Need help with a 555 one shot

[Iawia]

I am triggering a 555 timer with some logic. i have the output feeding an led so i can see how long the duration of the output is. however, no matter how i seem to tweak the resistor or capacitor, the output led always remains 'on' while the logic is high.

long story short, i want the one shot to be triggered and go off before the logic changes again.

is there a way to do that with the one shot? another method would be fine too.

 

[k7elp60]

A schematic of your circuit would be most helpful. I would assume you are applying a negative trigger to pin 2 and have the LED from pin 3 ground, with a resistor in series with the LED.

 

[alec_t]

It would help if you posted a schematic. You may not have the 555 circuit configured correctly.
One solution (dependent on the configuration) is to apply the logic output via a capacitor to the trigger input of the 555.

 

[Ian Rogers]

You need a monostable.. 555 circuits

 

[RodSTAR]

Iawia,
I don't know if understood clearly your problem, but sounds like maybe this helps you, which is a problem I had, and it was solved there. (well I have to test it tomorrow)

 

[Iawia]

Rodstar, this is exactly what i need, with the exception that i don't need a pulse on negative edges. i will post a schematic asap.

 

[Iawia]

Schematic

Here is my schematic. sorry about the rough drawn. i figured it would be faster. I am having the output of the j/k flip flop to flip power off to most components when the counter reaches 5. but the counter continues counting and nothing turns off.

i have hooked up an alarm to trigger the 'J' value to high (min alarm time is 60s) when the alarm goes off, thus i figured that when the k value is triggered by the counter turning '5', it would toggle the Q state, the components (except for the timer and the jk), then the alarm would run out, and it would be able to start all over again on the next alarm signal.

i noticed that the jk doesn't change until it is clocked, so i figured it best to hook up another timer whose clock width is much smaller to catch the output '5'.

Help!

 

[RodSTAR]

Iawia,
In the electronic stuff I can't help what you are asking, but I can tell you, -just in case you didn't figure it already- that a cheap 8-bit microcontroller and a few lines of code you can easily reach your task (and lots more complexity you can do). Anyways I know your task can be done in the old multi-chip style as you expose.

In my case I needed external support due to the heavy realtime multitasking in my 32-bit microcontroller (UART and SPI TX/RX communications, sensors capturing, heavy math algorythms, motors monitoring and controlling all that in "realtime" processing).

I hope you the get help you need soon.