What has to happen is for the trigger to be low when the '555 supply is active.
The capacitor on the trigger pin holds the trigger low while the supply gets established.
If the supply rises slowly then the trigger pin voltage is not held low long enough to trigger the '555.
Increasing C4 increases the time the trigger is delayed allowing the supply to establish.
If you switch the power on with a switch then the rise time will be pretty fast so this will not be a problem, though you will still need a capacitor on the trigger pin but it can be a smaller value.

 

VG1 in this latest version still is fairly slow rising.
To cope with that you could increase C4 to 10uF to hold down the trig pin longer.

Instead I graphed the LED current on the "OUT" pin, which is a result of the voltage.

Also, when i change R4 (pull-up at Trig pin) from 100kohm to 10kohm, it doesn't work anymore properly. Any thoughts why?

 

Now that i have the 8s timer with 555 working, what is the combination connections required to have the output blink every 0.5s for 8 seconds total? i understand i need to use a dual timer chip like 556 but any reference circuit design out there that i could look at or if you guys have any input on how to do that?

 

i thought uploading the latest schematic would be the better for any new people that are viewing the post, NO?

It's considered poor form to edit a post in a way that makes someone responding to the original post appear to be a dufus.

When I edit a post, I add an "EDIT:" line with the nature of the edit.

What was confusing is that your first post wasn't indicating that it was edited by the forum software when I looked at the schematic.

 

Now that i have the 8s timer with 555 working, what is the combination connections required to have the output blink every 0.5s for 8 seconds total? i understand i need to use a dual timer chip like 556 but any reference circuit design out there that i could look at or if you guys have any input on how to do that?

Use the 8s one shot to enable the second timer that blinks.

 

Also, when i change R4 (pull-up at Trig pin) from 100kohm to 10kohm, it doesn't work anymore properly. Any thoughts why?

The lower resistor value reduces the delay on the trigger signal.

 

Now that i have the 8s timer with 555 working, what is the combination connections required to have the output blink every 0.5s for 8 seconds total? i understand i need to use a dual timer chip like 556 but any reference circuit design out there that i could look at or if you guys have any input on how to do that?

Something like the image below except that the right hand rimer should be connected as in your existing circuit.

 

Something like the image below except that the right hand rimer should be connected as in your existing circuit.

OK, so the left hand circuit is in astable (0.5s blink) & right hand timer is mono-stable (8s).

 

It's considered poor form to edit a post in a way that makes someone responding to the original post appear to be a dufus.

When I edit a post, I add an "EDIT:" line with the nature of the edit.

What was confusing is that your first post wasn't indicating that it was edited by the forum software when I looked at the schematic.

Got it thanks!

 

OK, so the left hand circuit is in astable (0.5s blink) & right hand timer is mono-stable (8s).

You will need to set the timing values for the left hand timer for 0.5s - see the datasheet.

 

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.

 

Post deleted

 

Keep in mind that all of the components must be rated for the temperature extremes seen in an automobile.

The capacitors are especially critical. Aluminum electrolytic caps can get very leaky at high temperatures and lose their capacitance below freezing temperatures. The 4.7 uF and 100 uF caps will probably have to be a tantalum cap because of this.


Edit: Added 4.7 uF cap in addfition to 100 uF cap.

 

Another thought on temperature extremes...

I would recommend that you test your circuit at high and low temperatures before installing it in the automobile.

First, heat it with a hair dryer to get it hot but not too hot to touch. If it still works properly then let it cool down and then put it in the freezer for an hour. Take it out and test it before it can warm up. (Too keep it from warming up too quickly, you can put ice cubes in a plastic sandwich bag and wrap your circuit it in a towel with the ice cubes).

 

Now that i have the 8s timer with 555 working, what is the combination connections required to have the output blink every 0.5s for 8 seconds total? i understand i need to use a dual timer chip like 556 but any reference circuit design out there that i could look at or if you guys have any input on how to do that?

Any reason you can't use a blinking led for this, just turn it on and it'll blink by itself..

 

Any reason you can't use a blinking led for this, just turn it on and it'll blink by itself..

i didnt know an LED that blinks existed. can you share a part # preferably Surface mount?

 

Another thought on temperature extremes...

I would recommend that you test your circuit at high and low temperatures before installing it in the automobile.

First, heat it with a hair dryer to get it hot but not too hot to touch. If it still works properly then let it cool down and then put it in the freezer for an hour. Take it out and test it before it can warm up. (Too keep it from warming up too quickly, you can put ice cubes in a plastic sandwich bag and wrap your circuit it in a towel with the ice cubes).

thanks for your input!

 

i didnt know an LED that blinks existed. can you share a part # preferably Surface mount?

Here's a few: https://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=blinking+led+smd&_sop=15

 

https://www.aliexpress.com/item/100...7956a7d&transAbTest=ae803_2&priceBeautifyAB=0

Another thing..If you use a blinking led in series with standard leds, they will all blink..

 

Hello all,

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 NE 556 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.
When I prototyped it I used two LM555 on the breadboard. 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.