematic are going to restrict the base current enough to limit the number of LEDs that can be used. Darlington transistors have much higher gains so they will switch the same LED current without having to increase the base current.

For example:

BC327, max gain = typically 250 at 0.1A, max load 0.1A
BD682, max gain = typically 750 at 2A load, max load 4A

Generally, the gain drops as the current increases.

Brian.

### olof102

Points: 2
Re: LED Strobe Circuit

Thanks Brian!

Sadly I don't have any BD682 but I do have some "BD679 TO-126 NPN Darlington 80V 4A".

Do you think one of these would work?

Re: LED Strobe Circuit

I also use BD679 in some designs but they are NPN. The original schematic uses a PNP transistor.

With a simple change it should work fine:

Connect the emitter to ground.
Connect the collector to the LED resistor.
Remove R3 completely.

Note that the on/off of the LEDs is reversed but as it is a flasher, that shouldn't make much difference.

Brian.

Re: LED Strobe Circuit

Thanks again!

I've actually reconsidered building this one instead:

Do you think this circuit will work? And yeah, I'll use the BD679, I just couldn't find it in Multisim.

Is R1, R2 and R7 correctly dimensioned?

Re: LED Strobe Circuit

The diagram shows the transistor the wrong way up but otherwise it looks OK.
With an NPN transistor, the emitter goes to ground and the collector to the LEDs.

You can save a few components by putting 3 LEDs in series and using one common resistor for all 3.

Brian.

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