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555 Astable Multivibrator

The 555 Timer shown below is configured as an 555 Astable Multivibrator previously discussed. However additional components have been added to enhance the operation of the circuit. As soon as the circuit is powered up the 555 will begin producing a square wave determined by the value of Capacitor C1 and Resistors R1 and R2, as determined by the timing equation below.

Graphic 555 IC Astable multivibrator circuit schematic with Capacitor Tolerance Adjustment
555 Multivibrator Capacitor Adjustment

This particular circuit uses a few additional [optional] components to allow for adjustment or compensation for variations in the tolerance of the timing capacitor [C1].

Capacitor Tolerance Compensation

The 555 Integrated Circuit is an extremely accurate device. However it's output is only as accurate or stable as the external components used. Standard value resistors normally come with a tolerance of either 1% or 5%, of course other resistor tolerances are available. For more information refer to Resistor Values, Tolerance and Package Sizes. Capacitors on the other hand came in a large variety of tolerances which could change by the quality of the part or by the dielectric used. There are three common capacitor types used with a 555 times, Ceramic, Tantalum, and Electrolytic dielectric. However regardless of the style used, the best tolerance value will be in the range of 10 to 20%. Also see Capacitor Dielectric Materials and their characteristics.

A potentiometer [R3] can be placed on the Threshold pin to adjust the 2/3 Vcc line [recall that the potentiometer will be in parallel with the bottom two 5k resistors]. This effectively changes the trip point of the internal resistor ladder. The additional potentiometer can be used to change the trigger point and be used to compensate for variations in the Trigger Capacitor [C1]. As each new circuit is fabricated and a different capacitor used the potentiometer can be changed to adjust the circuit to compensate for differences in capacitance. The amount of compensation would depend on the values of resistors used in the circuit. Resistor R4 is used to keep the resistance from reaching zero ohms or ground. So the sum of resistors R3 and R4 will be in parallel with the two internal 5k resistors.

The capacitor [C3] in parallel with the adjustment resistors is just a by-pass capacitor and is not required. Capacitor C2 is also a by-pass capacitor. Refer to this page for addition data on Potentiometers. An additional design hint; capacitor values also change with temperature, which this circuit will not compensate for; refer to Film Capacitor Data for additional information.

555 Operational Description

The 555 uses two comparators to control the operation of the timer. The lower comparator is set to trigger at 1/3 Vcc while the upper comparator is set to trigger at 2/3 Vcc. Three internal 5k resistors placed in series between Vcc and ground are used to provide voltage references. The 1/3 Vcc voltage is generated between the bottom and middle 5k resistors and the 2/3 Vcc voltage is generated between the middle and upper 5k resistors.

The Trigger input [TRIG] is applied to the first comparator which is referenced to the 1/3 Vcc. So as long as the trigger voltage remains above 1/3 Vcc the 555 will not trigger [operate]. The second comparator is referenced to 2/3 Vcc and is used to discharge the timing capacitor C1 once the voltage across the capacitor reaches 2/3 Vcc.

555 Timing.
In addition to the external capacitor two external resistors are also required to complete the timing circuit. Resistor [R1 and R2] and the capacitor [C1] form an RC circuit which define the length of time it takes the capacitor to charge [defined by this RC Charging Curve]. The discharge of the capacitor is completed through resistor R2 [defined by this RC Discharging Curve].

Before the Trigger pulse occurs [capacitor discharge] the internal transistor is 'ON' [conducting] so the capacitor is effectively shorted to ground and is not allowed to charge.
Once the trigger is applied and falls below 1/3 Vcc the comparator trips and turns the transistor off. When the transistor shuts off the capacitor is allowed to charge toward Vcc. In effect the above text describes the operation of a monostable circuit. As the output will toggle once the negative trigger pulse is applied and will remain high until the capacitor reaches 2/3 Vcc and is again shorted out [discharged] by the internal transistor. The output will switch back to its resting state [low].

Trigger: Pin 2 is connected to the input trigger voltage for the circuit. Each time a Trigger [capacitor discharge] is applied the output will toggle high for a predetermined amount of time, based on the value of the resistor [R1] and capacitor [C1] used. A high to low transition [low pulse] on the trigger input will cause the output to switch; however, the trigger should be taken high before the output pulse times out. In other words the output pulse width should be longer than the input trigger width.

555 Timer Supply Voltage.
Note the term Vcc is used because the 555 will operate with any Vcc [pin 8] between 4.5 volts and 16 volts. The actual voltage used by the 555 does not effect the timing circuit because both the charging rate of the capacitor and the threshold of the capacitor use the same voltage.
As with any integrated circuit always Bypass the Vcc pin to ground [pin 1] with a 0.1uF ceramic capacitor [C2]. Read more on By-pass or Decoupling Capacitors.

555 Output Drive.
Pin 3 is the output of the circuit. The output will toggle between ground and Vcc each time the capacitor C1 charges and discharges. The output pin is capable of about 200mA of sink current [depending on the load], with a rise or fall time in the range of 100nS.

555 Reset.
The reset pin [4] is held high [Vcc], so no reset will occur.

Timing Equations:
Charge Time; t1 = 0.693*(R1 + R2)*C1
Discharge Time; t2 = 0.693*R2*C1
Total period; T = t1 + t2 = 0.693*(R1 + 2*R2)*C1
Oscillation frequency; f = 1/T = 1.44/[(R1 + 2*R2)*C]
Duty cycle = D = R2/(R1 + 2*R2)

Dual Version: Dual 555 timer in a single IC; LM556, SE556, NE556
Generic Packages: 14-pin SOIC, 14-pin DIP. DIP Sockets

Editor note: even though this circuit uses more components than the previous one, as circuit designer you can always add more components to compensate for changes in other components. When a capacitor used has a negative temperature coefficient, you can always add another with a positive temperature coefficient to compensate or off-set any change in temperature.

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