The control voltage
The control voltage(Vcont) changes the threshold value of the 1/3 Vcc and 2/3 Vcc for the internal comparators. With the control voltage, the upper value is Vcont and the lower value ½ Vcont. When the control voltage is varied, the output frequency also varies. An increasing in Vcont increases the charging and discharging time of the external capacitor and causes the frequency to decreases. A decreases in Vcont decreases the charging and discharging time of the capacitor and causes the frequency to increases.
Duty cycle =(tH/T)100%=(tH/tH+tL)100%
Duty cycle=(R1+ R2/R1+2R2)100%
Features of 555 timer IC
• It operates from a wide range of power supplies ranging from + 5 Volts to + 18 Volts supply voltage.
• Sinking or sourcing 200 mA of load current.
• The external components should be selected properly so that the timing intervals can be done in several minutes along with the frequencies exceeding several hundred kilo Hertz.
• The output of a 555 timer can drive a transistor-transistor logic (TTL) due to its high current output.
• It has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005 %/ °C.
• The duty cycle of the timer is adjustable.
• The maximum power dissipation per package is 600 mW and its trigger and reset inputs has logic compatibility.