Today we'll turn our attention to yet another dedicated purpose chip: the 555 timer.  While not the newest or fanciest way of generating signals, it is cheap, reliable, and well-known. While they're kind of a meme in the world of hobbyist electronics, you can do some surprisingly cool things with them like drive ultrasonic transducers to levitate things or amplify signals as part of a geiger counter. Or you can do less cool things like turn a disposable vape into a Synthesizer. People are still coming up with new uses for them, such as a 1MHz voltage to frequency converter. Whatever the case, let's dig into what the chip does.

The 555 timer is an integrated circuit (IC) that is primarily used to make square waves of a known frequency.  It is a powered device, and therefore requires connections from pin 1 to ground and from pin 8 to a supply voltage ($V_{CC}$, typically +5 V) in order to function.

A diagram of the the pin connections critical for the operation of the 555 timer as a square-wave generator. The positions of the connections have been shifted to comply with typical circuit conventions (positive powering voltages to the top, negative/ground to the bottom, signals in on the left, and signals out on the right).

The 555 timer has two states: on and off. 

• The chip begins in the on state, in which $V_{out}$ is equal to the supply voltage ($V_{CC}$), and the discharge ($\mathrm{D}$) is not connected to ground (i.e., there is an open switch between the discharge pin and ground). 
• When the chip is off, the discharge ($\mathrm{D}$) is connected to ground (i.e., the switch between the pin and ground is closed) and $V_{out}$ is also equal to ground. 

The 555 timer has two inputs, the trigger ($\overline{\mathrm{TR}}$ ) and the threshold ($\mathrm{TH}$), which are typically wired together.  The values of these inputs are compared to specific voltages, and the outcomes of these comparisons determine whether or not the 555 timer changes state. 

• The trigger ($\overline{\mathrm{TR}}$ ) voltage is compared to $\frac{1}{3}V_{CC}$; if $V_{TR} < \frac{1}{3}V_{CC}$, the chip switches to the on state. 
• The threshold ($\mathrm{TH}$) voltage is compared to $\frac{2}{3}V_{CC}$; if $V_{TH} > \frac{2}{3}V_{CC}$, the chip switches to the off state. 

Although the threshold and trigger voltages may change the 555 timer’s state, there is no current in or out of either pin (since they effectively have infinite input impedance). There may, however, be current at the other four pins, depending on the configuration.  The reset ($\mathrm{R}$) pin will disable or reset the timer if it is at ground or lower, so it generally is tied to $V_{CC}$ to prevent unwanted behavior.

Alright, now that we've talked about what the chip's designed to do, we'll put it to work making an oscillator for us. 

A typical 555 timer astable oscillator.  Note that the pins are not shown in clockwise order around the circuit here.  They have been arranged according to conventions of putting higher voltages towards the top of a diagram and having signals propagate from left to right.	The same circuit, drawn with the pins in counter-clockwise order. The topology is the same, but it requires drawing long, looping connections and doesn't take as well to modifying.

Let's start with a couple of predictions (If you did the tutorial these will be familiar).  Assume the chip initially starts in the on state.

How long will it take the capacitor to charge up to the threshold voltage $\left(\frac{2}{3}V_{CC}\right)$?
After the chip is off, how long will it take the capacitor to discharge enough to turn the chip back on?

Finally, how long will it take in the next cycle for the capacitor to charge from $\frac{1}{3}V_{CC}$ to $\frac{2}{3}V_{CC}$ ?

What is the resulting period of oscillation for this circuit's output?

Characterize the signals you see at both $V_{out}$ and across the capacitor, $V_C$.  Do they behave as expected?

Your signal will be distorted somewhat when you attach your speaker, but the effect will be greatly mitigated with the 10$\Omega$ resistor inline with it. This also results in the sound volume being reduced greatly, as you're effectively making about a 10:1 voltage divider. A properly designed transistor circuit would be able to boost the volume substantially, if you want to experiment with this sort of thing in your copious free time.

Our breadboard-ready speaker. There are two pins on the underside, and the entire thing is about the diameter of a quarter.	A button designed to straddle the middle channel of the breadboard. The left and right sides are connected when the button is pushed.	A schematic of a resistor-speaker network that will not overload our poor 555 timer, and that will be muted when the button isn't pressed.

What do you hear when the speaker is connected?
Predict how the frequency of the sound would change (increase or decrease) if $R_2$ were replaced with a 2.2k resistor

Keep your circuit built, you'll be modifying it throughout the lab. 

As you noticed, you get a somewhat lopsided square wave out of this circuit, because the capacitor charges through both resistors but only discharges through $R_2$.  To fix this, we can use our good friend the diode (Use a 1N914, the smaller ones) to make it so that the capacitor charges through $R_1$ and discharges through $R_2$, letting us create a wider array of signals.

A modified circuit that allows for more flexibility in waveform.  Note that the diode will result in slightly slower charging through $R_1$ due to the 0.6 V drop.

What effect did adding the diode have on the output's waveform (without the speaker attached)?

What effect did adding the diode have on the speaker's sound?

Remember that pin 5 that we told you not to worry about before?  It is the control pin, and it is used to modify the typical trigger and threshold settings by altering a voltage divider circuit.  If we feed a different voltage threshold, then when our input signal is low it will result in the capacitor charging to a lower voltage, (causing quicker oscillations) and when it is higher it will take the capacitor longer to charge, resulting in slower oscillations.

See the Wikipedia page or page 3 of the datasheet for a schematic of the internals. In both diagrams, there's a voltage divider made from three resistors. The threshold and trigger pins are compared to the to 2/3 and 1/3 of the power rail voltage via comparators. We'll get to flip-flops and inverts in later weeks, but for now we'll focus on how the control pin connects directly to the voltage divider / inverting input of a comparator.

Our modulated square wave generator.  We removed the diode here, but it should otherwise be similar to what you've used before.

Do your tests support the idea that higher control voltages lead to the output staying high longer?

Set the function generator to make a 10 Hz, 300mV peak-to-peak square wave with a 3.333V Offset, but leave it off for the moment

Predict what parts of the square wave will correspond to higher/lower frequencies.

What happens when you change the frequency and amplitude of the modulating signal?

Finally, let's try using the speaker to listen to the output signal. Note that this will alter it somewhat; speakers take some work to use without loading other circuitry due to their small resistance and significant inductance.

What difference does the modulating signal make?

This technique is also know as Frequency-Shift Keying (FSK). It was one way of making early Morse code signals more robust, and a modified version of it is used for Bluetooth devices to send binary data robustly.

You've probably noticed that the output for your circuit can jitter about in some odd ways on the scope, and that its hard to observe sometimes.  Since scopes are mostly designed for signals with a single, known period, combinations will always prove problematic. We got around this a bit by judicious triggering settings, but there's another option. These scopes have a built-in Fast Fourier Transform FFT function, which will let us get information about the frequency components of our signals even when they're hard to observe otherwise.

The Fourier transform display is a plot of the amplitudes of different frequency components of your signal.  For a pure sine wave, you'll expect to see a sharp peak at a single frequency (middle) and then some low amplitude noise.  Since the 555 timer circuit generates a few kHz square wave, the largest component will be the first peak shown, followed by some higher-order components.

The FFT of a square wave. The horizontal scale is 2.5 kHz per horizontal division, so the peak intensity at 1.8 divisions corresponds to ~4.5 kHz. You can also see higher order harmonics here, albeit at lower amplitudes.	The FFT of our modulated square wave. Here we've split the major frequency components into 4.3 and 4.9 kHz. Note that you can use the cursors in this mode to measure frequencies too

Your mileage may vary depending on how you've set up your system. The most useful things to do here are to change settings while listening to the audio output, and also going back and forth between the $V$ vs $t$ view and FFT to get an idea of how they're correlated.

Portions of this page are adapted from “Flexible Resources for Analog Electronics” by Stetzer and Van De Bogart.