NE555; output not turning off after 8s

Thread Starter

pcbenthusiast

Joined Mar 24, 2017
62
I am working on a new project which requires an LED light to turn ON for 8 seconds and turn off automatically after 8 seconds. This is for a seat belt minder. During those 8 seconds, flashing LED every 0.5s would also be nice to have, but optional per the safety standard. So I tried making a circuit using the NE555 as attached to generate a single shot 8s timer. The model I have should theoretically produce 8s ON (no LED blinking) but the output waveforms don’t represent that. Something is wrong in the circuit. Any suggestions on why i'm not seeing an output that stays ON for 8s as soon as powered and then should turn off itself even though the power to the IC remains. Right now, the output stays LOW the whole time.

I understand that i have to use 556 dual timer if i wanted to do the LED blinking functionality as well. but for now, i just want to troubleshoot this one shot 8s timer using NE555 hopefully.

*** update:having the power supply VG1 turn ON faster (0.01 s rise time) made my output turn ON for 8s and turn off properly, thanks to AlbertHall)***
 

Attachments

Last edited:

AlbertHall

Joined Jun 4, 2014
12,190
If I read your graphs correctly then the '555 output only goes up to about 100mV - i.e. it never triggers.
Connect a 100nF (value not critical) between TRIG and 0V. This will hold the TRIG pin low when power is first applied to trigger the '555.
 

Thread Starter

pcbenthusiast

Joined Mar 24, 2017
62
If I read your graphs correctly then the '555 output only goes up to about 100mV - i.e. it never triggers.
Connect a 100nF (value not critical) between TRIG and 0V. This will hold the TRIG pin low when power is first applied to trigger the '555.
Thanks for catching that i uploaded the wrong schematic/waveform. I have uploaded the correct one now. Basically, i had to reverse the LED connection to get the output to go HIGH; but the 8s timer is still NOT working. The output stays HIGH as long as the power does.
"New schematic/waveform is uploaded now"
 

AlbertHall

Joined Jun 4, 2014
12,190
Nope. The LED is now the wrong way round and its reverse breakdown is what is pulling up the '555 output.
Turn the LED back round, and it should also have a resistor in series with it, say 470Ω.
Then add the capacitor on the trigger pin.
 

dl324

Joined Mar 30, 2015
15,476
The circuit in the first post won't give 8s. It's going to be more like 0.1s.

Try increasing C4 to give a longer trigger pulse.
 

Thread Starter

pcbenthusiast

Joined Mar 24, 2017
62
Nope. The LED is now the wrong way round and its reverse breakdown is what is pulling up the '555 output.
Turn the LED back round, and it should also have a resistor in series with it, say 470Ω.
Then add the capacitor on the trigger pin.
The new schematic & results have now been uploaded. the "V555out" which is the "OUT" pin stays LOW..
 

Thread Starter

pcbenthusiast

Joined Mar 24, 2017
62
The circuit in the first post won't give 8s. It's going to be more like 0.1s.

Try increasing C4 to give a longer trigger pulse.
so from the schematic i attached, R2=73kohm and C1=100uF together provide the timer of 8s (per the equation 1.1RC). C4 is the bypass cap that @AlbertHall asked me to add between TRIG & GND
 

AlbertHall

Joined Jun 4, 2014
12,190
You still need a resistor in series with the LED.
The problem now is that VG1 takes about 1 second to get up to voltage.
Make VG1 rise quickly and all will be well.
 

Thread Starter

pcbenthusiast

Joined Mar 24, 2017
62
Stop editing your first post. Attach a new schematic when you make changes.

Are you breadboarding this circuit or simulating?
i'm only simulating it now.
Do I just need to name the new schematics i add as rev.1 rev. 2 etc as i upload? i thought uploading the latest schematic would be the better for any new people that are viewing the post, NO?
 

Thread Starter

pcbenthusiast

Joined Mar 24, 2017
62
i am suspecting the trigger pin connection. is it connected properly? When the VG1 is ON, i want the LED to turn ON and stay ON for 8 seconds,. after 8 seconds, the VG1 will still be ON but the LED would have been turned off.
 

Thread Starter

pcbenthusiast

Joined Mar 24, 2017
62
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.
You're right! I got it. so when i go make the circuit in a breadboard, the input is a switch when will turn the power on from a 12V lead acid battery, will this slow turn still be an issue?
also, i want to understand why exactly did the slow turn on cause issues with timing!?
 
Top