Take note of the following datasheet parameters:
- Maximum Supply Voltage = 15V
- Control Voltage = 2/3 Supply voltage
- Threshold Voltage = 2/3 Supply voltage
- Trigger Voltage = 1/3 Supply voltage
- Output Voltage when LOW = approx. 0V
- Output Voltage when HIGH = 2/3 Supply voltage
- Maximum R for 15V operation = 20MΩ
- Maximum R for 5V operation = 6.7MΩ
Internal Block Diagram
Variable Duty Cycle
Since the formula for tw_LOW uses only RB for discharge time, and tw_HIGH uses RA + RB for charge time, it is obvious that the astable configuration always produces a duty cycle > 50%. When an application requires a duty cycle < 50%, one of the ways it can be achieved is by adding a diode across RB so that it is bypassed during the charge cycle while remaining in the discharge path.
When this is combined with a potentiometer in the charge path,, the duty cycle can be made to vary greatly.
PWM Circuit suitable for motor control. Q is the current source for charging the capacitor; Q is also the destination for discharging the capacitor.
When the capacitor is charging, the Discharge pin internal transistor is Off so the pull up resistor turn the external transistor and motor ON; when the capacitor discharges, the Discharge pin transistor is ON sinking current turning the external transistor and the motor OFF.
Varying the R1 pot changes the duty cycle from almost 0% to almost 100% with the frequency remaining constant. The frequency is the reciprocal of .7 X R1 X C1.
While the circuits shown above are the traditional configurations to connect the 555, unconventional configurations are also possible; this circuit is but one of them.
Can you predict what output would be realized with this circuit?
Perhaps you should build it and see if your prediction is accurate.
The 555 timer is an extremely versatile IC with applications limited almost only by ones imagination. We will explore more applications in future labs. In the meantime, the website 555-timer-circuits.com lists some additional applications - there are many more. If you have the parts, please experiment with some of these circuits.