# Figure 3 555 timer astable configuration when the

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Figure 3 - 555 Timer Astable Configuration When the circuit is first turned on, the discharge pin is disconnected from ground (internally) and the output pin is set high since the trigger pin is below 33.3% V cc (no charge in the capacitor). The capacitor, C, starts to charge through resistors R1 and R2. The threshold pin detects when the voltage across the capacitor reaches 66.6% V cc . When the voltage across the capacitor reaches 66.6% V cc , the output pin is set low and the discharge pin is connected back to ground. When the discharge pin is connected back to ground, the capacitor starts discharging though resistor R2 until the capacitor reaches 33.3% V cc . The cycle then repeats and creates a series of output pulses. Compared to the Monostable circuit, the Astable circuit connects the trigger pin to the capacitor and adds a resistor between the discharge and threshold pins. An Astable circuit triggers from previous output pulse whereas a monostable circuit requires an externally applied trigger. In short, the output pin oscillates from high to low creating a series of pulses as the capacitor charge oscillates from 33.3% to 66.6% V cc , without any external triggers. The duration the output pin stays high, t HIGH , is given below: 𝑡 ???? = 0.693 𝐶 (𝑅 1 + 𝑅 2 ) The duration the output pin stays low, t LOW , is given below: 𝑡 𝐿𝑂𝑊 = 0.693 𝐶 𝑅 2 The frequency, f, of the series of pulses is: 𝑓 = 1 𝑡 ???? + 𝑡 𝐿𝑂𝑊

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The Astable 555 timer circuit can be used in the following applications: a) Modulate transmitters such as ultrasonic and IR transmitters b) Create an accurate clock signal (Example: There is a pulse accumulator pin on the 68HC11 microcontroller that counts pulses. You can apply an Astable 555 timer circuit set at 1 Hz frequency to the pulse accumulator pin and create a seconds counter within the microcontroller. The pulse accumulator will be covered in later in the course.) c) Turn on and off an actuator at set time intervals for a fixed duration 3. Remotely Controlled LED using a 555 Timer An Astable 555 Timer circuit can be used to generate a square wave signal with an accurate frequency. This signal can drive an infrared Light Emitting Diode (LED) through the output pin of the 555 timer. Adding a switch to the IR LED circuit allows a user to choose when to power the IR LED with the signal.
• Spring '16
• KBH
• VCC, Infrared, 555 Timer, 555 timer IC

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