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Line Clock

Introduction

In many digital circuits, we need a regular and continuous train of pulses to control the timing of various circuits. In some cases, we want two separate pulse trains, one the inverse of the other. In other cases, we may want two square wave pulse trains in quadrature (phased 90° apart).

The astable multivibrator you demonstrated in a previous experiment was quite able to produce a pair of complementary clock pulses. The clock frequency and duty cycle could be readily set by adjusting the values of the two capacitors cross-coupling the transistors. However, there is a limitation to this: the operating frequency is only approximate; it isn't really accurate and is subject to drift over time and with component aging.

While this is not a problem with many applications, it is a serious problem when two separate circuits communicate with each other, or when accurate timing is required. In such cases, we need an accurate clock whose frequency is known and stable. Many circuits, especially if they're battery powered, make use of the natural properties of shaped quartz crystals. We won't deal with those at present.

For now, we will make use of a very simple circuit to derive a pair of complementary clock signals from the secondary winding of your power transformer. This is known as a line clock, because it is derived from the AC power line and operates at your line frequency of 60 Hz (in the US) or 50 Hz (in Europe). Most countries use one of these two frequencies for their AC power systems. Because many generators in different places feed power into the power grid, the frequency is maintained very precisely, and is kept quite stable.

You'll find that this line clock is quite satisfactory for a wide range of practical demonstrations.



Schematic Diagram

Schematic diagram of a line-based clock generator.

As we can see in the schematic diagram shown here, the line clock consists of a voltage "conditioning" circuit followed by an inverter. We use two such circuits to provide complementary clock signals.

The voltage "conditioning" circuit consists of a 10K resistor and a 5.1 volt Zener diode. The Zener diode conducts normally in the forward direction, but has the useful property of also conducting current in the reverse direction if the applied voltage exceeds a specific threshold. Of course, any diode will break down and conduct if the applied reverse voltage exceeds the capabilities of the diode. But the Zener diode does this in a controlled manner, and will maintain a nearly constant voltage across itself so long as its power dissipation rating is not exceeded.

A very common application of a Zener diode is for voltage regulation. In this case, we're using it to clip the output voltage of the power transformer secondary winding to legitimate logic levels and then running that clipped signal through an inverter to give it the same output characteristics as all the other experimental gates we've been exploring. The 10K resistor serves to limit the current through the Zener diode to a safe value. The result is that the sine wave output of the transformer is clipped to logic levels, without upsetting the operation of the power supply in any way.

The inverters serve to isolate the clock signals from the power transformer, and provide standard logic signals to your experimental circuits.

To get the inverted clock signal, we could simply pass the CLK signal through another inverter. However, due to the slight time lag required for an inverter to change state, this would mean a very slight overlap between the two clock signals. Often this doesn't matter, but sometimes it does. By using two Zener clipping circuits (very easy because Radio Shack supplies Zener diodes packaged in pairs anyway) we can ensure that there will be no overlap between the two pulse trains.



Parts List

To construct and test the line clock circuit on your breadboard, you will need the two unused inverters in the 4049 IC already on your breadboard socket, plus the following experimental parts:



Constructing the Circuit

To avoid taking up any of the experimental space on the right side of your breadboard socket, we will construct the line clock circuitry in the unused contact columns between the power supply and the 4049 IC driving the LED indicators. This IC still has two inverters that we aren't using as yet, so we only need to add the Zener diode conditioning circuit. We'll simply form the resistors and jumpers to bridge the gaps as needed, and make sure to properly insulate these connections from other component leads.



Circuit Assembly

Start assembly procedure










Starting the Assembly

The line clock will be installed on the left side of your breadboard socket, in the unused space between LED driver IC and the power supply. There is enough room here so that all components will fit without moving anything already in place, other than the leads to the power transformer. However, there's no extra room, so take care that you fit your components carefully into the space you have.

Click on the `Start' button below to begin. If at any time you wish to start this procedure over again from the beginning, click the `Restart' button that will replace the `Start' button.

Remove Transformer Connections

Remove the three jumpers that connect your power supply to the transformer secondary. You will re-install these a little differently in order to more easily accommodate the components you'll add shortly.

Click on the image of the jumpers you just removed to continue.

0.3" Black Jumper

Locate or prepare a 0.3" black jumper and install it in the location indicated in the assembly diagram to the right.

Click on the image of the jumper you just installed to continue.

0.1" Bare Jumper

Locate or prepare a 0.1" bare jumper and install it in the location indicated in the assembly diagram.

Again, click on the image of the jumper you just installed to continue.

0.2" Bare Jumper

Locate or prepare a 0.2" bare jumper and install it in the location indicated to the right.

As before, click on the image of the jumper you just installed to continue.

0.5" Yellow Jumper

Locate or prepare a 0.5" yellow jumper, and install this jumper in the location indicated in the assembly diagram.

Once more, click on the image of the jumper you just installed to continue.

5.1V Zener Diode

Locate a 5.1 volt Zener diode (type 1N4733A or equivalent). Form its leads to a spacing of 0.3". In this application you need not worry about heat dissipation, so clip the leads to a length of ¼" and install this diode flush with the breadboard socket in the location shown to the right. Be careful to note the orientation of the diode, and install your diode with the cathode band in the same direction as the assembly diagram.

Click on the image of the diode you just installed to continue.

5.1V Zener Diode

Locate a 5.1 volt Zener diode (type 1N4733A or equivalent). Form its leads to a spacing of 0.3". Clip the leads to a length of ¼" and install this diode flush with the breadboard socket in the location shown to the right. Be careful to note the orientation of the diode.

Again, click on the image of the diode you just installed to continue.

Black Transformer Lead

Reconnect the black transformer lead to the location indicated to the right.

Click on the image of the lead you just installed to continue.

10K, ¼-Watt Resistor

Locate a 10K, ¼-watt resistor (brown-black-orange) and form its leads to a spacing of 0.5". Clip the formed leads to ½" to hold this resistor above the surface of the breadboard socket and allow room for other parts. Next, locate a ¼" length of yellow insulation (or else remove a ¼" length of insulation from your supply of yellow hookup wire) and slide this insulation over one end of the resistor lead. The insulation will help make sure that the ground lead of the 1000µf capacitor cannot accidentally touch the resistor lead. Install this resistor in the position indicated in the assembly diagram, with the insulated end oriented to the left.

Click on the image of the resistor you just installed to continue.

Yellow Transformer Lead

Reconnect one of the yellow transformer leads to the location indicated to the right.

Click on the image of the lead you just installed to continue.

10K, ¼-Watt Resistor

Locate a second 10K, ¼-watt resistor (brown-black-orange), form its leads to a spacing of 0.5", and clip the formed leads to ½". As with the previous resistor, slide a ¼" length of yellow insulation over one lead. Again, the insulation will protect the resistor lead from accidental contact with the ground lead of the 1000µf capacitor. Then, install this resistor in the location indicated in the assembly diagram, with the insulated lead oriented to the left.

Click on the image of the resistor you just installed to continue.

Yellow Transformer Lead

Reconnect the other yellow transformer lead to the location indicated to the right.

Click on the image of the lead you just installed to continue.

Assembly Complete

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. The CLK and CLK' outputs are taken from the inverter outputs, as shown to the right.

You will need two of the white jumpers you prepared in earlier experiments, in order to complete this experiment. When you are ready, proceed with the experiment on the next part of this page.

Restart assembly procedure
Continue assembly procedure


Verifying the Clock Signals

Turn on power to your breadboarding system, and verify that the LEDs remain off, even though the CLK and CLK' outputs use inverters from the same IC.

Use one of your white jumpers to connect the CLK output to the L0 input. How does L0 respond to this? Use a second white jumper to connect the CLK' output to the L1 input. How does L1 respond?

When you have made these determinations, turn off the power to your experimental circuit and compare your results with the discussion below.



Discussion

The fact that the CLK and CLK' outputs use inverters from the same IC as the LED drivers had no effect. The IC keeps these circuits separate, even though they use the same power connection. Therefore, there is no interaction to any significant degree.

When you connected the CLK output to the L0 input, L0 turned on and was apparently on constantly. The same held true for L1 when you connected the CLK' output to the L1 input. However, we know that the CLK and CLK' outputs cannot possibly both be at a constant logic 1. After all, they are driven from opposite ends of the power transformer, which are necessarily of opposite phases. The power supply is working or the LEDs could not be turned on. So what's happening here?

What's happening is a phenomenon called "persistence of vision." Your eyes will continue to respond for a short while to a visual stimulus, even after that stimulus is removed. When the LED turns off, your eyes will take a fraction of a second to register this change. If the LED turns on again during this time, you will perceive the LED as if it never turned off.

There is a second factor here, augmenting the persistence of vision effect. When current ceases to flow through the LED it still leaves some of the electrons in the LED crystal in an augmented energy state. Some of these will emit visible photons as they return to their normal energy levels, thus delaying the visible turn-off of the LED slightly.

These two phenomena both cause the LED to appear to remain on constantly when it is flashing at a high-enough frequency. The LED need not even be on half the time; as long as it is turned on again before the persistence effects wear off, it will appear to have never turned off. We can and will make use of this phenomenon in future experiments.

When you have completed this experiment, remove the two white jumpers you used to monitor the CLK and CLK' outputs and put them aside for use in future experiments. Then, make sure power to your experimental circuit is turned off.


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