The NE555 astable circuit

The NE555 Integrated Circuit (IC) is an easy to use timer that has many applications in Meccano modeling. It is widely used in many circuits and this means it is also very cheap to buy, typically only 40p. Other versions of the NE555 are available for low current applications or use in extreme temperatures. A ‘dual’ version with two NE555s in one package is also available, called the NE556.

In this and the next two issues of Electronics in Meccano, three of the most common NE555 circuits will be introduced, starting with the astable circuit which was used in issue 2 as part of the stepper motor driver circuit.

Figure1: Pin-out diagram of the NE555First, we need to know what the NE555 looks like. A top view of it is shown in figure 1. It has eight connections (called pins) to its plastic case, arranged as four on one side and four on the other, as shown. From this diagram, you can see that pin 1 is the pin on the bottom left when the IC is held horizontal with the writing the correct way up. The little notch in one side also helps to identify pin 1, as does the small white dot next to this pin.

Click here to read this datasheet in Adobe Acrobat format 555 Timer Datasheet
141Kb

The most important pins on the NE555 are the two power supply pins and the output pin.

The Power Supply
Pin 8 is where you connect the positive power supply (+Vs), which can be any voltage between 3V and 15V DC. Pin 1 is the 0V connection to the power supply.

The Output
Pin 3, the Output of the NE555, is where you connect up components that you wish the NE555 to control. These could be an LED, lamp or motor, and the way they are connected will be discussed later on.

The Output only has two ‘states’ — it is either ‘on’ or ‘off’. When it is on, it’s voltage is the same as the power supply voltage at pin 8. When it is off, it’s voltage is 0V.

There are several other names that may be used instead of ‘on’ and ‘off’ in these types of circuit, so don’t be confused if you see different terms being used:

on — high, mark, logic 1
off — low, space, logic 0

Figure 2: Timing diagramThe astable circuit

In an astable circuit the output continually switches on and off with no intervention from the user, producing a ‘square wave’ as shown in figure 2.

This type of circuit could be used to give a mechanism intermittent motion by switching a motor on and off at regular intervals.

Figure 3: Circuit diagram of NE555 circuitIn the stepper motor driver circuit, this regular square wave was used as a ‘clock’ for the SAA1027 so that as the output of the NE555 changed state from high to low, the stepper motor moved one step.

When we use the NE555 for an astable circuit, the lengths of time that the output is ‘on’ and ‘off’ can be changed by changing the values of the two resistors and one capacitor connected at pins 2,6, and 7 (see figure 3).

The following two formulas are used to determine the ‘on’ and ‘off’ times...

Equation Ton = 0.7(R1+R2)C
Equation Toff = 0.7R2C

...where R1 and R2 are the two resistor values in Ohms and C is the capacitor value in mF. As you can see from these formulas, the ‘on’ time Ton cannot be equal to or less than the ‘off’ time Toff..

The sum of Ton and Toff is known as the ‘time period’, and since the frequency of a square wave is given by the reciprocal of the time period, another formula can be written to give the frequency of the square wave...

Equation f = 1.44 / (R1 + 2R2)C

...where f is the frequency measured in Hertz (Hz). 1Hz is one on/off cycle per second.

Doing the calculations

If the formulas look daunting, don’t worry! The following steps and example {in curly brackets} should make it simple for you to work out the components you need for your own astable circuit:

  1. Firstly, decide on the length of the ‘off’ period Toff..
    This can be very small (milliseconds) or large (several minutes), but it must be expressed in seconds for the equations to work.
    {I choose Toff=1s}

  2. Next, guess a value for the capacitor C in micro-farads. For starters, try 10mF. {I choose C=10mF}

  3. Put the values of Toff and C into the equation below and calculate resistor R2...

    Equation R2 = Toff / 0.7C

  4. Next, decide on the ‘on’ time period Ton which you require {I choose Ton=1.5s} and calculate resistor R1...

    Equation R1 = (Ton / 0.7C) - R2

You should now have the component values required to build the astable circuit in figure 3. However, if any of the resistor values calculated are smaller than 1kW or larger than 1MW, re-do the calculations with a different value of capacitor until you get resistor values in the acceptable range.

If you want Ton and Toff to be almost equal you should ensure that R2 is at least 10kW, then omit step 4 of the calculation and just make R1 much smaller than R2, for example, the minimum value of 1KW.

Need some help with these equations? Why not get the astable wizard to do the work for you!
Only works with Microsoft Internet Explorer version 4.0 and above.

Figure 4: Using the NE555 outputThe NE555 Output

In figure 3 a load is shown connected between the Output pin 3 and 0V. When the Output goes high, current will flow through the load and switch it on. This load could be anything that can be switched on and off, such as an LED, lamp, relay, motor or electromagnet. Unfortunately, these components have to be connected to the Output in different ways because the Output of the NE555 can supply a current of only 200mA.

LEDs
Typically LEDs consume only 20mA, so they can be connected directly to the NE555 Output via the usual current limiting resistor as shown in figure 4a. LEDs were covered in
issue 2, but just to remind you, the formula to work out the resistor required is...

Equation R = Vs - 2 / 0.02

...where R the value of resistor required in Ohms and Vs is the voltage of the power supply.

Lamps
Small low voltage lamps may be connected directly to the NE555 output, although it is best to switch them using a relay as shown in figure 4b. You can of course control several lamps, and higher voltage lamps using an appropriate relay.

Figure 5: Stripboard layout for the NE555 astable circuitMotors and Electromagnets
These must be connected via a relay because of the high current they consume. To just switch a motor on and off, use the wiring in figure 4c. To allow the relay to change the direction of the motor, use the wiring in figure 4d. This is the same as the ‘reversing switch’ from issue 1.

Notice the diode in the circuits that use the relay. This is there to stop the NE555 being damaged when the relay switches off and produces a ‘back-emf’. The diode used is called the 1N4148 signal diode.

Building the circuit

To try out the circuit and your own ideas, you can build the circuit on breadboard (see issue two) so you can easily move components around if you need to without soldering. When you have a finished design, build the circuit on stripboard. Figure 5 shows a example stripboard layout for the NE555 astable circuit. Here are a few tips for building the circuit:

Contrast with: The NE555 monostable circuit , The NE555 bistable circuit

The Electronics in Meccano Circuits Shop Buy 555 Astable Modules from the Electronics in Meccano Circuits Shop

Click here for details

 

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What's Vs and 0V?

Vs is the name given to the positive lead from your power supply (marked '+' or coloured red.)

0V is the reference voltage for the circuit and is the negative lead from your power supply (marked '-' or coloured black.)

Some power supplies for electronics have three outputs which are +Vs, 0V, and -Vs. Ignore the -Vs output.
Use a multimeter on it's DC voltage range to check the voltage from a power supply before you connect it to a circuit.

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Figure 7: Driving high power stepper motorsDriving more powerful stepper motors
By John Hewes

Because some smaller stepper motors such as the one available from Maplin may not be powerful enough for use in Meccano models, you may wish to use larger motors (such as those salvaged from old dot-matrix printers.)

Unfortunately, the SAA1027 will not directly drive any stepper motor that consumes more than 500mA per coil, so if you have a stepper motor that consumes more than this, you will need to add an extra transistor per coil as shown in figure 7. A suitable transistor is the TIP32C, a pin-out drawing for which is also shown in figure 7.

Alternatively, you could gear down a smaller stepper motor so that it can drive larger loads. This would also have the benefit of making the rotation smoother and increasing the number of steps per revolution.

The four transistors will need to have heatsinks fitted as they become fairly hot in use. Meccano parts could be used for this. However, don't use one heatsink for all of the transistors because the metal case of each transistor is actually connected to one of the input leads, and so connecting them with a metal heatsink would cause a short circuit.

The 1000mF capacitor in the SAA1027 driver circuit may need to be replaced with one of a higher value to make the circuit work, or you could run the stepper motor and the SAA1027 circuit from separate power supplies, only linking the 0V wire.

See also: Stepping out: Using stepper motors

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Title: Practical Matters -- Stripboard

In issue two we discussed how breadboard can be used to design your circuits easily without soldering. When you have finished the design, you can build the circuit for real on stripboard, which is similar to breadboard in that it has copper strips with a matrix of holes at a 0.1 inch pitch. However, in stripboard the copper tracks run in one direction only. Component leads are pushed through the holes and then soldered to the copper track on the underside of the stripboard. The remaining part of the lead can then be cut off at the top of the solder joint using good quality side cutters so that the soldered joint is not damaged. The leads can fly off quite fast, so eye protection is advisable.

ICs such as the NE555 are placed at right-angles to the direction of the track so that each pin has a section of track on each side of the IC. Always remember to cut the copper track between the pins of the IC (as shown by the X's on the diagrams) so that there are no short circuits . You can buy a special track-cutting tool to do this neatly.

Don’t solder ICs directly onto the stripboard as you may need to remove them in the future — use an ‘IC holder’ instead and then insert the IC into this when you have finished soldering. Using an IC holder also avoids the IC getting damaged by heat from your soldering iron.

Always design your stripboard layout on paper first!
Included with the printed edition are two copies of the EiM ‘Stripboard Layout Planning Sheet’ which should make designing stripboard layouts easier. (You can download
strpplan.pub, 109Kb, the Stripboard Layout Planning Sheet in Microsoft Publisher '97 format)

Start by putting the +Vs power supply on a track at the top and the 0V power supply on a track at the bottom, since most components will be connected between an IC pin and one of the power supply tracks. Next, position the ICs (mark crosses where you will break the tracks between the ICs), and connect up each pin in turn to the components. Don’t be too fussy about the exact positioning at this stage. Use tinned copper wire, or surplus leads from components, to connect up tracks. On your second attempt, try to save board space by placing components as close together as possible.

Finally, when you have built the circuit, remember not to mount it where any Meccano parts can touch the tracks!

Stripboard layout examples in Electronics in Meccano

Although example stripboard layouts of circuits will be given in EiM, it is always best to re-draw them yourself since you will invariably be using some components that are not the same size as the ones I’ve used. A typical example is capacitors, which vary in size dramatically depending on their value. Also, you may need to incorporate extra components or other circuits onto one board, which will usually mean a bit of component re-arranging. For example, you may wish to combine the SAA1027 and NE555 astable circuits so that the astable clocks the SAA1027.

See also: Practical Matters: Veroboard

Contrast with: Breadboard

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Title: Shopping ListThe following lists the electrical parts that are discussed in the articles. Prices and order codes given are taken from the current Maplin catalogue, which is the probably best source of electronic components for the hobbyist in the UK.

If you have access to a company account with Rapid Electronics or RS Electronics you may find these companies are cheaper.

The NE555 astable circuit
 
Parts needed for the NE555 astable circuit in Figure 3 to be constructed on stripboard
Maplin Order Code Price Page
1 x NE555 Timer IC QH66W 29p 292
1 x 8-pin IC Holder BL17T 14p 304
1 x 220mF Capacitor VH41U 22p 94
1 x 0.01mF Capacitor BX70M 15p 91
Stripboard (at least 24 holes x 12 rows) JP47B £1.79 204

Also needed are suitable values for resistors R1 & R2, and capacitor C.

Driving more powerful stepper motors
Maplin Order Code Price Page
4 x TIP32C PNP transistor UM83E 80p 248
4 x 1N4148 signal diode QL80B 8p 246

For a parts list for the stepper motor driver circuit on stripboard, please see the Shopping List in issue 2

Practical Matters: Stripboard
Maplin Order Code Price Page
Stripboard (39 holes x 29 rows) JP47B £1.79 204
Stripboard track cutting tool FL25C £6.49 205
Side wire cutters GW96E £4.99 1019

Maplin charge £2.50 for delivery on orders under £30.00 inc. VAT.
Prices are taken from the September 2000 - August 2001 Maplin catalogue, and include VAT at 17.5%

Contact their order line on 0870 264 6000 or visit one of their shops.
Their customer service line is 0870 264 6002 and they have a website at www.maplin.co.uk where on-line ordering is available.

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Electronics in Meccano March 1999 -- Issue 3

Edited by Tim Surtell
E-mail: timsurtell@eleinmec.freeserve.co.uk


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