# Driving a large LED using a flickering LED (artificial candle)

#### Mellisa_K

Joined Apr 2, 2017
391
I have got two LEDs that I scavenged from a dollar store.

The first LED (LED 1) is shown in this photo. It flickers to simulate a candle (3v, 18mA).

The second LED (LED 2) is from a cupboard light. (4.5v, 350mA). Its much brighter than LED 1.

(The voltage specs above came from the batteries in their respective housings. I measured the currents when I connected my power supply to each one.)

My goal is to make the bigger LED flicker like a (big) candle using a common power supply—a 9V battery. I saw a Youtube video where a fellow just connected, in series, two LEDs (one like LED 1 in my photo, and the other, a plain (non flickering), similarly rated LED. By “in series“ I mean he connected the Anode of the flickering LED to the positive terminal of a battery (via a resistor) and its Cathode to the Anode of the plain LED. The Cathode of the plain LED went to the negative terminal of the battery. The second, plain LED flickered in sync with the first. Not a bad hack.

Here is the Youtube video. Warning: Please start your view at no earlier than 1minute, 49 seconds.

However, this only worked because of the similarity of the ratings of the Youtube man’s 2 LEDs. Its not as simple in my case because my second LED is so much more high powered than the first.

If I am to use this hack, I have to have LED 1 connected to the battery, followed by LED 2. However, without a resistor before the battery I will destroy LED 1. With an adequately sized resistor I wont get enough power to LED2.

I need a transistor in between?

#### Wolframore

Joined Jan 21, 2019
2,610
A transistor should work. You will need a sufficient DC power supply and current limiting for your larger LED.

#### Mellisa_K

Joined Apr 2, 2017
391
Thankyou can you suggest a circuit please?

#### ci139

Joined Jul 11, 2016
1,898
? you can try a current multiplier (although to make a power efficient design takes some more thinking)

https://wiki.analog.com/university/courses/electronics/text/chapter-11 versus (https://www.researchgate.net/figure/Linear-current-multiplier_fig4_3131936)

exact matching = "don't care" - ver. : ← it's only for illustrative purposes because the real world circuits usually perform a lot of worse may require "more effective" regulation . . . e.g. you should use fuses and or current limiting to avoid "unexpected surprises"
_________
more idealistic wave tracing :: less efficient more efficient (but still far from what can be achieved) about TLV2371
. . . ↑ but these are still only the Spice experiments = you (might but by default) can not copy/paste these to a pinboard ↑
+ yet another way to do it (not so efficient as the prev.) (i don't know how it works - so can't tell how adjust it in real )
a poor(out of specs and dangerous to components) but efficient PWM test

Last edited:

#### Alec_t

Joined Sep 17, 2013
14,417
If LED2 is rated at 350mA, a little 9V battery won't be able to provide that much current for very long, if at all.
To convert from 9V down to 4.5V efficiently a switch-mode DC/DC buck converter (such as one of these) is called for.

#### Bernard

Joined Aug 7, 2008
5,784
If it needs to be portable, then a battery box with a switch for 3 AA Alk. batteries is suggested. ' Found this
circuit in my file which shows the waveform of the flickering LED. An added emitter follower to drive the transistor , or if a logic level FET is available like FDD6530A, then the circuit could be used as is. Looks like an
old 9V battery was used. How bright does the LED need to be ?

#### iimagine

Joined Dec 20, 2010
511
Use a LDR circuit and mimic a real candle!

#### Wolframore

Joined Jan 21, 2019
2,610
this will be fun for halloween... coming up!

#### Tonyr1084

Joined Sep 24, 2015
8,006
this will be fun for halloween... coming up!
Get a bunch of superbright yellow LED's and put them in a fireplace. Will look like a fire is burning in the firebox. Won't look like flames, but you'll get the ambient light that of a fireplace.

Or set it up on the front porch hidden from view and make it look like the house is on fire. Just let the fire department know you're doing this and if there's a report of a fire, first confirm before rolling the big trucks.

#### Mellisa_K

Joined Apr 2, 2017
391
If it needs to be portable, then a battery box with a switch for 3 AA Alk. batteries is suggested. ' Found this
circuit in my file which shows the waveform of the flickering LED. An added emitter follower to drive the transistor , or if a logic level FET is available like FDD6530A, then the circuit could be used as is. Looks like an
old 9V battery was used. How bright does the LED need to be ?

View attachment 184438
Hi Bernard,

I got your circuit to work! And with these modifications - see below

I'm happy with the brightness of the large LED.

Thanks ever so much for your help.

Regards
If it needs to be portable, then a battery box with a switch for 3 AA Alk. batteries is suggested. ' Found this
circuit in my file which shows the waveform of the flickering LED. An added emitter follower to drive the transistor , or if a logic level FET is available like FDD6530A, then the circuit could be used as is. Looks like an
old 9V battery was used. How bright does the LED need to be ?

View attachment 184438

#### Mellisa_K

Joined Apr 2, 2017
391
A transistor should work. You will need a sufficient DC power supply and current limiting for your larger LED.
hi Wolfrmore,

I had trouble with this circuit. I built it according to my transcription of the image you referred me to in the link. A snapshot of my interpretation of the circuit is attached.

It doesnt flicker the big LED.

Im still troubleshooting.

Ill let you know if I can improve on it.

Regards

Mellisa

#### Wolframore

Joined Jan 21, 2019
2,610
Melissa you should put a current limiting resistor between 9v and your larger led. If you’re using a 9v battery it won’t last long but little danger of supplying too much current because they are pretty weak. But if you decide to power it with a wall plug it would burn out that led. Start with a 100 Ohm 1/4 watt resistor. It should be plenty bright.

#### Wolframore

Joined Jan 21, 2019
2,610
hi Wolfrmore,

I had trouble with this circuit. I built it according to my transcription of the image you referred me to in the link. A snapshot of my interpretation of the circuit is attached.

It doesnt flicker the big LED.

Im still troubleshooting.

Ill let you know if I can improve on it.

Regards

MellisaView attachment 184506 View attachment 184506
With an NPN transistor you need the LED on top (low side switch). That 47 ohm bypasses the transistor so it should be removed. But it’s awesome that’s you’re trying things. Change the 10 Ohm to 100. This will give you just under 1/4 watt using a good power source (not 9v battery) the base of the transistor should be connected between the candle led and its resistor going to ground This allows voltage to rise when the candle led is on and allow current to flow. Basically you have it right on the first diagram you posted. But add a resistor.

Last edited:

#### Tonyr1084

Joined Sep 24, 2015
8,006
With an NPN transistor you need the LED on top (low side switch). That 47 ohm bypasses the transistor so it should be removed. But it’s awesome that’s you’re trying things.
I 100% agree. With the LED following the 2222, you get current through the BE junction. Even if you have no power applied to the collector, the BE junction can still light the LED. I discovered this the first time I built a chase light circuit using 2222's and a decade counter. Having zero volts on the collector, my LED's were still lighting. Not as brightly as when I powered the collector, but they were still lighting. In my circuit I was strobing the collector. With the speed of the chase and the strobe I could make it appear as though the chase lights would move left to right or right to left. At certain frequency combinations I could make chase lights appear to be moving in both directions at the same time. I don't have that diagram any longer. That was back in the 80's.

But I agree, the transistor should follow the LED. That way you have full control of the LED's brightness.

#### Bernard

Joined Aug 7, 2008
5,784
It appears from the photo that a current limiting resistor is included in LED package if operating on 4.5
Post # 12, operate on 4.5 V, remove R1, bypass R3.
How much current do you wish for LED; 200 mA is pretty bright.

#### oz93666

Joined Sep 7, 2010
739
The easiest way is just to up the voltage and put many leds in series ... Putting non flickering in parallel is no good since they will get dimmer ...

So if you had 30V ... you could put aprox 10 non flickering in series with 1 flickering ...

Incidentally , how do you think they get that led to flicker???

#### Mellisa_K

Joined Apr 2, 2017
391
With an NPN transistor you need the LED on top (low side switch). That 47 ohm bypasses the transistor so it should be removed. But it’s awesome that’s you’re trying things. Change the 10 Ohm to 100. This will give you just under 1/4 watt using a good power source (not 9v battery) the base of the transistor should be connected between the candle led and its resistor going to ground This allows voltage to rise when the candle led is on and allow current to flow. Basically you have it right on the first diagram you posted. But add a resistor.
Thanks for your encouragement and advice wolframore. I will check this out after a few days. I am tied up with work presently.

#### Tonyr1084

Joined Sep 24, 2015
8,006
how do you think they get that led to flicker???
Do you know? I don't.