BC547 - Need to drive several LEDs

Thread Starter

MozzyMarr

Joined Dec 23, 2017
5
I am making some disco lights for NYE, I have made a very simple circuit consisting of two BC547s, a 47k pot (really needs to be a low value but it is all I have in), and an electret microphone. It works well but the maximum current of the BC547s is only 100MA which to be on the safe side is only 4 LEDs. Ideally I want to add more, can I simply connect a MOSFET to the collector of the BC547 and use the MOSFET to drive the LEDs? Most my electronics hobby consists of Arduino stuff and it has been many years since I studied anaologue electronics. Thanks.
 

Thread Starter

MozzyMarr

Joined Dec 23, 2017
5
That would be the easiest thing, but I don't have any of them, only IRF and IRLs :( With it being Christmas week I won't be able to source anything for at least a week. So I was thinking for now could I just connect an IRF540 to the BC547 and use the 547 to switch on the MOSFET? I know it is needless complexity but I just need to work with what I have in my collection. Of course the MOSFET would need to be very quick too as the LEDs dance to the music.
 

Thread Starter

MozzyMarr

Joined Dec 23, 2017
5
Thanks, just quickly made this. I ideally want to drive around 3V LEDs.

dancing leds2_MozzyMarr.png


Moderator's Note:
It had been cut the space part of the uploaded circuit.
 

dl324

Joined Mar 30, 2015
13,857
Thanks, just quickly made this. I ideally want to drive around 3V LEDs.
You shouldn't be connecting LEDs in parallel as they won't all have the same forward voltage and one will hog current. If it fails, it will likely cause a cascading failure.

If that potentiometer is for adjusting brightness, that's the wrong way to do it. If you don't drive the transistor into saturation, you have to mind it's power dissipation.
 

Thread Starter

MozzyMarr

Joined Dec 23, 2017
5
All the LEDs are identical for that reason (same colour and batch), I have checked and they all draw the same amount of current. The POT adjusts the sensitivity but I may replace that with a fixed resistor once I have worked out a happy medium figure. It really needs a 20K POT but I don't have any. If I was to wire the LEDs in series I would have to dramatically increase the voltage which means I wouldn't be able to use the 4 x AA battery pack want to use. I am not too concerned with overall reliability as it only needs to work for one night and even if it fails we have plenty of other lights. It is really just a bit of fun.

I know what you mean about wiring different LEDs in parrerel I did this on another project mixing LED types with very odd results :D
 

Audioguru

Joined Dec 20, 2007
11,249
You do not say how much current "more" LEDs need.
The datasheet for your IRF540 shows that it completely turns on when its gate voltage is +10V.
Some IRF540 Mosfets conduct only 0.25mA when the gate voltage is 4V (the max threshold voltage).
Then an IRF540 Mosfet might not work when your 4 AA batteries produce only 4V to 6V.

Try a few IRF540 Mosfets, you might find a sensitive one since the datasheet shows that a "typical" one conducts 3A pretty well when its gate is 4.5V.
 

Thread Starter

MozzyMarr

Joined Dec 23, 2017
5
Sorry I meant to say above around 20 LEDs at 20ma each although I can use less LEDs if I really have to. I have some IRL MOSFETs too which are probably more suitable as I think they have a switch on voltage of around 5v.
 

dl324

Joined Mar 30, 2015
13,857
All the LEDs are identical for that reason (same colour and batch), I have checked and they all draw the same amount of current.
They're not as identical as you think. Even LEDs from the same wafer will have a range of forward voltages, colors, and luminous intensities. Manufacturers test and bin them according to characteristics they choose to sell.
upload_2017-12-23_9-13-32.png
upload_2017-12-23_9-14-5.png

upload_2017-12-23_9-12-50.png

If you buy in large enough quantity, you can have the manufacturer select for whatever characteristics they can test (and you'll pay) for .
 

ian field

Joined Oct 27, 2012
6,539
Depending on your circuit you may be able to replace the BC547 with, for instance, a 2N7000 MOSFET.
Why stop there - a TO220 package MOSFET wouldn't break a sweat even without a heat sink.

The 7000 has a low VGSthr, so a logic level device would be needed to equal that. Older PC motherboard onboard regulator MOSFETs have low voltage ratings and VGSthr is often less than 2V.
 

ebeowulf17

Joined Aug 12, 2014
3,282
All the LEDs are identical for that reason (same colour and batch), I have checked and they all draw the same amount of current. The POT adjusts the sensitivity but I may replace that with a fixed resistor once I have worked out a happy medium figure. It really needs a 20K POT but I don't have any. If I was to wire the LEDs in series I would have to dramatically increase the voltage which means I wouldn't be able to use the 4 x AA battery pack want to use. I am not too concerned with overall reliability as it only needs to work for one night and even if it fails we have plenty of other lights. It is really just a bit of fun.

I know what you mean about wiring different LEDs in parrerel I did this on another project mixing LED types with very odd results :D
You don't need to run multiple LEDs in series, but each LED should have its own series resistor, instead of parallel LEDs sharing one resistor.
IMG_3971.PNG
 

be80be

Joined Jul 5, 2008
2,059
I dont no why people don't like these P2N2222A it would work with 20 leds set to draw 20 mA each like post 13
some 2n3904 would work some there not all the same spec.

ebeowulf17 post 13 best way

Don't see not liking it. Series leds one goes they all stop
That's how the led lighting I use a lot of works. Some has 3 led set's too but I like the one with just one resistor per led it works with no problems.
 
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