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Astable Timer

Astable (free-running) mode – the 555 can operate as an electronic oscillator. Uses include LED and
lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation
and so on. The 555 can be used as a simple ADC, converting an analogue value to a pulse length
(e.g., selecting a thermistor as timing resistor allows the use of the 555 in a temperature sensor and
the period of the output pulse is determined by the temperature). The use of a microprocessor-based
circuit can then convert the pulse period to temperature, linearize it and even provide calibration means.
 
The Basic Circuit:
Technical:
In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor.
In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C1:
                                                                            


The high time from each pulse is given by:
                                                

and the low time from each pulse is given by:
                                                         

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.

The power capability of R1 must be greater than

      .
To have an output high time shorter than the low time (i.e., a duty cycle less than 50%) a small diode (that is fast enough for the application) can be placed in parallel with R2, with the cathode on the capacitor side. This bypasses R2 during the high part of the cycle so that the high interval depends only on R1 and C, with an adjustment based the voltage drop across the diode. The voltage drop across the diode slows charging on the capacitor so that the high time is a longer than the expected and often-cited ln *R1C = 0.693 R1C. The low time will be the same as above, 0.693 R1C.

With the bypass diode, the high time is
                                              
where Vdiode is when the diode's "on" current is 1/2 of Vcc/R1 which can be determined from its datasheet or by testing. As an extreme example, when Vcc= 5 and Vdiode= 0.7, high time = 1.00 R1C which is 45% longer than the "expected" 0.693 R1C. At the other extreme, when Vcc= 15 and Vdiode= 0.3, the high time = 0.725 R1C which is closer to the expected 0.693 R1C. The equation reduces to the expected 0.693 R1C if Vdiode= 0.

Practical Circuit:
This circuit allows you to adjust the Pulse Time (ON with Preset R1) and the Pause Time (OFF with Preset R2). The capacitor is a 100mF & with the combination of R1 at 47K and R2 at 1M this will give you times from 2.5 seconds to 60 seconds.
This circuit gives a visual indication of when it is ON via the LED and is capable of driving a Relay.
This drawing shows the output at Pin 3 which will drive the relay,
and how to adjust the times. R1 will adjust the Pulse (On) time
and R2 will adjust the Pause (OFF) time.

By replacing the C1 Capacitor with a higher value it is possible to
extend the times, however there is a limit, before a redesign is
required.

Pin layout of the 555 Integrated circuit.









Now before you embark on the next stage it must be mentioned there are a number of manufacturers who supply timers in either Kit form of premade.

On a Strip board:
This is the above circuit can be built on a 74 x 35mm strip board, (check you have 29 x 14 holes).
First cut all the breaks in the strips as shown in the second image.
Then fit all the links shown in Red, Black, & purple.
Then start to fit the other components. It is recommended that you fit a IC socket rather than solder the IC to the board. IC's do not like too much solder heat.
Once complete set the 2 presets to mid point and switch on.
The LED should start to flash and the relay start to click.
Adjust the ON - OFF time with the two presets.
The PULSE preset sets the LED ON time, the PAUSE preset sets the LED OFF time.














The next image shows all the breaks in the copper track which need to be made before you start placing components on the board. Please note how the strip board has been turned over. (example the 0v terminal is now at the top of the image.)
The components are shown in X-ray as they would be placed.















Parts List for Strip board:




Make your own PCB:
If you want to produce your own Printed Circuit then the following will help. The Component parts are the same as the strip board, except for the PCB and the equipment to produce the PCB. See the Parts List Later.
The Best way is to use the actual size drawing as a template and stick this to a cut piece of Board on the copper side, now drill all the holes 0.8mm. Now remove the tracing and drill the fixing holes 3.2 to 3.5mm. Use the etch resistant pen to copy the circuit from the tracing onto the copper side of the board. Now you can etch the board in Ferric Chloride, once the copper is removed, the etch resistance can be removed with white spirit. It is now time to put the components into the board. Follow the image below for the position of the components, it is recommended that an IC socket be used rather than soldering the IC directly to the board.



















Make your own PCB:
If you want to produce your own Printed Circuit then the following will help. The Component parts are the same as the strip board, except for the PCB and the equipment to produce the PCB.
See the Parts List Later.
The Best way is to use the actual size drawing as a template and stick this to a cut piece of Board on the copper side, now drill all the holes 0.8mm.
Now remove the tracing and drill the fixing holes 3.2 to 3.5mm. Use the etch resistant pen to copy the circuit from the tracing onto the copper side of the board.
Now you can etch the board in Ferric Chloride, once the copper is removed, the etch resistance can be removed with white spirit. It is now time to put the components into the board.
Follow the image below for the position of the components, it is recommended that an IC socket be used rather than soldering the IC directly to the board.





















The following image is the board turn upside down with the terminal block now on the right hand side, use this to double check your tracing before you etch the board.





















PCB Parts List:












The following is a drawing of the copper track on the copper side, use this as a drilling template first.