Tech research group 2 - Tom's PM class - fall 2002
taku, yuriko, & scott
Part 1 - A brief overview of oscillators/ timers
Part 2 - The 555 Timer
Part 3 - Applications for oscillators and timers
Oscillator/ Timer Introduction
What is an oscillator?
An oscillator is a general term for something that sends a signal back and forth. A good example is a pendulum of a clock. If you push it to start the movement, it will swing at a steady frequency for a curtain amount of time. By adding a little bit of energy for each cycle it will sustain this movement. This is how the clock counts time.

An electronic oscillator works on the same principle. By creating an integrated circuit you can have energy moving back and forth between two components at steady rate. The simplest form to create an integrated circuit would be to connect a capacitor to an inductor. Both a capacitor and inductor store energy.

If you charge the capacitor it will start to discharge when it reaches its capacity through the inductor. The inductor that receives the energy will build an electromagnetic field, inhabiting the current until the field is fully created. When the capacitor is discharged, the magnetic field will try to keep the current moving in the circuit sending enough energy to charge the capacitor again. This creates a cycling process called a positive feedback. In theory this cycle should last forever, but do to the resistance in the wire, energy is gradually lost, so additional energy must be added like the pendulum of the clock. The frequency will depend on the size of the capacitor and inductor. Because electronic oscillators are able to send steady pulses, it is used as a timer in many electronic devices.

How does a Theremin work?
The Theremin creates its sound by changing the frequency between two oscillators. This is called beat-frequency. When two frequencies exist at a different rate they create a pulse or beat. The bigger the difference is the more pulses. The two oscillators in the Theremin are initially tuned at a high frequency that inaudible to the human ear. But by disrupting the electromagnetic field of an antenna that influences the current to the oscillator, a difference is made between the two frequencies generating an audible pulse. Because our body possesses an electrical charge, the proximity of the body is critical the disruption the electromagnetic field. This is why the sound of the Theremin is controlled without being touched to the instrument. This process is called heterodyning. Usually the vertical antenna controls the pitch and the horizontal one the volume.

Part 1 - A brief overview of oscillators/ timers
Part 2 - The 555 Timer
Part 3 - Applications for oscillators and timers
The 555 Timer (or 'How I Learned to Count with an IC')
available at Radio Shack
additional links

What is it?
"A highly stable controller capable of producing accurate time delays, or oscillation."
-Philips Components and Semiconductors

The 555 is a 6-pin intergrated circuit with 2 modes of opperation. The time delay (stable) mode is controlled by one capacitor and one resistor. The oscillation (astable) mode is controlled by a capacitor and two resistors. This page will focus on it's astable capabilities.

The timer was first made in the eary 70s by the Signetics Corporation and was called "The IC Time Machine". It was the first commercial IC timer available.

The 555 circuit is consists of two comparators, one ohmic ladder (consisting of three 5k resistors), one flip-flop and a discharging transistor. The inside of this complex array of switches looks like this:

A block diagram of the pins is a bit clearer and easier to read:

The pins perform the following functions:
1) ground
2) trigger - the input to the lower comparator which sets the latch that causes output to go high
3) output
4) reset - resets the latch and returns output to low regardless of the state of the other inputs. when not in use, connect it to V+
5) control - serves as the reference for pin #6.
6) threshold - input to the upper comparator. if 2/3V+ pin 5 is applied, output (pin 3) goes low.
7) discharge - typically shorted to ground. it is on(low resistance to ground) when output is low and off(high resistance to ground)when input is high
8) V+

How to measure freq.
The external resistors and capacitor create the frequency at which the timer oscillates. In a simple astable circuit, as we built, the frequency can be measured by the following equation:

f= -----------------------------
    .693 * C * (Ra + 2(Rb))

>Where R is in K (Ohms) and C is in F (Farads)

it looks like this:

(see our example here)

the trigger (2) and the threshold (6) pins (attached to the comparators) are connected together and to the external capacitor. when power is supplied, the capacitor charges towards the supply voltage through R1 and R2. pin7 (discharge) is externally connected to the junction of the 2 resistors.

when power is first applied to the circuit, the capacitor is uncharged. the lower comparator sets the output to high. this allows the capacitor to charge through R1 & R2. When the capacitor reaches 2/3 of the supplied voltage, the upper comparator is triggered, causing output to go low. when the voltage across the capacitor is 1/3, the lower comparator is triggered, setting output to high.

animation of the flow through the circuit



TLC555 Timer
(276-1718)                 Specifications             Faxback Doc. # 31977

Very Low Power Consumption:............................1 mW Type at V^DD=5V
Capable of Very High-Speed Operation:.......Typically 2 mHz in Astable Mode

Complementary CMOS output Capable of Swinging Rail-to-Rail

High Output-Current Capability:.............................Sink 100 mA Typ
                                                           Source 10 mA Typ

Output Fully CMOS-, TTL-, and MOS-Compatible

Low Supply Current Reduces Spikes During Output Transitions

High Impedance Inputs:.......................................10^12 Ohms Typ
Single-Supply Operation:......................................2 to 18 volts

  Functionally Interchangable with the NE555; has Same Pinout


RESET         TRIGGER           THRESHOLD         OUTPUT         DISCHARGE
              VOLTAGE+          VOLTAGE+                         SWITCH

Low           Irrelevent        Irrelevent        Low            On
High          < 1/3 V^DD        Irrelevent        High           Off
High          > 1/3 V^DD        > 2/3 V^DD        Low            On
High          > 1/3 V^DD        < 2/3 V^DD        As previously

+ Voltages levels shown are nominal.

absolute maximum ratings over operating free-air temperature range
(unless otherwise noted)

Supply voltage, V^DD (see Note 1):.....................................18 V
Input voltage range (any input):.............................-0.3 V to 18 V
Continuous total dissipation at (or below):..........................600 mW
25 degrees C free-air temperature
Operating free-air temperature range:...........0 degrees C to 70 degrees C
Storage temperature range:...................-65 degrees C to 150 degrees C
Lead temperature 1.6 mm (1/16 inch)from the case for 10
seconds:......................................................260 degrees C

NOTES:  1.  All voltage values, are with respect to network ground

Electrical characteristics at 25 degrees C free-air temperature, V^DD=5
V to 15 V (unless otherwise noted)


Threshold                                         66.7%
voltage level as
a percentage of
supply voltage

Threshold         V^DD = 5 V                       10              pA

Trigger voltage
level as a
percentage of                                      33.3%
supply voltage

Trigger current   V^DD = 5                         10              pA

Reset voltage                                      0.7              V

Reset current     V^DD = 5                         +/-10           pA

Control voltage
(open-circuit) as                                  66.7%
a percentage of
supply voltage

Low-level output
voltage           V^DD = 15 V   ^IOF = 10 mA       0.1
                                ^IOL = 50 MA       0.5
                                ^IOL = 100 mA        1              V

                  V^DD = 5 V    ^IOL = 5 mA        0.1
                                ^IOL = 8 mA        0.16

                                ^IOL = -1 mA       14.8
High-level        V^DD = 15 V   ^IOL = -5 mA       14
output voltage                  ^IOL =  10 mA      12.7
                  V^DD = 5 V    ^IOL = -2 mA        4
                  V^DD = 5 V    ^IOL = -1 mA      4.5

                  V^DD = 15 V                     360
Supply Current    V^DD = 5 V                      170          microA

Operating Characteristics, V^DD = 5V

Initial error of  V^DD = 5 V to 15 V,              1%             %/V
timing interval

Supply            R^A = R^B = 1 kohm to 100 kohms.
voltage           C^T = 0.1 microF,
sensitivity of    See Figure 1
timing interval                                    0.1

Output pulse      V^DD = 5 V.   R^L = 10 Mohms,    20
rise time

Output pulse      C^L = 10 pF                      20              ns
fall time

frequency in      R^A = 470 Ohms,   R^B = 200 Ohms, 2.1           MHz
astable mode      C^T = 200 pF


Additional 555 Timer links
Radio Shack

Part 1 - A brief overview of oscillators/ timers
Part 2 - The 555 Timer
Part 3 - Applications for oscillators and timers

Oscillators are very important in many different types of electronic equipment. They appear many different places in our everyday life.

*Quartz Watch:
uses quartz oscillator to keep track of what time is it.
*AM radio : 
uses an oscillator to create the carrier wave for the station,  AM receiver uses a special form of oscillator called a resonator to tune in a station.
a specialized oscillator, called the clock, serves as a sort of pacemaker for the microprocessor.  The clock frequency (or clock speed ) is usually specified in megahertz(MHz), and is an important factor in determining the rate at which a computer can perform instructions.
The motherboard inside the computer's case has an oscillator that is running at anywhere from 300 MHz to 1,000 Mhz. The keyboard has its own processor and oscillator as well. The video card has its own oscillators to drive the monitor. All of these oscillators have the potential to broadcast radio signals at their given frequencies.
*Wireless receivers, and transmitters:
cellphones, pagers, Global Positioning System.
*Cordless computer peripherals:
cordless mouse is a common example; keyboards and printers can also be linked to a computer via wireless
*Audio- frequency equipment:
music synthesizers, theremin, drumbuddy
There are many types of oscillator devices, what they have in common is that they all operate according to the same basic principle: an oscillator always uses  a sensitive amplifier whose output is fed back to the input in phase.  Thus, the signal regenerates and sustains itself.  This is known as positive feedback.  It is the same process that sometimes causes unwanted "howling" in public-address systems.The frequency at which an oscillator works is unually determined by a quartz crystal. 

Our Circuit : a geek tune creator, our board fed the oscillating current into the bx. a variable resistor on the timer modulated the time base (the time between resets) and a variable resistor on the bx input (the 555 output) allowed us to modulate the frequency using FreqOut on the chip to generate sound (through an external amp.) here's the code:

dim timerVar as integer
sub main()
		call Freqout(17,timerVar,timerVar,2560)
		debug.print cStr(timerVar)
end sub