555 timer delay circuit + light sensor?

by AdlietMeyer   Last Updated May 16, 2019 04:25 AM

I am quite new to electronics and I would like to combine these 2 circuits together so that during complete darkness on the LDR, the LED would turn on instantly and when light falls on the LDR there would be around a 1 or 2 second delay before completely shutting off. I would like this to keep running and not just a one-time thing where once the LED completely shuts off, the LED would not light up again even in complete darkness.

The circuit would be running of a 5V DC power supply and powering an LED array.

I have the 2 circuits below working separately, but have no idea how to combine them together.

I tried connecting the delay circuit to the output of the dark sensor (LED) but it did not work.

555 Timer Delay Circuit Dark Detector Circuit



Answers 1


In the 555 circuit the capacitor controls the wait time, if the capacitor is short-circuited the circuit will wat forever.

In the LDR circuit the transistor acts like a switch but unfortunately it's switching to ground but the capacitor in the 555 circuit is connected to +9V

To resolve this I swapped the parts in the 555 circuit upside down to have the capacitor to ground. Then it was simple to I merge the two circuits.

schematic

simulate this circuit – Schematic created using CircuitLab

I could have instead built the upside-down version of the LDR circuit using a BC557 transitor (or other similar PNP type) instead of the BC547 NPN transistor and merged that with the original 555 circuit.

Jasen
Jasen
May 16, 2019 04:14 AM

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