Long signal to short with a twist using 555?

Thread Starter

Maveric77

Joined Nov 5, 2021
3
I am attempting to send a signal triggered by a LDR that reads a light that will be on for 2-12 seconds then off for 2-8 seconds, with the goal of a short signal each time the light comes on. I built the normal light detection circuit using the LDR, resistors and a BC547 transistor and it functions as it should with a signal when the light is on. But I need the signal to be a short (.5 sec. or so) burst, which means a timer (I think). I ordered some 555 timers, but can't wrap my head around getting it to send a short signal with a long trigger (everything I read says the 555 out stays high with the trigger) then resetting until the next trigger. Or is there an easier way to do this without a timer?
 

Thread Starter

Maveric77

Joined Nov 5, 2021
3
Thank you for the reply. I have read that information, but it does not address the long trigger problem. It states " Trigger pulse at PIN 2 must be shorter enough to the OUPUT pulse, so that the capacitor gets enough time to charge and discharge.". Trigger will be multitudes of the output.
 

AnalogKid

Joined Aug 1, 2013
9,630
If the width of the low trigger pulse is longer than the width of the high output pulse, the 555 output will stay high until the trigger input goes high. That is a well known quirk of the way a 555 works internally. The solution is a differentiation circuit. this "converts" the leading edge of the input trigger signal from a transition to a low level to a short negative-going pulse. Here is a grab from an image search that shows the basic approach.

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Thread Starter

Maveric77

Joined Nov 5, 2021
3
Thank you! I've been reading up on leading edge differentiation on the 555 and that should work. I appreciate it! I'm learning as I go.

But I was also wondering if a simple timer can be done with transistors and a small capacitor. Can the LDR trigger a transistor, causing a capacitor to discharge, and then recharge when the light is off? That would give me the short signal I need.
 

ElectricSpidey

Joined Dec 2, 2017
2,027
Sure this kind of thing can be done with transistors, caps and resistors, but you wont get that clean logic output without some doing.

And, by the time you get it working, you will be wondering why you didn't just use the 555.
 
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