I have attached a schematic of the timer configuration. through testing and power cycling the 555 timer outputs 60% of the time but the other 40% the 555 will not output. What could be the reason for this? (SC_12V is just 12V)

 

The trigger time constant is very short so triggering will depend a great deal on the rate of rise of the supply voltage. I suggest increasing the time constant of the trigger RC by a factor of 10 to 100.

Normally discharge is connected directly to the threshold pin and the cap for one-shot operation, though this is not strictly necessary if you don't mind the long discharge time and potential for having a shortened output pulse if the timing cap has not been fully discharged.

 

Looks like the intent is to trigger the one shot on power on. When trying to retrigger, are you sure the power source is fully discharged?

 

As noted, the R1C1 time constant seems short, at only 0.47ms, to trigger from the power supply power up.
Increase the value of R1 and/or C1 to give an RC value at 20 times of what you have.

 

As noted, the R1C1 time constant seems short, at only 0.47ms, to trigger from the power supply power up.
Increase the value of R1 and/or C1 to give an RC value at 20 times of what you have.

And perhaps add a diode across R1, cathode to 12V, to discharge C1 when power is removed so the circuit will react to a brief power down.

 

I have attached a schematic of the timer configuration. through testing and power cycling the 555 timer outputs 60% of the time but the other 40% the 555 will not output. What could be the reason for this? (SC_12V is just 12V)

View attachment 153548

Connect a 0.1uf bypass cap across the 555 power supply pins.

eT

 

Your output time is 2.5 seconds, trigger time is 470nS , discharge time is 2.2 seconds , have you got a scope handy?

 

Your output time is 2.5 seconds, trigger time is 470nS , discharge time is 2.2 seconds , have you got a scope handy?

Yes I do

 

Do you used this circuit for your car?

 

Do you used this circuit for your car?

Yes, the supply for the 555 is a Shorai LFX14L2-BS12 lithium iron phosphate battery

 

Yes I do

Ok can you look at pins 2, 3 on power up to see whats going on with your scope.

Then pins 3, 6 ... And note the times.

 

Ok can you look at pins 2, 3 on power up to see whats going on with your scope.

Then pins 3, 6 ... And note the times.

For Pins 2 and 3, they both rise to roughly Vcc(12V) and then after a few seconds Pin 3 falls and Pin 2 is still high.
For Pins 3 and 6, Pin 3 rises to roughly 12V while Pin 6 is slowly climbing to about 7V and once around 7V, Pin 3 drops to zero and Pin 6 slowly falls to zero. However, When I connect the scope to pin 6 the output stays higher than it normally should

As for time, I got rise time of trigger pin anywhere from 40us to 960us.

When ever the 555 does not output the scope waveform form pin 2 rises a very small bit and falls or doesn't move at all.

 

You can get a faster discharge by linking out pins 6,7, and put your charging resistor on pins 7,8.

Sounds like its working ok.