====== Lab Work 3 (Part 2) ======

====== One-Shots and Oscillators ======

We will be using the versatile 555 timer as both a monostable multivibrator (one-shot) and an astable multivibrator (oscillator). The internals of a 555 IC is shown below.

{{:archive:pgt104:timer555.jpg?direct&350|}}

//**Disclaimer**: The image above is extracted from resources available for Digital Fundamentals 11th Edition (Global Edition)//

The main internal components of a 555 timer are 2 voltage comparators that are configured by a voltage divider circuit that provides a trigger value of <m>{1/3}V_cc</m> and a threshold value of <m>{2/3}V_cc</m>. These values can be externally adjusted using the control voltage pin (5).

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====== 555 One-shot Operation ======

We just need a resistor and a capacitor for this (an extra decoupling capacitor is optional).

{{:archive:pgt104:timer555_monostable.jpg?direct&350|}}

//**Disclaimer**: The image above is extracted from resources available for Digital Fundamentals 11th Edition (Global Edition)//

Initially, the circuit will settle down to its stable state.

{{:archive:pgt104:timer555_monostable_0.jpg?direct&350|}}

//**Disclaimer**: The image above is extracted from resources available for Digital Fundamentals 11th Edition (Global Edition)//

When triggered, it will set the internal R-S latch, which consequently enables the circuit to charge the capacitor.

{{:archive:pgt104:timer555_monostable_1.jpg?direct&350|}}

//**Disclaimer**: The image above is extracted from resources available for Digital Fundamentals 11th Edition (Global Edition)//

Once the capacitor is fully charged (actually, once the voltage is over threshold) and the trigger signal is deasserted, comparator A should cause the latch to reset and consequently cause the discharging of the capacitor.

{{:archive:pgt104:timer555_monostable_2.jpg?direct&350|}}

//**Disclaimer**: The image above is extracted from resources available for Digital Fundamentals 11th Edition (Global Edition)//

Notice that the pulse width generated at the output pin should be around <m>{t_w}=1.1{R_1}{C_1}</m>

====== 555 Oscillator Operation ======

This time we need an extra resistor.

{{:archive:pgt104:timer555_astable.jpg?direct&350|}}

//**Disclaimer**: The image above is extracted from resources available for Digital Fundamentals 11th Edition (Global Edition)//

This is how the the oscillator works - the capacitor is charged through the two resistors (<m>R_1</m> and <m>R_2</m>) when the internal transistor <m>Q_1</m> is off, and discharged through <m>R_2</m> when transistor <m>Q_1</m> is on.

{{:archive:pgt104:timer555_astable_0.jpg?direct&400|}}

//**Disclaimer**: The image above is extracted from resources available for Digital Fundamentals 11th Edition (Global Edition)//

The frequency of the generated signal at the output pin should be <m>{f}={1.44/({R_1}+{2R_2}){C_1}}</m>. The time that the output is at <m>V_CC</m> should be <m>{t_H}={0.7({R_1}+{R_2}){C_1}}</m>, while the time for the output to be at GND should be <m>{t_L}={0.7{R_2}{C_1}}</m>. Thus, the duty cycle is given by <m>({R_1+R_2}/{R1+2R_2})100%</m>.

To get a duty cycle of less than 50%, we need a diode in order to bypass <m>R_2</m> when charging the capacitor.

{{:archive:pgt104:timer555_astable_1.jpg?direct&350|}}

//**Disclaimer**: The image above is extracted from resources available for Digital Fundamentals 11th Edition (Global Edition)//

Thus, the duty cycle is now given by <m>({R_1}/{R1+R_2})100%</m>.

====== Things To Do ======

**THING1** Build a one-shot circuit that produces 1ms pulse. Determine a suitable R & C values. Verify.

**THING2** Build an oscillator circuit that produces 1kHz (50% duty cycle) square-wave signal. Verify.

**THING3** (Optional) Build an oscillator circuit that produces 50Hz signal, and <m>{t_H}=1ms</m>. Verify.