Home > Circuit Bricks > Logic Gate Monstable UPDATED:.21:09 21 October 2013
 FUNCTION BLOCK DESCRIPTION THIS USES LOGIC DEVICES Logic 1 = High = +V Logic 0 = Low = 0V This monostable is built around 2 LOGIC gates. It produces an output pulse, either high or low by using NOR or NAND gates. The time of the pulse is set by 2 components. Useful for producing brief pulses for latches and clocks, from 1us to 1 second, for longer durations use a 555 monostable.
 CIRCUIT DETAILS Input NOR GATE MONOSTABLE - Circuit Diagram Trigger This is activated by a low to high input signal. A high to low transistion has no effect. Output When triggered, the output will pulse high for the pre-determined time. The output can only sink/source about 2-3mA, just enough for an LED (but this will reduce the output voltage). It should really only be used to trigger other logic devices - but you can add a driver for higher current. Input NAND GATE MONOSTABLE - Circuit Diagram Trigger This is activated by a high to low input signal. The opposite to the NOR monostable. Output When triggered, the output will pulse low for the pre-determined time. Again the output is only 2-3mA and should be connected to another logic device.
 CALCULATING THE TIME... The duration of the pulse is calculated as follows: Time (secs) = 0.7 x C1 x R1 It can be tricky to choose values to start with, so try the following: 1 Choose one of the following values for the capacitor, depending on the time you require: Less than 1 ms. = 1nF, Up to 100 ms. = 47nF, Up to 1 sec. = 470nF 2 Now rearrange the formula to work out the required resistor value R1 R1 = Time / (0.7 x C1) To ensure that the caluclations are correct, use either Mohms & uF or Ohms & Farads as units Try using a preset or a potentiometer for R1. Then you can adjust the time manually.
 PACKAGE DETAILS Both the NOR and NAND gates come in a 14pin DIL package. Any gate can be used for any purpose. CMOS types have been chosen since their supply voltage can range from 3 to 15v. 4001B 2ip NOR GATE 4011B 2ip NAND GATE When using ICs, it is always best to use an IC socket so the IC can be removed easily if needed. Unused gates should have their inputs connected to 0v
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