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Power Supply for Logic Circuits


In order to continuously power modern experimental logic circuits, you'll need a power supply capable of delivering a regulated +5 volt output with a current-handling capacity sufficient to power any experimental circuit you may wish to build and test. Fortunately, such a power supply is easy to build with inexpensive parts, which in turn may be readily purchased at an electronics parts store such as Radio Shack.

We will not go into experimental demonstrations in constructing this power supply, since you probably want to just build it and get on with your experiments. Such experiments will be reserved for a set of pages on analog electronic circuits, which will be designed for the purpose.

If you happen to have a spare power supply from an a typical PC, you might be tempted to try to use it instead of building one. Unfortunately, that's not a practical option. These power supplies are rated for 20 amps or more from the +5 volt source, and require a significant current drain (3 amps or more) to maintain proper regulation. Your breadboarding system won't draw enough current to allow the PC power supply to work reliably.

You can build a perfectly good, serviceable logic power supply on approximately the last half inch at one end of a large breadboard socket. Or, if you prefer, you can build the same circuit on a separate board and then run the power connections over to your breadboard socket for use with your experimental circuits. The layout on this page is for building the power supply on the left end of the socket.

Schematic Diagram

Schematic of the +5 volt logic power supply.

Since every experiment involves a particular circuit, the schematic diagram of that circuit will be included and explained. In the case of the logic power supply, most of the important circuitry is contained within the 7805 IC, which is a standard three-terminal voltage regulator.

For the most part, this schematic is here for the sake of interest and completeness. However, one point needs to be addressed at once: the transformer primary is shown connected only to the line cord and plug. This is because this circuit is to be constructed on your breadboard socket, for experimental use only. We call for the use of a power strip having both a switch and a circuit breaker, to provide the appropriate protection against miswires and short circuits.

If at any time you should decide to build this power supply or any other as a separate, stand-alone circuit, be sure to include a power switch and an appropriate fuse or circuit breaker in the hot side of the primary circuit. This power supply is double-insulated — there is no electrical connection between the primary and secondary sides of the transformer — so most jurisdictions do not require a grounding plug. However, you should check your local electrical codes in such matters.

Parts List

In order to construct the logic power supply you will need the following parts and components. Most components listed are available from Radio Shack in the US; you folks in other parts of the world will have to locate your own parts, I'm afraid:

Tools Required

To construct the power supply on your breadboard socket, you'll need a pair of diagonal cutters, longnose pliers, and a basic wire stripper. If you're practiced at it, you can use the diagonal cutters in place of the wire stripper.

You'll also need your voltmeter to verify the correct operation of the completed power supply.

Preparing Jumper Wires

One thing you'll need to do constantly for any experiments on a breadboard socket is to construct and install jumper wires. You'll need a number of different lengths, of course, but two common lengths will be 0.3" and 0.5". These lengths match the spacing between the main component area in the middle and the bus strips along the top and bottom of the breadboard socket.

The traditional way to create a jumper is to cut a piece of insulated wire from the roll or bundle, and then remove ¼" of insulation from each end. This works fine for longer jumpers (2" or more), but is a problem when you try to remove insulation from the end of a 1" length of wire. You're almost guaranteed to pull off all of the insulation.

A looped jumper wire.

One answer is to make the jumper a bit longer and bend it as shown to the right. The added length is enough to let you hold the body of the jumper firmly while removing the ¼" of insulation at each end. This has the added advantage that it is easy to insert and remove the jumper from the breadboard socket, or to move it from place to place during an experiment. It also has the downside that the breadboard socket can get filled up with loops of jumper wire all over the place, making it more confusing in some cases.

A minimum-size, squared-off jumper wire.

Since the power supply will be on your breadboard socket for some time, it makes sense to build it as neatly and compactly as possible. Therefore, we'll make the jumpers as shown to the right: as short as possible and with ends bent at right angles to just fit where they need to go.

To easily make jumpers this way, start by removing about 4 to 5 inches of insulation from the end of a spool of hookup wire with the appropriate color insulation. Throw this away. Then, bend ¼" of wire at the end into a right angle.

Now, use the wire stripper to separate the required length of insulation from the main body still on the wire. As a rule of thumb, make this length to the nearest 1/16" that is shorter than the desired length of the jumper. For a 0.3" jumper, for example, you can cut ¼" (0.25") of insulation from the current end and slide this short length of insulation up to the bend. For a 0.5" jumper, make the insulation 7/16" long. This makes it easy to use a standard ruler, marked in sixteenths of an inch, to measure the required length of insulation.

Next, bend the wire again at the end of the cut length of insulation. Finally, cut the wire ¼" from the second bend. This will leave you with a jumper that will fit precisely into place, and will sit snugly on the surface of the breadboard socket.

Other Components

Other components are handled similarly to short jumpers. However, for components such as resistors, capacitors, and diodes, it is important not to bend the leads of the component directly at the body of the component itself. That could break the casing of the component or even tear one of the leads loose, thus destroying that component's usefulness. Diodes require slightly different treatment. Rectifier diodes may dissipate a significant amount of heat, while signal diodes often have glass bodies that require gentle handling. Therefore, diode leads should be cut to ½" rather than ¼", to space them above the surface of the breadboard socket. Transistor leads are generally short enough that they need not be clipped after forming.

Use the following figures as a guide to forming small jumpers and component leads to fit readily on your breadboard socket. For each component, first bend the component leads to the correct spacing without bending them too close to the component body; then clip the leads to the length indicated.

Resistors Diodes Transistors
Forming short jumpers. Forming resistor leads. Forming diode leads. Forming transistor leads.

Circuit Assembly

Start assembly procedure

7805 voltage regulator

Beginning the Assembly

This is an example of the assembly diagrams that will be used in all experiments in these pages. The image to the right represents the left end of your breadboard socket, which is where we will construct the power supply circuit. Most components will fit easily on the socket itself, and the remaining components will be placed nearby so as to avoid using up any additional space on the socket. The construction display is designed to fit inside an 800x600 or larger window. I'm sorry, but 640x480 is just too small for this; you'd be forced to scroll the page horizontally.

To begin the assembly procedure, click the 'Start' button below. If at any point you want to start over, simply click the 'Restart' button that will appear as soon as you begin.

0.3" Black Jumper

Prepare a 0.3" black jumper as described above. Insert it into the location shown in the assembly diagram to the right. Black jumpers are typically used to make ground connections to the various circuits. We'll use black jumpers here to connect the power supply ground reference to the inner bus strips on your breadboard socket.

When you have this jumper in place, click on its image in the assembly diagram to move on to the next assembly step.

0.3" Black Jumper

Prepare a second 0.3" black jumper as described above. Insert it into the location shown in the assembly diagram.

As before, when you have installed this jumper correctly, click on its image to continue.

0.3" Black Jumper

Prepare a third 0.3" black jumper as described above. Insert it into the location shown in the assembly diagram.

As before, when you have installed this jumper correctly, click on its image to continue.

0.5" Red Jumper

Prepare a 0.5" red jumper wire and install it in the location shown. This jumper will connect your +5 volt output to the upper bus strip on your breadboard socket, to make +5 volts readily available to experimental circuits.

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

0.3" Red Jumper

Prepare a 0.3" red jumper as described above. Insert it into the location shown in the assembly diagram to the right.

Again, click on the image of the jumper to continue.

0.5" Red Jumper

Prepare a 0.5" red jumper wire and install it in the location shown. This jumper will connect your +5 volt output to the lower bus strip on your breadboard socket, to make +5 volts readily available to experimental circuits.

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

0.1" Jumper

Cut a ½" length of bare hookup wire and bend it in half as shown here. Don't bother trying to keep insulation on this jumper; that's almost impossible and isn't worth the effort. Use this jumper to connect the leftmost two columns of contacts on the lower half of the breadboard socket as indicated,

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

Silicon Rectifier Diode

Before preparing any rectifier diodes for your power supply, look at the body of one diode. You'll see that there is a color band painted around the diode body, next to one end. This band marks the cathode end of the diode. It is essential that you observe and follow the indicated orientation of each diode you install. Incorrect installation can cause damage to circuit components.

Diodes should always be spaced above the surface of the breadboard socket. To accomplish this, form the diode leads to the required 0.3" spacing and then clip the leads to ½" length. When you insert the diode, the longer leads will keep it spaced above the breadboard socket and allow air circulation.

Click on the image of the diode to continue.

Silicon Rectifier Diode

Locate a second silicon rectifier diode and form its leads to a spacing of 0.3". As before, cut the formed leads to a length of ½" and install this diode in the location shown to the right. Again, be careful to observe the orientation of the diode in the assembly diagram.

As usual, click on the image of the diode to continue.

7805 +5 Volt Regulator IC

Install the 7805 voltage regulator IC as shown. Be sure to mount this IC so that the metal back faces the top of the breadboard socket, and the front side with the printed markings faces the lower edge. In this orientation, pin 1 of the IC is closest to the left edge of the breadboard socket, pin 2 is connected to your black jumpers, and pin 3 is connected to your red jumpers.

You may find it easier to insert this IC if you twist the three leads 90° to align with the contacts inside the breadboard socket.

Click on the image of the regulator you just installed to move on to the next step.

.01µf Disc Ceramic Capacitor

Locate a .01µf disc capacitor and examine it. If you don't have any .01µf disc capacitors, similar ones of higher capacitance may be substituted. The capacitors in the experimental package I picked up from Radio Shack included 13 .047µf capacitors (labelled 473, or 47000 pf) as shown in the pictorial. A .01µf capacitor may be labelled either ".01" or "103." Any other characters, such as "Z5U" or "YF2," may be safely ignored here.

In many cases the capacitor leads will already be short enough to use without clipping them. If they are longer than 3/8", clip them to a length of 3/8". Then install this capacitor in the location indicated in the assembly diagram. The spacing between leads on such capacitors is typically either 0.2" or ¼". Either spacing will fit pretty well on this diagonal.

As you probably expect by now, click on the image of the capacitor body to continue.

The Power Supply Reservoir Capacitor

The 1000µf electrolytic capacitor you obtained for the +5 volt power supply is much too large to fit on the breadboard socket itself, but it can easily be placed just off the left end of the breadboard for convenience. Its leads are long enough to reach the points where they must go.

The arrow markings on the body of this capacitor point to the negative lead. It is important to observe the polarity of an electrolytic capacitor when you install it in a circuit. Insert the negative lead into a ground point as shown, and the positive lead into the common connection between the two rectifier diodes and pin 1 of the 7805 regulator IC.

Click on the capacitor image to continue, once you have installed it.

The Power Supply Output Capacitor

Examine the 10µf electrolytic capacitor you obtained for the +5 volt power supply. Both leads project from the same end of the capacitor body, but one is shorter than the other. In addition, the plastic outer case shows arrows pointing to the shorter lead. As with the 1000µf capacitor, the arrows indicate the negative lead.

Clip both capacitor leads to a length of 3/8", and install this capacitor as shown in the assembly diagram. Be sure to observe the capacitor polarity: the negative lead must be oriented to the left and connected to ground (black jumpers). The positive lead is connected to +5 volts (red jumpers). Allow this capacitor to "lean" forward a bit so it does not touch the body of the 7805 IC.

Click on the capacitor image to continue, once you have installed it.

Preparing the Transformer Secondary

Your transformer for this power supply has a primary side and a secondary side. The secondary side has three wires associated with it. In my sample, obtained from Radio Shack, the center lead is black while the other two leads are yellow. Your transformer will have a similar means of identifying the three secondary wires.

Cut a 6" length of black hookup wire and remove ¼" of insulation from each end. Also trim the exposed wire from the transformer to ¼" if necessary. Connect one end to the black center tap lead using a small wire nut. Make sure that both wires are physically secure within the wire nut, as you twist the wire nut tightly over both wires.

Repeat with two 6" lengths of yellow hookup wire, connecting them to the two yellow leads. Then, click on the transformer image to the right to continue.

Connecting the Transformer Secondary to the Breadboard

Insert the two yellow wires from the transformer secondary winding into adjacent leftmost contact strips on the breadboard socket as shown to the right. It doesn't matter which wire goes in which contact strip.

Next, insert the black wire from the transformer secondary into the indicated ground contact. Electrically, any ground contact will do, but this placement keeps the wire out of the way.

When all three wires are correctly inserted, click on them to continue your assembly.

Wiring the Transformer Primary

Trim the exposed conductors from the transformer primary wires and your line cord to ¼", and use two wire nuts to connect each transformer primary lead to one of the line cord wires. It doesn't matter which lead goes to which wire.

The connections shown here are acceptable without grounding in most jurisdictions, because the primary connections are completely isolated from your experimental circuitry. This arrangement is described as double insulated, since the 120 volt (or 240 volt) primary circuit is completely insulated from all other circuitry.

Click on the transformer image to continue.

470, ¼-Watt Resistor

Locate a 470, ¼-watt resistor (color code yellow-violet-brown) and form its leads to a spacing of 0.3" as shown above. Clip the leads to ¼" length as indicated, and install this resistor on your breadboard socket as shown in the assembly diagram.

As usual, click on the image of the resistor to continue.

Green LED

Locate a green LED (you can substitute another color if you prefer, and you can use any shape you have available, according to your preferences) and examine it. As shown in the pictorial here, one lead is longer than the other, and the shorter lead is adjacent to a flat spot on the side of the case. The flat spot and shorter lead indicate the cathode lead.

Note carefully which of the two leads is the cathode, and then trim the LED leads to a length of ¼". Then insert the LED as shown, with the cathode to the right (connected to the 470 resistor you just installed). You should find that while there is no extra space, the LED will just fit in this location. Do not force the LED into place; some have leads formed to space them above the mounting surface.

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

Assembly Complete

This completes the assembly of the +5 volt logic power supply on your breadboard socket. Before you go on, please check your layout against the assembly diagram, and make sure all components are installed as shown. Make any corrections that may be required.

For the experiments on these pages, the next item down the list will be the actual experimental procedure. In this case, however, we will merely test the power supply and make sure it is working correctly. When you are ready, scroll on down and follow the instructions there.

Restart assembly procedure

Testing the Power Supply

Plug the line cord into a commercially available power strip, which includes both a switch and a circuit breaker for overload protection. This provides an easy way to protect your household wiring against accidental short circuits and have a power switch at the same time.

Set your voltmeter to read DC voltages in the range of 0-20 volts. If your meter does not have a 20 volt range, select the lowest possible range above 20 volts. Connect the ground (typically black) lead from the voltmeter to the negative lead of the 1000µf capacitor. Connect the input lead (typically red) from the voltmeter to the exposed end of either red jumper wire. Turn on your voltmeter, if necessary.

Turn on the power switch. The green LED should immediately turn on and remain on. At the same time, your voltmeter should indicate a voltage between +4.75 volts and +5.25 volts (within 5%, or 0.25 volt, of +5 volts). If the voltage is correct but the LED remains off, you probably have it installed backwards. If the voltage is incorrect, you have very probably miswired something. In either case, turn power off and wait 15 seconds. Then go back up to the assembly diagram and recheck your installation.

Once you have verified the correct output voltage, move the red lead of your voltmeter to the positive lead of the 1000µf capacitor. Here you should measure a voltage of about +17 volts. This will vary somewhat depending on your exact line voltage and component tolerances, but will be close to this value.

If your voltages are correct, observe the LED as you turn power off. What does its behavior suggest to you?


The heart of this circuit is the 7805 voltage regulator IC. This device does the hard part of making sure that you get a stable +5 volt power source for all of your experimental circuits. Of course, there is room for some variation, due to component tolerances and other variables that cannot be readily controlled. The actual output voltage may vary by 5% (0.25 volt) from the nominal +5 volt value and still be within acceptable range. The power supply for the test circuit built here to verify all experiments measures 4.85 volts.

When you turned on power, the green LED turned on. Thus, it serves as a pilot light to let you know the circuit is powered up. However, it serves two additional purposes as well. It performed both of these functions when you turned power off.

You should have noticed that the LED stayed on for several seconds after you turned off power. This shows that the 1000µf capacitor holds enough of a charge to keep your circuits powered in spite of changes in load conditions or your household line voltage. The test setup showed a voltage of 17.5 volts on this capacitor, which is quite sufficient to allow the regulator to function properly.

You should never insert or remove any components while that green LED is on, unless instructed to do so in an experiment. Wait until it goes completely out before you construct or modify a circuit. This will help to prevent possible damage to some sensitive components.

The second function of the LED here is to provide a minimum load on the power supply, so the electrolytic capacitors will be sure to have a discharge path. Without that discharge path, these capacitors could easily hold their charge for a longer period of time.

As wired here, your power supply is capable of providing up to about 150 milliamperes (mA) of current to experimental circuits. This is quite sufficient for all of the experiments on these pages. The 7805 IC may get quite warm to the touch, but this is perfectly acceptable. If you provide a heat sink for the 7805, this circuit is quite capable of delivering a full ampere of current to its load. Thus, you can use this circuit as-is for a practical, stand-alone experimental power supply. If you do so, however, remember to provide a power switch and a fuse or circuit breaker in the transformer primary circuit.

Now that you know your power supply is working properly, you are ready to move on to the next phase of your preparations for these experiments: building a circuit to monitor the states of multiple digital outputs. Proceed when you are ready.

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