Lesson 2

Introduction

Our next lesson will cover simple timing circuits. In electronics, timing is very important. At the component level, complex systems will need to have some timing capabilities. For example, a central processing unit (CPU) on a computer will have several operations running in parallel. This would not be possible without strict timing dictating when instructions can be executed. Another example is a typical monitor. The refresh rate of a monitor in the United States is 60 Hz. This means that a unique frame is drawn every 60th of a second. Additionally, within that 60th of a second, each row is activated to display the pixel at each column before being turned off. This means that at any given instance, the screen is mostly off, but appears to be on due to how quickly it refreshes.

Overview

  1. Background Knowledge
  2. Schematic Design
  3. Theory of Operation
  4. Bonus: Shift Registers

555 Timer

The 555 timer is a common chip that is used to create a square wave at various frequencies and duty cycles. The 555 timer has a wide range for input voltage and operating modes.

Square Wave

A square wave is a common wave form in digital circuitry. When the wave is at its peak, the output is considered high or a digital 1. Furthermore, when the waveform is low, the output is considered low or a digital 0. The duty cycle of a waveform is the percentage of time a waveform is high. For instance, if a 60 Hz square wave has a 50% duty cycle, it is on for 17 mS and off for 17 mS before repeating indefinitely.

Timer Design

The formulas at left will be used to find the values of the resistors and capacitor you will use. You should pick a T that will be visible to the human eye. Be aware that capacitors have a small standard value. This means the capacitor will have the greatest impact on T, the period. Rb impacts the time off mostly and Ra impacts the time on.

Theory of Operation

In the circuit at left, an example of the circuit we will design is shown. When the switch is closed, the circuit is powered and begins to oscillate. The time at which it oscillates is largely determined by the size of the capacitor. When all of the math is done this circuit will flash at about a tenth of a second which will just barely appear as a blink and may look more like a dim light. A common capacitance is 4.7uF and this will bring our period to 476mS. The resistors give a duty cycle of about 70%, meaning our on time will be about a third of a second and an off time of about a tenth of a second. A larger capacitor will continue to increase this period. If capacitors are put in parallel, they will add their capacitance. During the normal operation of this circuit the top LED will be on while the during the low end of the duty cycle and the bottom LED will be on during the top end of the duty cycle. This will be an alternating flasher similar to the ones seen at a rail road crossing. To make it more like the rail road crossing, reduce the duty cycle to about 50%. It is important to note that C2 is not necessary to the design.

Bonus: Shift Register

The shift register is an integrated circuit that is very common in modern electronics. The most easily available shift register is the 74LS595. This is a code that gives us information about the chip. The 74 denotes that it is of the 7400 series, LS is short for Low-power Schottkey, and the 595 refers to the model/shift register. Anytime a bar appears over an input or output, it should be read as NOT variable. This means it is active low or the effect attributed to the variable is achieved when the pin is low. The shift register is a powerful Integrated Circuit (IC). Similar to the 555 timer, it comes in a sleek black dual in line package (DIP). The function of this circuit is to take an input signal, SER, and shift it to each pin. SER is sampled on the positive edge of SRCLK and RCLK which are tied together. For example if the input SER is 1101 and both the CLKs are pulsed 4 times, then QD will be 1, QC will be 1, QB will be 0, and QA will be 1. As you can see the input signal is decomposed into into individual bits and shifted through the register.It is important to note that OE must be tied low or to ground as this will enable the output. Additionally, SRCLR must be tied high to prevent the register from being cleared. Further information can be found by looking up the datasheet for this chip.