How to build Monostable Flip Flop

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.

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.

Circuit diagram

circuit from http://www.bowdenshobbycircuits.info/page9.htm#mono.gif