Fundamentals of Electrical Engineering and Monostable Multivibrator

Posted on Feb 6, 2014

This is one of the most basic 555 circuits. This circuit is part of this chips datasheet, complete with the math needed to design to specification, and is one of the reasons a 555 is referred to as a timer. The green LED shown on the illustration lights when the 555 output is high (i. e. , switched to Vcc), and the red LED lights when the 555 output

Fundamentals of Electrical Engineering and Monostable Multivibrator
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is low (switched to ground). This particular monostable multivibrator (also known as a monostable or timer) is not a retriggerable type. This means once triggered it will ignore further inputs during a timing cycle, with one exception, which will be discussed in the next paragraph. The timer starts when the input goes low, or switched to the ground level, and the output goes high. You can prove this by connecting the red wire shown on the illustration between ground and point B, disconnecting it, and reconnecting it. It is an illegal condition for the input to stay low for this design past timeout. For this reason R3 and C1 were added to create a signal conditioner, which will allow edge only triggering and prevent the illegal input. You can prove this by connecting the red wire between ground and point A. The timer will start when the wire is inserted into the protoboard between these two points, and ignore further contacts. If you force the timer input to stay low past timeout the output will stay high, even though the timer has finished. As soon as this ground is removed the timer will go low. Rt and Ct were selected for 3 seconds timing duration. You can verify this with a watch, 3 seconds is long enough that we slow humans can actually measure it. Try swapping Rt and Ct with the 27 KO resistor and the 100 µF capacitor. Since the answer to the formula is the same there should be no difference in how it operates. Next...

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