# 555 1.5Hz LED Flash with LDR as Dark Detector

#### FranciscoB

Joined Feb 8, 2014
107
I would like to blink a string of leds under the dark time at a frequency around 1.5Hz

The 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.html

Here 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 1.5Hz

Is there a simpler, more elegant way to do this?

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#### boostbuck

Joined Oct 5, 2017
570
Perhaps leave the 555 powered on and connect the LDR/pullup resistor to pull down the reset line when light, if you can support continuous supply current to the chip.

#### FranciscoB

Joined Feb 8, 2014
107
What is the minimum resistance for the LDR?
Measured 400 ohm under direct sunlight. Maybe had 100ohm resistor in series just in case?

#### ericgibbs

Joined Jan 29, 2010
19,118
hi Fran,
The value of the LDR resistance must be over 1K5 when dark in order to enable the 555.
Check this LTS sim.
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#### FranciscoB

Joined Feb 8, 2014
107
I dont understand the implications. Should I change the circuit I'm using at the moment?

The led begins to blink when it's very dark, remains on for close to 1 seconds and stays off for close to 1 second too, as was my intention.

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#### ericgibbs

Joined Jan 29, 2010
19,118
hi Fran,
From that simulation it shows On for ~0.35 Seconds, Off for ~0..35Seconds.

Have you actually built the 555 Astable and the NMOS FET to check the flash rate.?

Measure the resistance of the LDR when it is just getting to the Darkness level you expect the LED's to start to flash, let me know.
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#### FranciscoB

Joined Feb 8, 2014
107
Hi, Eric. Thanks for your time.

I have a dimmable led strip above my workbench, my only light source for the test. I began by testing the circuit with 4.7K resistor in series with the 100K pot (together with the LDR as voltage divider connected to RESET). It started to blink while it was still too bright or my intent. 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. The LDR measure 85K ohm. I replaced the resistor and the pot with a 220k resistor and now it starts blinking just when the led strip is really low, which was my intention.

For blink rate, I experimented using this site as reference: https://ohmslawcalculator.com/555-astable-calculator

I ended up using C=10uF, R1=10K and R2=100K

This project doesnt need to be terribly accurate, it's just meant to make those "fairy lights" start to blink at dusk.

#### ericgibbs

Joined Jan 29, 2010
19,118
Hi Fran,
So is the project now working OK as you expected?

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