Monday, 13 February 2012

Changing the pulse width

Changing the pulse width
The previous assembly gives a rectangular signal with a duty cyclic equal to 50 %. We can need to change it to produce pulses for example. A simple means is to use a variable RC circuit and to reshape the signal obtained by means of a Schmidt trigger , here a CD 4093 which allows to use a wide range of voltage. (The CD 4093 is in fact a quadruple logic gates NAND which we use here only the inverter function).
Pin Assignments for CD4093
  1. In1
  2. In1'
  3. Out1
  4. Out2
  5. In2
  6. In2'
  7. Ground
  8. In3
  9. In3'
  10. Out3
  11. 1Out4
  12. In4
  13. In4'
  14. +Vcc

Components List
Pot. : P1 = 20 kΩ
Capacitors : C1, 33 pF
digital circuit : CD 4093
Power supply VCC : 5 to 18 V
Modifying the value of P1 we make vary the height of the signal (B) which is not any more squarewave but sawtooth. The trigger activates when a threshold is reached and we so obtain a modified duty cycle (C and D).

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