Digital Circuits

Timer Circuits

18 questions By Tony R. Kuphaldt

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Question 7 of 18

A sequential timer circuit may be constructed from multiple 555 timer ICs cascaded together. Examine this circuit and determine how it works:

Can you think of any practical applications for a circuit such as this?

Reveal answer

Each 555 timer’s cycle is triggered by the negative edge of the pulse on the trigger terminal. A passive differentiator network between each 555 timer ensures that only a brief negative-going pulse is sent to the trigger terminal of the next timer from the output terminal of the one before it.

Follow-up question: when timer circuits are cascaded like this, do their time delays add or multiply to make the total delay time? Be sure to explain your reasoning.

Notes:
Practical applications abound for such a circuit. One whimsical application is to energize sequential tail-light bulbs for an automobile, to give an interesting turn-signal visual effect. A sequential timer circuit was used to do just this on certain years of (classic) Ford Cougar cars. Other, more utilitarian, applications for sequential timers include start-up sequences for a variety of electronic systems, traffic light controls, and automated household appliances.
Question 8 of 18

Predict how the operation of this astable 555 timer circuit will be affected as a result of the following faults. Specifically, identify what will happen to the capacitor voltage (V_C1) and the output voltage (V_out) for each fault condition. Consider each fault independently (i.e. one at a time, no multiple faults):

Resistor R₁ fails open:

Solder bridge (short) across resistor R₁:

Resistor R₂ fails open:

•Solder bridge (short) across resistor R₂:

Capacitor C₁ fails shorted:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Resistor R₁ fails open: Capacitor voltage holds at last value, output voltage holds at last value.

Solder bridge (short) across resistor R₁: Timer IC will become damaged at the first discharge cycle.

Resistor R₂ fails open: Capacitor voltage holds at last value, output voltage holds at last value.

Solder bridge (short) across resistor R₂: Oscillation frequency nearly doubles, and the duty cycle increases to nearly 100%.

Capacitor C₁ fails shorted: Capacitor voltage goes to 0 volts DC, output voltage stays “high”.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
Question 9 of 18

This circuit uses a “555” integrated circuit to produce a low-frequency square-wave voltage signal (seen between the “Out” terminal of the chip and ground), which is used to turn a pair of transistors on and off to flash a large lamp. Predict how this circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Transistor Q₁ fails open (collector-to-emitter):

Transistor Q₂ fails open (collector-to-emitter):

Resistor R₃ fails open:

Transistor Q₁ fails shorted (collector-to-emitter):

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Transistor Q₁ fails open (collector-to-emitter): Lamp remains off, no current through any terminal of Q₂.

Transistor Q₂ fails open (collector-to-emitter): Lamp remains off, no current through any terminal of Q₂, normal base current through Q₁, no current through collector of Q₁.

Resistor R₃ fails open: Lamp remains off, no current through any terminal of Q₁ or Q₂.

Transistor Q₁ fails shorted (collector-to-emitter): Lamp remains on, full “on” current levels through terminals of Q₁ and Q₂.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.