Hi guys,
I'm a newbie to circuit building but was hoping for some guidance to build a circuit that I could use to determine at what frequency people start to see flicker vs no flicker (the critical flicker fusion frequency).
I need a LED (preferably red) that has a 50% duty cycle that I can adjust the frequency between 1-100Hz with a button or dial. I also need to know at what frequency the light is flickering (so some form of digital output). I'm guessing from what I've read that I need a 555 timer but beyond that I'm lost.
I couldn't find any old projects doing the same and I apologize if this is a stupid sounding project.
Any suggestions would be greatly appreciated
Cheers.

 

hi,
A 555 timer can be set for a 50% duty cycle.
The variable frequency could be achieved by using a potentiometer, which could have a frequency calibrated dial.
It may be necessary to have selectable ranges over the 1Hz to 100Hz band.

The 555 could drive a transistor/led circuit.

A word of caution, at around a flash rate of 16Hz some people can be adversely affected by the flashes.

 

There is a lot more to flicker than meets the eye. You might find Joyce Farrell's An analytical method for predicting perceived flicker interesting and maybe useful in preparing your experiments.
http://scien.stanford.edu/jfsite/Papers/ImageRendering/Displays/Farrell_Flicker_1986.pdf

A problem with the NE555 is that it is not a simple matter to vary the frequency and maintain a fixed duty cycle. You could set up the NE555 as a variable frequency oscillator as shown below then follow it with a D, JK, or T flip-flop to divide it by two to get a nice square wave (shown below the NE555 circuit).

Another approach might be to use my fixed duty cycle LED strobe circuit. It is based on an LM393 dual comparator

Edit: Corrected URL per ScottWang's correction.

 

hi Dick,
There is a 555 configuration that will give a 50% duty over a range of frequencies, ref image.
E

 

hi,
A 555 timer can be set for a 50% duty cycle.
The variable frequency could be achieved by using a potentiometer, which could have a frequency calibrated dial.
It may be necessary to have selectable ranges over the 1Hz to 100Hz band.

The 555 could drive a transistor/led circuit.

A word of caution, at around a flash rate of 16Hz some people can be adversely affected by the flashes.

Thanks Eric. Hopefully I shouldn't have anyone with a photosensitive epilepsy. I'll make sure to exclude people with a history of seizures but if someone did have a seizure, they couldn't be in the better place to have their first. Cheers

 

hi josh,
I would say a starting value for the timing Cap and Variable resistor would be around 3.3uF and 100K.
The CMOS version of a 555 eg: 7555 would be my choice.

What intensity of light flash are you planning.?

 

Indeed, don't try that with the bipolar version.

 

Thanks Eric. Good question about intensity. I'm thinking 400mcd to start with but

 

Thinking 400-1000mcd but may need to adjust in the future.
Many thanks

 

hi josh,
It would be an interesting exercise to have the option of flashing a Blue, Red or Green LED , all set for the same intensity,

 

Completely agree and I'm sure there would be a different frequency for different colours and different intensities but I can't test my patients all day so have to go for most bang for my buck. Red is the winner

 

If you have a way to measure it, the correct quantity to measure would be luminance (Ft. Lmaberts or Nit).

 

you mayget some strange results via beat freq. if the room has any AC powered lighting. I assume you will be in a darkened room, but if you want to have any light use sunlight or DC powered lights

 

I heard a well-known expert in human factors tell his audience that flicker from high refresh rate computer monitors (you can tell the time frame in which this took place) can beat with florescent lighting to create visible flicker. The idea was floating around the industry at the time.

Unbelieving, I asked where the nonlinear mixing would occur and he replies "In the eye.". Some days later I took two LEDs, one running at about 120 Hz and another connected to a function generator and I slowly and carefully swept the function generator around 120 Hz and could not detect any flicker in any part of my retina. This was repeated at various luminance levels in a dimly lit room always with no flicker being observed. I would be very excited to learn the details of an experiment that actually produced the effect.

 

I'd use an op-amp based square wave generator for this one. I think it's simpler to get the ~50% duty cycle over a wide frequency range, at the low end frequencies of this project.

One thing about the flicker is that it is more easily detected in peripheral vision than in the fovea. A moving light in our periphery will appear to flash - even from the movement of the eye itself - whereas a light you are staring at will appear steady at the same frequency.

The few times in my life that I was approaching a faint, I was very aware of the flicker of the fluorescent lights around me. I never noticed them normally.

 

Indeed, don't try that with the bipolar version.

Why? With the feedback timing resistor circuit, the bipolar 555 duty cycle will not be a perfect 50%, and it will change with load. Is that what you're referring to, or is there some other reason?

ak

 

Those are the reasons of which
I was thinking.

 

@ScottWang pointed out that the URL I gave for Farrell's paper is incorrect and he provided the correct URL, which is below.
An analytical method for predicting perceived flicker
http://scien.stanford.edu/jfsite/Papers/ImageRendering/Displays/Farrell_Flicker_1986.pdf

Thank you Scott!

 

There is a lot more to flicker than meets the eye. You might find Joyce Farrell's An analytical method for predicting perceived flicker interesting and maybe useful in preparing your experiments.
http://scien.stanford.edu/jfsite/Papers/ImageRendering/Displays/Farrell_Flicker_1986.pdf

A problem with the NE555 is that it is not a simple matter to vary the frequency and maintain a fixed duty cycle. You could set up the NE555 as a variable frequency oscillator as shown below then follow it with a D, JK, or T flip-flop to divide it by two to get a nice square wave (shown below the NE555 circuit).

Another approach might be to use my fixed duty cycle LED strobe circuit. It is based on an LM393 dual comparator

Edit: Corrected URL per ScottWang's correction.

Have you try this: