Below is the LTspice simulation of two 555 timers (or one 556) to give 8 seconds of LED pulses at 0.5 second intervals.
The value of R4 determines the LED OFF time and the value of R4 in parallel with R5 determines the ON time, which is set here for about 100ms ON and 400ms OFF.

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i have an update. I was able to get my prototype circuit to work with 2 LM555's on a beadboard so i went ahead and designed & ordered the PCbs. I got my PCBs today. I used NE556 instead of 2 LMs.. It does not work as expected (led total 8s, 0.5s on and 0.5s off). It just turns on for 1 second and turns off.
I'm curious if the LM555 series is not interchangeable with NE? any other input would be helpful too. I have attached the latest circuit.

 

Are you really, really sure that R7 is 620k and C3 is 10uF?
And you have double, triple checked that the PCB layout is correct?
The problem is that once you think these things are correct you don't really check anymore - that is human nature and we've all done it.

 

It's unlikely to be a 556 problem.
More likely is a wiring error or a component value error.
Double-check them both.
Make sure you have the pinout correct for the 556.

 

I checked the component values by replacing each one of them with a new one. the layout looks good because i did continuity checks... the only thing is i have not prototyped NE556 on the breadboard and proved it out yet. i used two LM555s.. i can try that next but if you guys can think of anything else i can troubleshoot with, let me know.

 

Can you post pictures of both sides the PCB?

 

 

Can you post pictures of both sides the PCB?

the board is populated with parts so i figured the layout from eagle osftware would be helpful?

 

So i still haven't solved the problem yet. In efforts to troubleshoot the NE556 circuit, I replaced it with an SA556, the result: First time I applied power, its blinked twice and then stopped. So I applied power again, the led stays ON for ~ 6 seconds and it starts to get hot and smoke a little. I think the chip itself is getting hot. This makes me wonder if the resistor values I chose have an effect of driving higher current than the chip can handle? The same values work perfectly on a breadboard with an exception of using 62kohm as R7 and 100uF as C3 instead of the current schematic which has 620kohm as R7 and 10uF as C3. Any thoughts?

 

If something is getting hot enough to smoke then you very likely have a wiring error.
The value of R7 and C3 would have nothing to do with that.

 

i have an update. I was able to get my prototype circuit to work with 2 LM555's on a beadboard so i went ahead and designed & ordered the PCbs. I got my PCBs today. I used NE556 instead of 2 LMs.. It does not work as expected (led total 8s, 0.5s on and 0.5s off). It just turns on for 1 second and turns off.
I'm curious if the LM555 series is not interchangeable with NE? any other input would be helpful too. I have attached the latest circuit.

Hi

So...if I understand your timer configuration, timer1 is a monostable set for about 6-7 sec, and timer2 is a astable set for about 100 Hz.
When Timer1 triggers it applies +V to the reset of timer2, then returns to GND, holding Timer2 in reset? So the LED will flash (so fast it will look like it is lit steady on) for the duration of timer1 then go dark, right? Is that how its supposed to work?

eT

 

So I individually tested the 2 timers inside 556 and they both work correctly when powered separately. The 1st timer gives a 1-shot 8s On and the other timer gives 0.5s on and off blink infinitely. When I connect the OUT of timer 1 to RESET of timer 2, the light turns ON for 1 second and stays OFF. This is happening on both breadboard & pcb. Any thoughts on whether there is enough current to drive the 2nd timer from the 1st ? I tried 500ohm, 1kohm resistors between OUT & RESET and result is same. However when I use 10kohm, the light blinks for a second and then stays ON. Finidng it hard to interpret the result.

 

Hi

So...if I understand your timer configuration, timer1 is a monostable set for about 6-7 sec, and timer2 is a astable set for about 100 Hz.
When Timer1 triggers it applies +V to the reset of timer2, then returns to GND, holding Timer2 in reset? So the LED will flash (so fast it will look like it is lit steady on) for the duration of timer1 then go dark, right? Is that how its supposed to work?

eT

No. the timer 1 will give a 1-shot 8 second ON (monostable) and the timer 2 will blink for 0.5s and 0.5s off for total of 8 seconds. i have attached my timer calculation excel for mono & astable. Please let me know if something looks incorrect.

 

Hi

Try pulling down 2reset with a 5-10k resistor to ensure the pin is at ground.
1out should connect directly to 2reset.

eT

 

You may have something there. The NE555 reset threshold is very low.