555 circuit design help.

[sonkidd]

I'm trying to design a LED circuit where a LED beings to flash after the light is turned off.

For the light detection part, I followed the design of the following ciruit, without any problems. Light-sensitive Alarm Project

Afterward, in order to make the LED flash, I used a simple astable 555 circuit, as follows: **broken link removed**

Both the circuits work without problem by itself. However, it fails to work when I join the 3rd leg of the 1st IC (the Light dependent circuit) as the input voltage of the 2nd circuit.

How am I suppose to connect the two ICs in order to make the circuit work?

Thanks a lot!

 

[ronv]

You didn't say where you tied the output of the 1st circuit to the second circuit. I would suggest pin 4 (Reset) that way when the sensor is in the dark the oscillator will run.

 

[sonkidd]

I attached the 3rd leg of the IC directly as the input voltage of the 2nd IC.

So I should just attached the 3rd leg directly to the 4th leg of the 2nd 555? (Like below)



But wouldn't the 555 have a constant oscillation output even without a current going through leg 4?

Thanks!

 

[ericgibbs]

View attachment 45398

I attached the 3rd leg of the IC directly as the input voltage of the 2nd IC.

So I should just attached the 3rd leg directly to the 4th leg of the 2nd 555? (Like below)

View attachment 45399

But wouldn't the 555 have a constant oscillation output even without a current going through leg 4?

Thanks!

hi,
The problem is the light/dark detector is a monostable, so it will only have it output pin3 high for the timing period, then it will go low.
While the light detector 555 pin3 is high and connected to the LED flasher 555 pin 4, the led will flash, but as soon as the light detector times out the LED will not flash

EDIT:
What is the resistance of the LDR when it changes from light/dark/light.?

 

[ronv]

It should have worked unless as Eric said the second timer is longer than the one shot in the first stage. The pulse output from the first stage is positive so the second timer is only on for 10 seconds max. Don't know about the timing for the second timer??? If you power the second stage from the first you should tie pin 4 to V+. I guess I assumed the second stage was like 1/2 second so it had a few flashes before it shut off.

 

[sonkidd]

thanks everyone! I tested again and it worked perfectly when I attached it to pin 4.

but I'm wondering why the initial design didn't work. Shouldn't the output from pin3 of the IC provide enough current to drive the second 555?

Thanks again =).