DIY shutter speed meter made of logic chips - Page 1
Forum section: electronics / projects

author: plazma
author_rank: Frequent Contributor
timestamp: February 05, 2020, 05:50:04 pm
content: I needed to verify the shutter speed of a CMOS camera. I ended up making a quick'n'dirty circuit of D-type flip flops.There are 30 SMD leds in a row. One illuminated led is stepped every clock pulse. The led low side is controlled with an N-fet which is also connected to the clock signal. One led is on when the clock is high. This way the led duty cycle & brightness can be controlled by adjusting the clock duty cycle.If the clock is 1kHz the led is stepped every 1ms. With a 10ms integration time there are 10 leds illuminated in the picture.

author: David Hess
author_rank: Super Contributor
timestamp: February 06, 2020, 08:19:45 pm
content: Why didn't you use decoder/demultiplexers like the 74138 and binary counters? One 7493 and two 74138s (or one 74154) could drive 16 LEDs by themselves with very little wiring.

author: plazma
author_rank: Frequent Contributor
timestamp: February 06, 2020, 08:49:55 pm
content: There are many ways to do a stepping led circuit with logic chips. I had D-type flip flop chips in my garage from other projects. It was easy to cascade them as many as wanted. I used 30 leds since that's how many fits to the protoboard in one line.First chip in the cascade is 74HCT74 since it got the set-direct pin (preset).

author: Caliaxy
author_rank: Frequent Contributor
timestamp: February 06, 2020, 09:52:41 pm
content: Cool! Obviously, you don’t own an analog oscilloscope

author: plazma
author_rank: Frequent Contributor
timestamp: February 07, 2020, 08:23:14 am
content: Quote from: Caliaxy on February 06, 2020, 09:52:41 pmOnly DSOs in use. An analog scope would give much cooler pictures for the report

author: Caliaxy
author_rank: Frequent Contributor
timestamp: February 07, 2020, 05:00:40 pm
content: I'm wondering how fast you can strobe the LEDs before they seem continuously lit and how this compares with the persistence of the phosphor on the CRT of an analog oscilloscope. You might win

author: graybeard
author_rank: Frequent Contributor
timestamp: February 07, 2020, 08:03:15 pm
content: Quote from: Caliaxy on February 07, 2020, 05:00:40 pmThe persistence of a LED is limited by the storage time of the minority carriers, typically in ns to the low tens of ns. GaN LEDs other than blue or UV use phosphors with a longer decay time, typically in the low 100s of ns that limits how fast they can turn off.The decay time of CRT phosphors varies from a few ms to tens of seconds.The turn off time of LEDs can be limited by the driving circuit turn of time as well.

author: graybeard
author_rank: Frequent Contributor
timestamp: February 07, 2020, 08:10:46 pm
content: Quote from: plazma on February 05, 2020, 05:50:04 pmThat is a clever solution.

author: Peabody
author_rank: Super Contributor
timestamp: February 08, 2020, 05:28:07 pm
content: Can you demonstrate rolling shutter with this setup by setting the LEDs vertical, or even diagonal? Or would all the lines be straight because the subject isn't actually moving with respect to the camera?

author: plazma
author_rank: Frequent Contributor
timestamp: February 08, 2020, 06:29:57 pm
content: I can try it and see if there is any interesting effect caused by the rolling shutter.