Red light frequency

Hi, I'm looking on how can i read the speed of a shutter with a digitalread and a light sensor... The idea is to be able to know how many RPS I'm using on the shutter.

The shutter is placed in front of the light, so i can read just ON and OFF of the light and calulate how many I have in a second.. and write it on a small LCD.

So, did someone know wich sensor i can use to work on this project??



I'd use a photodiode. (your thread title isn't very descriptive of the problem IMHO)

Upss :drooling_face: sorry I haven't see the mistake...

thanks for your answer and did you think that a photo diode can read a frequency of around 72 input for second?

best Ki

72Hz is not a problem for a photodiode.

photodiodes will typically handle MHz - phototransistors may be a bit slower due to storage effects. For really high speed a PIN diode is used. 72Hz is no problem. LDRs can be very slow though, don’t bother with them.

If the light levels are fairly fixed you may get away with chosing a load resistor that can directly produce logic level outputs and you might be able to use pulseIn - however to do this properly a schmidt-trigger gate might be a good idea. analogRead() take some time (100us??) but ought to be fast enough here.

Thanks for all your answers... In general the speed will be really precise and constant, is for a 16mm projector, so the idea is to be able if is a 24 ips or if it change to 23.90 or what else...

The ply problem is that i'm not so familiar with electronic and did you some photos on how use a PIN DIODE ?

sorry and thanks