Download Circuits for pulse shortening

Circuits for pulse shortening.
Assignment:
1. Create circuit connections (Figures 1, 2 and 3) to shortening the pulse and determine
the length of this impulse.
2. Make a report from these measuring.
Theory:
Monostable flip-flops
These circuits have only one stable state, which is break by trigger pulse. Trigger pulse may
be longer or shorter than the output pulse. According to the type of connection is the output
pulse more or less steep, and therefore needs to consider the connection. Otherwise it can be
use to achieve the desired shape e.g., Schmitt flip-flop circuit. Most common monostable flip
flops are 74123 or 555 timer circuit. Involvement of flip-flops with these circuits can be found
in catalogs.
Basic connection for pulse shortening.
Basic connection, which uses the hazard states (variable length of pulse) of logical circuits, is
seen below.
&
&
&
Input
&
Output
Fig. 1
The output of this circuit gives steep pulses, suitable as an exciter of clock, reset and write
inputs at flip-flop. Length of pulse depends on the number of gates and theirs delays.
To get more dynamic, we can use RC element to the gate. These connections have worse
rising edge of output pulse. Connection is ideal for pulses to a length of 10 micro seconds. We
must also consider the time to charge the capacitor (regeneration time). Another possible
connection is on the right. Circuit reaches longer times for the same values of C.
&
R
C
Fig. 2
C
R
Fig. 3
D
Flickering of button
Button contact is never perfect. If operator presses a button, the contacts are first connected
and then disconnected several times and then combined before the contact stopped and the
button is connected firmly. Wiring diagram, which eliminates the flicker of mechanical
contact button shown in the picture below. The original button is accompanied by a low pass
filter and closes the entry gate equipped by the Schmitt flip flop circuit. Off frequency of lowpass filter is realized by a resistor and capacitor is determined by:
1
R1  R2 .C
Typically it can be assumed that the operator is unable to press the button for more than 10
times per second. So we get frequency 10 Hz. We choose the filter off frequency of about 5
times to 10 times higher. So f0 = 50 to 100 Hz.
f0 
Schematic diagram of non-flickering buttons using Schmitt flip-flop at input.
Schematic diagram of non-flickering buttons using reset-set circuit.
Datasheet:
7414
7400
74LS74