Make Electronics

Experiment 11: A Modular Project

82 Chapter 2

Experiment 11: A Modular Project

You will need:

• AC adapter, breadboard, wire, and meter.


• LED. Quantity: 1.


• Resistors, various.


• Capacitors, various.


• Transistor, 2N2222 or similar. Quantity: 2.


• 2N6027 programmable unijunction transistor (PUT). Quantity: 2.


• Miniature 8Ω loudspeaker. Quantity: 1.
  So far, I’ve described small circuits that perform very simple functions. Now
  it’s time to show how modules can be combined to create a device that does
  a bit more.
  The end product of this experiment will be a circuit that makes a noise like a
  small siren, which could be used in an intrusion alarm. You may or may not
  be interested in owning an alarm, but the four-step process of developing it
  is important, because it shows how individual clusters of components can be
  persuaded to communicate with each other.
  I’ll begin by showing how to use a transistor to make a solid-state version of
  the oscillating circuit that you built with a relay in Experiment 8. The relay, you
  may remember, was wired in such a way that the coil received power through
  the contacts of the relay. As soon as the coil was energized, it opened the con-
  tacts, thus cutting off its own power. As soon as the contacts relaxed they re-
  stored the power, and the process repeated itself.
  There’s no way to do this with a single bipolar transistor. You actually need two
  of them, switching each other on and off, and the way that this works is quite
  hard to understand. An easier option is to use a different thing known as a
  programmable unijunction transistor, or PUT.
  Unijunction transistors were developed during the 1950s, but fell into disuse
  when simple silicon chips acquired the ability to perform the same kinds of
  functions, more accurately and more cheaply. However, the so-called pro-
  grammable unijunction transistor is still widely available, often used in appli-
  cations such as lamp dimmers and motor controllers. Because its primary use
  is in generating a stream of pulses, it’s ideal for our purposes.
  If you put together the components shown in Figure 2-98, the LED should start
  flashing as soon as you apply power.
  Note that this circuit will work on 6 volts. You won’t damage anything if you
  run it with 12 volts, but as we continue adding pieces to it, you’ll find that
  it actually performs better at 6 volts than at 12. If you read the next section,
  “Essentials: All about programmable unijunction transistors,” you’ll find out
  how the circuit works.

R1

R2

R3

Q1 C1

D1

6V

DC

Figure 2-98. Assemble these components,
apply power, and the LED should start
flashing.

R1: 470K
R2: 15K
R3: 27K
C1: 2.2 μF electrolytic capacitor
D1: LED
Q1: 2N6027 programmable unijunction
transistor