555 ic 1.5Hz LED Flash with LDR as Dark Detector - Page 1
Forum section: electronics / projects

author: FBMinis
author_rank: Regular Contributor
timestamp: December 27, 2022, 12:25:56 am
content: I would like to blink a string of leds under the dark at a frequency around 1.5HzThe string of LEDs works at 5V and pulls a couple of hundred milliamps (dont have them with me, forgot to measure)https://pt.aliexpress.com/item/1005004854212280.htmlHere is the simulation:https://tinyurl.com/2fz787cj(I replaced the load with just one led for simplicity)When it's dark, the resistance of the LDR increases to about 150K, the gate of the AO3400 is pulled HIGH, pulling the gate of the P-Mosfet LOW, powering the 555 IC in a configuration that makes the load blink at about 1.5HzIs there a simpler, more elegant way to do this?

author: Thunderer
author_rank: Regular Contributor
timestamp: December 27, 2022, 12:54:59 am
content: Simpler for somebody's else may not be so simple for you.My solution, LDR in a voltage divider feeding an ADC in a 6-8 pin micro-controller driving PWM a transistor (maybe in a current source connection). Another button to change the blinking sequence, and so on.There will be 2-3 less components than the 555 version, but much more to offer.

author: ledtester
author_rank: Super Contributor
timestamp: December 27, 2022, 01:26:34 am
content: I find designing timing circuits with the CD4093 a lot easier than with the 555.This is a pretty good site with some 4093 circuits:https://www.homemade-circuits.com/12-simple-ic-4093-circuits-and-projects-explained/For instance, combine circuit 2 ("Automatic Street Light") with the basic Schmitt trigger oscillator (Figure A) like this:The first NAND gate turns the second one (the oscillator) on or off depending on the LDR resistance.For more ideas, just google "cd4093 circuits".(Updated schematic.)

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2022, 11:20:37 am
content: Pin 4 can be used as an enable pin for the 555, so those extra transistors are unnecessary. The values of R3 and R4 might need to be adjusted to give the desired response.Here's where I got the original circuit from.http://www.555-timer-circuits.com/light-detector.htmlA useful site for calculating the resistor and capacitor values.https://ohmslawcalculator.com/555-astable-calculator

author: FBMinis
author_rank: Regular Contributor
timestamp: December 27, 2022, 08:30:23 pm
content: This worked perfectly, thank you. I replaced R3+R4 with a 100K resistor for simplicity and it begins to flash when it's "darkish".

author: FBMinis
author_rank: Regular Contributor
timestamp: December 27, 2022, 08:31:20 pm
content: Thank you very much as well for your suggestion, it got me down the rabbits hole of CD40 chips

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2022, 09:27:44 pm
content: Quote from: FBMinis on December 27, 2022, 08:30:23 pmBe careful adjusting the 100k potentiometer. Setting it to too lower value might damage the LDR. The reason for R3 was to limit the minimum resistance to 4k7 to protect the LDR.

author: FBMinis
author_rank: Regular Contributor
timestamp: December 28, 2022, 11:23:29 am
content: Quote from: Zero999 on December 27, 2022, 09:27:44 pmI have a dimmable led strip above my workbench. I began by testing the circuit with 4.7K in series with 100K pot. It started to blink while it was still too bright. I replaced 4.7K with 100K, dimmed the lights really low and adjusted the pot until the led started to blink. Measuring the combined resistance of the potentiometer and the 100k resistor, it was close to 180k. I replaced the resistor and the pot with a 220k resistor and now starts blinking just when the led strip is really low, which was my intention.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2022, 11:47:29 am
content: Quote from: FBMinis on December 28, 2022, 11:23:29 amUsing fixed resistors is the best way to do it. Potentiometers need to be adjusted and can be easily altered accidentally.