Since he was the son of an Alabama minister, everyone expected that young Lee de Forest would follow in his dad’s footsteps, living a spiritual life and preaching to his congregations. But it turned out that de Forest was fascinated by science, not theology. He possessed the important gifts an inventor needs — not only was he good with tools, but he had a passion for making new things. In 1899 he got his Ph.D. from Yale and started work with the Western Electric Company in Chicago. He did well, but before long, he left in order to work independently on projects of his own choosing.
De Forest’s greatest achievement came in 1906 when he developed the first amplifying vacuum tube. This tube could do something new and extremely important: it could increase the power of radio signals. There was a big problem with early radio: the signals received by radio sets were so weak that they could only be heard through headphones. If radio were ever to become really popular, the sound had to be made loud enough to be heard by several people at once, which meant it needed a loudspeaker (see my previous column on building a moving coil loudspeaker) driven by powerful electrical signals.
Turn It Up
De Forest’s invention, which he called the Audion, was an electronic device that could transform a small electrical signal coming in one side of the device to a much larger one when it exited the other side.
The Audion tube was a triode, with three connections: the anode, the cathode, and the control grid. Current passing through the filament (cathode) heats it up, which causes it to emit a stream of electrons. The electrons, being negatively charged, are attracted to the positive plate (anode) at the top of the Audion. De Forest’s world-changing idea was to place a grid of wires between the filament and the positive plate. The grid becomes more or less negatively charged as more or less voltage is applied to it. As the signal to the grid varies, it controls the number of electrons flowing between the filament and the anode. This is how a tube amplifier increases the size of the signal — it uses a small signal to control a much larger voltage.
Tubes to Transistors
Once engineers figured out how useful amplifiers could be, they were incorporated into radios, telephone systems, scientific instruments, and much more. The vacuum tube business exploded. During the next 50 years, millions of tubes were manufactured. But tubes had a lot of shortcomings — most significantly they were big, expensive, and hot.
So, in 1945, Bell Labs put together a team of scientists to find a better alternative to the vacuum tube. The team came up with another sort of amplifier called the transistor. Like a vacuum tube, this silicon-based or solid-state amplifier also has three parts. Current applied to the middle part controls a much larger current or voltage between the top and bottom parts. Modern amplifiers are made with integrated circuits which contain great numbers of transistors in a single electronic component.
Build an Amplifier-Based Touch Switch
In this edition of Remaking History we’ll use electronic amplification, the concept pioneered by Lee de Forest, to make a touch switch. We’ll use a pair of simple solid-state transistors (although in principle, de Forest’s vacuum tube Audions could be made to work as well) to amplify a tiny bit of electrical current moving through your finger.
Your body has a fair amount of electrical resistance, which is why you can’t just grab two pieces of wire with your hands and complete a low-voltage (6 volts DC) circuit to make an LED light up. That’s not to say that no current flows at all, it’s just that your skin presents so much resistance (about 100,000 ohms depending on how clean and dry it is) that the amount of amperage coursing through you is exceedingly small. But if that small amount of electrical flow can be detected and then increased, say through the use of an electronic amplifier, than you can use your finger alone as a switch that completes a circuit to light LEDs or sound a buzzer.
To assemble your touch switch, refer to the circuit diagram above. If you’re using a solderless breadboard, then it’s a very easy setup, just follow the photos here. Otherwise you can connect the components directly together, and then solder them.
1. Examine the LED and identify the positive lead, which typically is the longer one.
2. Holding the transistor with the flat side toward you and the legs down, identify the collector (left leg), the base (center leg), and the emitter (the right leg).
3. Wire the transistors together so the emitter of the first stage transistor is connected to the base of the second stage transistor. Connect the emitter of the second stage transistor to a negative power rail on the breadboard. Add a short jumper from the base of the first transistor to an unused row on the breadboard (see below photo).
4. Place a 100kΩ resistor between the first transistor collector and a positive power rail. Connect the positive lead of the LED to the positive rail. Insert a 220Ω resistor between the negative LED lead and the collector of the second transistor (see below photo).
5. Now you’ll use 2 jumper wires placed vertically on the breadboard to create your touch switch. Place one of the vertical wires on the positive rail. Connect the other vertical wire to the little jumper from the first transistor base. Connect a buzzer in parallel with the LED if you want to. Finally, connect your 6V battery pack to the positive and negative power rails (see below photo).
Now Get Amped
In this circuit, the amplifiers are wired in series to provide a gain, or amplification, several hundred times the original current. When you press on the bare wire connected to the 6V rail simultaneously with the base of the transistor connected to the 100K resistor, only a tiny amount of current flows from the battery, through your finger, to the transistor base.
But that small current is amplified first by one transistor, then by the second, increasing it enough to light the LED and sound the buzzer. Congrats, you’ve made your own amplifying touch switch!