Electronics

Circuit Prototyping and Fabrication

Purpose

  • To prototype a circuit using a protoboard
  • To test and evaluate the circuit
  • To fabricate the circuit on an etched and drilled printed circuit board

Resources

  • Proto Board
  • Wire kit
  • PC Board
  • LM555 Timer
  • 8 pin DIP socket
  • Resistors
    • 100R
    • 10K
    • 100K
    • 1M
  • Capacitors
    • .1f
    • .01f
  • Audio transducer
  • 9v battery & battery snap
  • Alligator clip leads

Background

The 555 timer/oscillator integrated circuit has been around since 1971 and is the biggest selling IC in the history of the electronics industry.  They are in use in many capacities from toys to spacecraft and the original design has not changed since its invention.

The 555 timer can be used in a number of modes. In this activity it will be configured as an astable multivibrator or free running oscillator. In simple terms, the chip will produce a train of electronic pulses which when connected to a speaker will produce a tone. The pitch or more precisely, the frequency of the tone, is determined by the value of the resistors and capacitor in the circuit.

PART 1: Prototype

Since the 555 timer forms the focus of this circuit place it on the proto board first. This will act as a reference to place the connecting wires. Make sure the dot which indicates pin 1 is in the position as shown in the next illustration. Also make sure it is placed in the exact location shown. Place all the wires as well.

Next insert the electronic components:

  • Trim and shape the leads on the resistors and the capacitor.
  •  Use 100K for R1, 10K for R2 and .1f (green) for C1.
  • The audio transducer (speaker) can be plugged in directly, it doesn't matter which way. Make sure the leads are in in the same "column" as the green and brown wires.
  • The battery snap leads (red and black) must be inserted as shown.

Next, double check your circuit and make sure the wires and components are exactly placed as shown. Is the dot on the 555 timer (pin1) in the bottom left corner?

  • Connect the battery. You should hear a tone. If not, disconnect it immediately and re-check the circuit.

Change Components

  • If your circuit is working, note the pitch (frequency) of the tone. Disconnect the battery and swap the .01f capacitor (C1) in for the .1f capacitor. Reconnect the battery. Compare the frequency of the tone.
  • Disconnect the battery, swap back the .1f capacitor and change the 100K resistor (R1) for the 1M resistor. reconnect and compare the frequency.

Making Measurements

If you have a Vernier Labpro or a USB Oscilloscope you can measure the frequency output and  compare to the predicted value. The activity will open in a new window.

Circuit Analysis LoggerPro

Circuit Analysis Oscilloscope

Part 2: Fabrication

In this part you will make a permanent circuit using a pre-etched and drilled printed circuit board. Start by soldering the 8 pin DIP socket. The components are mounted on the top side of the board and the pins are soldered on the "copper" side. The socket is used in case the chip "burns out" and needs to be replaced.

Next insert and solder the connecting wires. Here are a few hints:

  • The wires can be held in place with masking tape or the bare leads (copper side) can be bent.
  • Make sure you apply heat to the copper trace and wire before applying solder. Remember too much heat and the copper trace will separate from the board. Too little and the solder will not flow and results in a "cold solder" joint.
  • Cut off excess wire with a pair of flush cutting "snips".
  • Some melting of the plastic insulation  is OK.
  • Finally, some of the distances between holes on the PC board are different than on the large proto board. You will need to substitute red wires for orange normally between the outside "rail" and the inner holes.

Next insert and solder the electronic components:

  • R2 -10K resistor
  • C1 - Use the smaller .01f capacitor and bend the leads so it is close to the PC board
  • Audio transducer - The leads should be in the same "column" as the green and brown wires.
  • Battery snap -  Make sure the red and black leads are as shown.
  • When all components are installed, mount the chip in the socket. Carefully line up the leads (dot in lower left corner) and press firmly until seated.
  • Note that the 100K resistor R1 is not mounted on the board. More on this later.

Since the circuit will be used as a test device, a pair of alligator leads will be used to connect external devices. solder the leads exactly where the 100K resistor should be. Make sure the wire insulation is close to the PC board. Note: In this exercise the 100K resistor (R1) has been soldered "inline" with one of the leads. It doesn't matter which way the leads are placed.

Finally, use a cable tie to secure the alligator clip leads to the PC board. If not, the leads will eventually be pulled out of the PC board. The battery can be secured to the PC board with a piece of double sided tape. Before you connect the battery read the following hints:

  • Inspect the underside (copper) of the PC board for solder "bridges" and loose or cold solder joints.
  • Double check the placement of wires and components.
  • Is the 555 chip inserted properly in the socket?
  • When you connect the battery, place a finger on the chip. If it gets extremely hot, disconnect the battery immediately.