An Interval Timer
(Designing a Printed Circuit Board)

Intro to Eng. II - Module 2
(Electrical Engineering)

Developed by
Robert J. Dirkman
Department of Electrical Engineering

Web Development by
Drew Pommet
Department of Electrical Engineering


Student's Guide - Part 1

PART 1. Constructing the Interval Timer on a Protoboard

Make sure your circuit kit contains the components shown in Figure 2. You will also be provided with the extra materials shown in Figure 3. In this part of the project, you will construct the Interval Timer on a protoboard.



Figure 2. Circuit Kit Components




Figure 3. Additional Materials

The Protoboard

The protoboard, shown in Figure 4, provides an quick and easy way to construct electronic circuits.


Figure 4. Protoboard for Constructing Electronic Circuits

The protoboard is a plastic block with sets of holes, spaced 0.100" apart, on both sides of a central slot. The holes are arranged in groups of 5 which are connected together electrically with an internal clip into which wires are inserted. If two components are to be connected in series, one side of each component would be inserted into the same metal strip. All of the holes along the top are connected together and all of the holes along the bottom are connected together as shown.

Connecting the 555 Timer

The 555 timer IC is contained in a mini-dip plastic package. The eight pins protruding from the case are connected to the internal electronics inside the chip. The timer is included in the circuit by connecting other components to its pins.

The pins are numbered from 1 to 8. It is very important to identify which pin is which. The IC has a mark which identifies the orientation of the chip. On some chips the mark is a slot as shown in Figure 5(a). In others, it is a small circular indentation as shown in Figure 5(b). The pins are numbered with respect to this marked end of the chip. Viewed from above (so that the pins are pointing away from you) and with the marked end to the left, the pins are numbered in a counter-clockwise sense with pin 1 located at the lower left of the chip.

Figure 5. 555 Timer IC Chip Pin Numbers (Viewed from Above)

The 555 timer is to be placed in the protoboard as shown in Figure 6. First you must straighten the pins.


Figure 6. 555 Timer Inserted into the Protoboard

The pins on a new IC chip point out at a small angle, as shown in Figure 7(a) and have to be bent, as shown in Figure 7(b), before the chip can be placed into the protoboard. This can be done by placing the pins on one side of the IC on a flat surface, holding the ends of the chip and rotating the chip as shown in Figure 7(c) until the pins are in the proper direction.

Figure 7. Straightening the IC Pins

To insert the chip in the protoboard, carefully place the pins in the desired holes with the IC over the slot. Make sure that all the pins are entering the holes properly and carefully press down until the chip is seated on the board.

If you need to remove an IC from the protoboard, don't try to pull it out with your fingers. For sure you will bend the pins. There is a tool specifically for doing this, but the chip can be removed easily with a small screwdriver. Placing the tip of the screwdriver in the slot alternately pry up each side a little until the chip is completely loosened so that it can be removed with your fingers.

Connecting The Transistor

The 2N2222 transistor has three leads as shown in Figure 8(a). The schematic of the device is shown in Figure 8(b). Notice that you can tell which lead is which by observing the tab (which is closest to the emitter).

(a) (b)


Figure 8. Transistor

Cut the leads so that they are about 1/2" long and insert the leads carefully in the protoboard in the position shown in Figure 12. Make the transistor is oriented so that the tab is in the position shown.

The LED

The LED has two leads as shown in Figure 9(a). One of the leads is longer than the other.The longer lead is on the side of the "flat" portion of the case. The schematic of the LED is shown in Figure 9(b).

(a) (b)

Figure 9. Light Emitting Diode (LED)

Place the LED into the printed circuit board as shown in Figure 12. Cut the leads so that they are about 1/2" long. Make sure you have the flat side of the LED case in the position shown.

The Timing Capacitor

The tantalum capacitor in the parts kit has polarity as shown in Figure 10; it is important how the two leads are placed in the circuit. It is important that the - side is always at a lower voltage than the other side (the + side). In our circuit the capacitor is placed with the minus side in the lower row as shown in Figure 12.


Figure 10. Polarity of the Tantalum Capacitor

The Pushbutton Switch

The pushbutton switch, shown in Figure 11 is a "normally-open momentarily-close" type switch. This means that if the button is not depressed, the contacts are open; when the button is pressed, the contacts are connected. "Momentary" means that when the button is released, the contacts automatically move to their open position by a spring inside the switch.


Figure 11. The Pushbutton Switch

Insert the pushbutton switch on the protoboard as shown in Figure 12.


Figure 12. Protoboard with Transistor, LED, Pushbutton, and Capacitor Added

Completing the Prototype

Add the four resistors and the other capacitor as shown in Figure 13. The battery cable isn't shown. Unlike the 555 timer, the capacitor, the LED, and the transistor, it doesn't matter in which direction these components are inserted. They work the same in either direction.



Figure 13. Protoboard with Additional Components Inserted

Finally add connecting wires as necessary to complete the circuit. The wires should be stripped about 1/4" as shown in Figure 14 so that the bared ends are long enough to make a good connection in the metal clip yet not so long that there is a danger of them touching some other component or wire in the circuit.

Figure 14. Connecting Wires

The Battery Clip

Batteries also have polarity, as you certainly know. Insert the 9-volt battery clip leads into the appropriate holes as shown in Figure 15. Make sure you get the colors right.

The circuit with some of the wires added is shown in Figure 15.


Figure 15. Wired Protoboard with Three Wires Missing

It is helpful, when connecting up a circuit, to indicate the connections already made on the circuit diagram. This is done in Figure 16 for the circuit connected as shown in Figure 15. However, notice that three wires are missing. It is necessary for you to figure out where these should go and add them to your prototype board.



Figure 16. Circuit Schematic Showing Connections Made So Far

The 9 volt battery can be attached to the protoboard using a small piece of double sided sticky foam tape as shown in Figure 17. Place the speaker on top of the battery so that the speaker magnetic holds it on.



Figure 17. Protoboard Circuit with Battery Mounted


Testing the Unit and Making it Work

When the pushbutton is pressed the LED should light for about 3 seconds. If it doesn't, something is wrong with your circuit. Try the following:

If this doesn't help, have a colleague check your circuit; often, you can't see your mistakes.

If this doesn't work, have a teaching assistant or instructor check your circuit.

Part 1 Exercise: Building the Prototype Interval Timer

Construct a working prototype of the interval timer using the components provided, and bring it to the next class meeting.

IMPORTANT: Don't forget that the protoboard shown in the instructions is missing 3 wires. Make sure that you add them.

Go To Student Guide ToC || Part 2 || Timer Main Page || EASNE Page
Last Updated: 8/15/96