Problems using LEDs as a strobe light

Thread Starter

measuretwice

Joined Oct 21, 2018
4
Hi,

I’m trying to make a strobe using the bank of leds from a cheapo flashlight. I was shooting for an on time of around 30ms but the flashes are less than overwhelming, seems very dim compared the brightness of the flashlight. Is there a ramp up with LEDs to reach full brightness that I should accommodating?

The application is to ‘freeze’ a rotating object as part of soft bearing dynamic balancer I’ve built. From an accelerometer mounted on the balancer I clean up the signal then trigger a 555 as a monostable timer setup for around 30 milliseconds. After the 555, I use a transistor as a switch. It works, just not bright enough compared to what I thought I’d get based on the flashlights brightness. Frequency will be 1-4 Herts and I want the light turned on for a short enough duration so you can see radial markings on the rotating part and thereby tell where it is its rotation when things were triggered.

I’m at the beginner end of things but try and muddle through. Maybe LEDS are a poor choice for this? I liked it because it was inexpensive, but am open to better ideas

thanks
 

ebeowulf17

Joined Aug 12, 2014
2,945
Is there a ramp up with LEDs to reach full brightness that I should accommodating?
The LEDs themselves should be plenty fast - in fact much, much faster than most other lighting options.

The circuit driving the LEDs could potentially slow things down and/or dim them, so @bertus is right that we'd need to see that.

Are you trying to judge this by eye, or take pictures or video? The eye also responds slowly to light changes. That's why PWM dimming can be used without us seeing the flicker (dimmable LEDs are often set up to turn off and on literally hundreds or thousands of times per second, but our eyes just perceive the average amount of light received, they don't see all the individual on/off cycles.)

It may be that a 30ms burst just isn't long enough to be perceived as full brightness by the human eye, even if it is in fact delivering the same brightness it does in the flashlight circuit.
 

Thread Starter

measuretwice

Joined Oct 21, 2018
4
Can you show us the actual schematic you are using?
Its all bread boarded but not written out (yet). I'll have to do so eventually, but was trying to get it working first. I'm thinking there is either lag in the LED coming to brightness or its physiology - a short duration flash needs to be a lot more powerful to look as 'bright'.

I'll try with a longer duration see what that looks like.
 

Audioguru

Joined Dec 20, 2007
11,251
The brightness of LEDs is determined by their current. A solderless breadboard cannot pass much current. A transistor without enough base current cannot pass much current. Cheap Chinese LEDs in a cheap flashlight are destroyed by a little more current.
Please show us your schematic with a measured while loaded battery voltage, transistor part number, base resistor value, 555 parts and quantity of LEDs. 30ms is short enough to "stop" slow motion but be seen bright. Less than 30ms looks dimmed.
 

OBW0549

Joined Mar 2, 2015
3,012
I’m trying to make a strobe using the bank of leds from a cheapo flashlight. I was shooting for an on time of around 30ms but the flashes are less than overwhelming, seems very dim compared the brightness of the flashlight.
Not surprising; LEDs are pretty underwhelming when trying to use them as strobe lights. Xenon photoflash tubes are driven with anywhere from 5 Joules per flash up to about a kilojoule, far more than any LED (or small LED array) could withstand in a 30 millisecond pulse. So the light power available from LEDs as flashlamps is much lower than Xenon.

Is there a ramp up with LEDs to reach full brightness that I should accommodating?
No. LEDs are extremely fast, and turn on or off on a time scale of a few nanoseconds to a few microseconds, not milliseconds. For a good discussion of LED usage as strobe lights, see this thread.

If you really want to make a strobe out of LEDs, I would suggest using a high-power illumination array, such as this, and drive it with a short pulse at possibly several times its continuous current rating; don't go too high, though, or LED lifetime may suffer.
 

Audioguru

Joined Dec 20, 2007
11,251
A cell phone has a bright short duration flash and it does not use 144 LEDs to do it. But it also does not use a cheap Chinese LED that is in a cheap flashlight.
 

Thread Starter

measuretwice

Joined Oct 21, 2018
4
sorry for going quiet, was a busy week and didn't get much hobby time. I like the illumination array idea and ordered a few.

On the project, it turns out I wasn't getting 30ms. I hadn't done the trigger properly - I was driving it from function generator and was able to get it working using an edge trigger like this

https://electronics.stackexchange.com/questions/324207/why-does-this-edge-trigger-work-on-input-of-555-monostable-config.

I'm working with low frequency, 1 or 2 hertz and it was tricky getting the right resistor and cap mix

30MS is a now very bright flash, probably sufficiently so.

I will try to get a schematic together, no doubt you guys will have many good suggestions for improvement. Thanks again for the help
 
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