Description
The monostable flip flop, sometimes called a 'one shot' is used to
produce a single pulse each time it is triggered. It can be used to
debounce a mechanical switch so that only one rising and one falling
edge occurs for each switch closure, or to produce a delay for timing
applications. In the discrete circuit, the left transistor normally
conducts while the right side is turned off. Pressing the switch grounds
the base of the conducting transistor causing it to turn off which
causes the collector voltage to rise. As the collector voltage rises,
the capacitor begins to charge through the base of the opposite
transistor, causing it to switch on and produce a low state at the
output. The low output state holds the left transistor off until the
capacitor current falls below what is needed to keep the output stage
saturated. When the output side begins to turn off, the rising voltage
causes the left transistor to return to it's conducting state which
lowers the voltage at it's collector and causes the capacitor to
discharge through the 10K resistor (emitter to base). The circuit then
remains in a stable state until the next input. The one shot circuit on
the right employs two logic inverters which are connected by the timing
capacitor. When the switch is closed or the input goes negative, the
capacitor will charge through the resistor generating an initial high
level at the input to the second inverter which produces a low output
state. The low output state is connected back to the input through a
diode which maintains a low input after the switch has opened until the
voltage falls below 1/2 Vcc at pin 3 at which time the output and input
return to a high state. The capacitor then discharges through the
resistor (R) and the circuit remains in a stable state until the next
input arrives. The 10K resistor in series with the inverter input (pin
3) reduces the discharge current through the input protection diodes.
This resistor may not be needed with smaller capacitor values.
Circuit Diagram
Note:
These circuits are not re-triggerable and the output duration will be shorter than normal if the circuit is triggered before the timing capacitors have discharged which requires about the same amount of time as the output. For re-triggerable circuits, the 555 timer, or the 74123 (TTL), or the 74HC123 (CMOS) circuits can be used.
Source http://www.bowdenshobbycircuits.info/page9.htm#mono.gif
Note:
These circuits are not re-triggerable and the output duration will be shorter than normal if the circuit is triggered before the timing capacitors have discharged which requires about the same amount of time as the output. For re-triggerable circuits, the 555 timer, or the 74123 (TTL), or the 74HC123 (CMOS) circuits can be used.
Source http://www.bowdenshobbycircuits.info/page9.htm#mono.gif
