555 timer latch on when trigger hold low

Thread Starter

bug13

Joined Feb 13, 2012
1,708
Hi guys

I have square output from a device (200ms high, 300ms low), I want to have it always high when I see this output. I am trying to do it with 555 one shot, but my one shot need to have a short (about 10ms) low to discharge the timer cap.

Is there a way to get rid of this so it can stay high all the time.

LTSpice simulation and screen capture are attached, please ignore circuit on the left hand side of Q1.

Capture.PNG
 

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Thread Starter

bug13

Joined Feb 13, 2012
1,708
Please tell us how you fixed it.
Pretty please with a cherry on top.
I should have properly given more info, the input to my circuit is a PWM signal driving LEDs. There are two modes:
  • dim backlight mode on standby(period = 500 ms, on = 5.3ms)
  • flashing mode when device is active(period = 500ms, on = 200ms and on = 5.3ms, that gives bright - dim, bright - dim)
my task is to get rid of the backlight (dim backlight mode). And when the leds are flashing, I need to change it to solid on.

And here my circuit, left part filter out the dim backlight signal, the part on the right turn flashing into solid on. (ignore the 555 timer circuit)

PS: my frist attempt was to use a 555 one-shot mode, but there is a short flicker, which is not acceptable for this application.

Capture.PNG
 
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