Hello,
I need to trigger a camera based on a signal from a photodiode.
When NOT triggered, the photodiode signal has a baseline of about 1V with two types on noise: 120Hz noise with amplitude of about 20mV (peak to peak) and high frequency noise consisting of very short spikes of a few microseconds width and amplitude of a few tens of mV.
A triggering event is defined when the photodiode signal drops by 20-30mV below the lower peak of the 120Hz noise. Yes, this is a relativly weak signal but this is the best I could get.
I built a circuit based on a comparator that outputs a logical 'true' (5V) when the photodiode voltage goes below a predetermined threshold. The comparator output is then fed into a 555 chip that generates a square pulse of 5ms, which in turn triggers the camera.
The problem is not the 120Hz noise but rather the high frequency noise that causes many false-triggering events. I first tried to filter out this noise using a low-pass filter, but found that the filter also distorts the signal I want to detect. The reason is probably that the signal consists of a sharp decrease in the photodiode voltage, which implies it has significant high frequency componenets. Hence, when going through the low-pass filter the overall amplitude of the signal decreses too much.
What I thought to do is to filter the high frequency noise AFTER the comparator, i.e. when the noise is already converted to square logical pulses. For this purpose I need a module that would reject/filter out square pulses that are shorter than some time (about 10 micro sec), and will pass longer pulses unchanged. From looking on various websites and forums I got the feeling that it is rather easy to do with a 555 chip, though I have not found a circuit design suitable for the job.
Could anyone tip me how to design such a module?
Thanks!
I need to trigger a camera based on a signal from a photodiode.
When NOT triggered, the photodiode signal has a baseline of about 1V with two types on noise: 120Hz noise with amplitude of about 20mV (peak to peak) and high frequency noise consisting of very short spikes of a few microseconds width and amplitude of a few tens of mV.
A triggering event is defined when the photodiode signal drops by 20-30mV below the lower peak of the 120Hz noise. Yes, this is a relativly weak signal but this is the best I could get.
I built a circuit based on a comparator that outputs a logical 'true' (5V) when the photodiode voltage goes below a predetermined threshold. The comparator output is then fed into a 555 chip that generates a square pulse of 5ms, which in turn triggers the camera.
The problem is not the 120Hz noise but rather the high frequency noise that causes many false-triggering events. I first tried to filter out this noise using a low-pass filter, but found that the filter also distorts the signal I want to detect. The reason is probably that the signal consists of a sharp decrease in the photodiode voltage, which implies it has significant high frequency componenets. Hence, when going through the low-pass filter the overall amplitude of the signal decreses too much.
What I thought to do is to filter the high frequency noise AFTER the comparator, i.e. when the noise is already converted to square logical pulses. For this purpose I need a module that would reject/filter out square pulses that are shorter than some time (about 10 micro sec), and will pass longer pulses unchanged. From looking on various websites and forums I got the feeling that it is rather easy to do with a 555 chip, though I have not found a circuit design suitable for the job.
Could anyone tip me how to design such a module?
Thanks!