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Issue 5 -- September 1999 -- See further down page


Front cover of issue 5The printed edition

Printed copies of each issue of Electronics in Meccano are available at the following prices:

Please contact me at timsurtell@eleinmec.freeserve.co.uk for more information and to order.

I will also be at meetings and exhibitions of the South East London Meccano Club, where demonstrations of the circuits discussed in EiM will be on show.


Do you have any comments about the articles in Electronics in Meccano or about this website?
Write to me, Tim Surtell, at:

timsurtell@eleinmec.freeserve.co.uk

Please suggest topics for future articles, or write an article yourself for inclusion in a future issue.


Win! Win a components pack in the Electronics in Meccano competition...

If you have used electronics in any of your Meccano models, or have knowledge of electronics that you would like to share, why not enter the Electronics in Meccano competition?

All you need to do is write an article for Electronics in Meccano. Your article can be long or short and can be on any electrical or electronic topic that might be useful to Meccano enthusiasts. If your article is accepted, you will win a Component Pack of resistors, capacitors, LED's and many other components.

Your article can contain anything that will help you explain the subject, such as diagrams, graphs, tables or equations. You don't need to worry about formatting the article -- plain text and hand drawn images will be fine!

If you wish, you can write an article to fit one of the Electronics in Meccano 'sections' such as Model Study, What's That?, and Practical Matters.

To enter, email the editor, Tim Surtell, at timsurtell@eleinmec.freeserve.co.uk with your ideas for your article and a rough layout for it.

Conditions -- please print out for your reference!


 

The NE555 bistable circuit

Figure 1: Circuit diagram of the NE555 bistableWe now have now seen two circuits that can be built with NE555, one with no steady states (the astable), and one with one steady state (the monostable).
The third circuit, shown in figure 1 has two steady states and is called a bistable.

The type of bistable that can be made with the NE555 is one with two inputs. A low pulse on the Trigger input (pin 2) makes the output go high; a low pulse on the Reset input (pin 4) makes the output go low. There is no timing involved in this circuit, so therefore there are no equations to work out the components. Notice in this circuit that pin 4, the Reset pin is used. In both the astable and monostable circuits this was connected to +Vs where it has no effect. Connecting it to 0V resets the NE555 so that it’s output goes low. Therefore, if Reset is used in the monostable circuit, the timing is stopped and the output returns to 0V. In the astable circuit, Reset will force the output to 0V. When Reset returns to +Vs, the astable timing will resume.

Figure 2: Timing diagram of the NE555 bistableApplications of the monostable circuit in Meccano could be an automatic reversing switch in a model such as a locomotive. The output would need to be connected to a relay to drive a motor, the relay being wired as a reversing switch (see page 2 for an example). When the locomotive reaches one end of the track, a switch makes the Trigger input to the NE555 go low, causing the output to change state and the locomotive to change direction. A similar situation will occur at the other end of the track, except another switch will make the Reset input go low.

Figure 6: A stripboard layout for the NE555 bistable circuit

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Figure 3: Reversing a DC motor using an AC supplySimple motor control
By Stan Leech

Reversing a DC motor using an AC supply

This simple circuit, shown in figure 3, requires only two 1A diodes and an SPDT toggle or slide switch with centre off. The AC supply from a transformer needs to be 1.5 to 2 times the voltage required by the DC motor that you use. The reason for this is that the diodes whilst converting from AC to DC lose half of the power in the process.

Momentary action switches

Mention was made in issue 1 of difficulty in obtaining momentary action switches. They are also known as biased switches and return to the centre off position when pressure is released from the toggle.

They are useful for controlling motors in exhibition models when the attention of an exhibitor is distracted by a person asking questions. Usually just a mechanism reaches the end of a traverse, and failure to stop or reverse results in either damage to the motor or a jammed up mechanism.

These switches can be mounted in a small black box (75mm x 50mm x 25mm) as shown in figure 4. Two terminal blocks form the power supply input at 6V or 12V, and six terminal blocks provide connections for three motors. These are mounted at each side of the case. This set up is ideal for, say, a block setting crane with three motors.

Figure 4: Momentary action switches used in a crane motor control box

Squires in Bognor Regis can provide the parts required to build these circuits. Although Squires prices are higher than Maplin’s, a minimum order of £7.50 is post free and their 332 page catalogue is also free. Their address is:

Squires Model and Crafts,
100 London Road,
Bognor Regis,
West Sussex PO21 1DD

Tel: 01243 842 424
Fax: 01243 842 525

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What's VA?

In electronic components catalogues you may sometimes see a number followed by the letters ‘VA’. This is simply another way of indicating the power rating, in Watts, of a component or appliance, and it arises from an equation called the Power Law...

Equation P = IV

...where P is power in Watts, I is current in Amps and V is voltage in Volts. VA is used simply because Volts times Amps gives Watts, so, for example 50VA is the same as 50W.

For those of you who like using mains voltage motors in your models…

...the Power Law can be re-arranged to find out the maximum current required by the motor or appliance given the power and voltage. So, for mains voltage motors or appliances, which normally specify their power rating somewhere on the casing, you can work out which fuse you need for the plug.

Equation I = P / 230V

The example {in curly brackets} shows a typical iron consuming 1000W, which therefore has a current of 5.6A flowing through it. Choose the minimum value fuse which can pass the appliance’s normal current and ensure that the mains flex can pass the fuse current (not the appliance current which will be lower.) If the flex cannot carry the fuse current it may overheat and burn out before the fuse blows, possibly causing a fire. Note that it is the fuse (not the appliance) that determines the flex rating, so the iron in the example above would require a 13A fuse (the smallest standard value which can pass 5.6A), so the flex must be capable of safely passing 13A.

The Power Law can also be used to work out the fuse required for low voltage circuits, and is a handy equation to remember in electronics. Note though that components such motor and large capacitors take a large surge or current when first switched on, which may blow fuses that are only just rated high enough, so circuits with these components in them may require fuses with higher ratings.

Earthing

It may be obvious, but if you are going to use mains voltage motors in a model, the metal parts of the model must be earthed to avoid the metal parts of the model becoming live in the event of a fault.

Remember to check that the earthing works in all parts of the model by using the continuity checking function of a multimeter. Remove the power to the model and connect one meter probe to the earth pin of the mains plug. Then touch every exposed metal part of the model with the other meter probe to check continuity.

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More about power supplies

The previous article about a simple power supply in issue 4 of Electronics in Meccano explained the various stages required to produce regulated DC from a 230V AC mains supply. This article deals with some of the more complex matters which you may need to consider when building a simple power supply.

Rectification loss

Firstly, the diodes used to rectify the low voltage AC supply to produce DC are not perfect. The output voltage from a diode will be about 0.7V less than the input voltage to that diode. This means that in the full-wave rectifier, where two of the diodes are in use at any one time, there will be a 1.4V voltage drop. So, if you put 12V in, you will get 10.6V out.

Smoothing

The next stage after rectification is smoothing, which is provided by a large value capacitor. The output from the rectifier has an average voltage somewhere between 0V and Vs since it is still alternating between these two voltages. The smoothing stage produces a voltage near to Vs, but with a slight ‘ripple’ voltage. This means that there appears to be a larger voltage after smoothing than before it.

Ripple voltage

The ripple voltage present after smoothing should be as small as possible since it can cause any IC’s (especially logic IC’s but not normally the NE555) in the circuit that you are connecting to the power supply to misbehave. If you find that your circuit is not doing what it should be, try using a smoothing capacitor with a larger value to see if that solves the problem. You can work out the ripple voltage if you need to using the formula…

Equation Vr = I / 2Cf

...where Vr is the ripple voltage in Volts, I is the current consumed by your circuit in Amps, C is the value of the smoothing capacitor in Farads, and f is the frequency of the AC supply in Hertz (which will be 50Hz in the case of the mains.) The ripple voltage Vr should not be more than 10% of Vs.

Regulation

If you are using a regulator IC as the stage after smoothing, then you shouldn’t need to worry about the ripple voltage, because the whole point of having a regulator is to generate a stable, accurate, known voltage for your circuit! However, if the ripple voltage is too large and the input voltage to the regulator falls below the regulated output voltage of the regulator, then obviously the regulator will not be able to produce a correct regulated voltage. The input voltage to a regulator should normally be at least 2V above the regulated output voltage of the regulator.

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Title: Practical Matters -- Connectors

In this issue Practical Matters takes a look a some of the different connectors available for multiple wire connections.

Pin strips
  • These inexpensive connectors come in three parts: PCB pins, socket housing and terminal pins.
  • The PCB pins are mounted 0.1” apart in a plastic spacer and are soldered onto a PCB or stripboard. Any number of pins can be used by simply snapping them off, but the number of connections is limited by the socket housing. Polarised PCB pins are also available so that the connection can only be made one way around.
  • The socket housing can be purchased in sizes ranging from 2-way to 17-way, and has slots for inserting the terminal pins. These pins have a spring at one end which engages with the PCB pins, and an area at the other end to solder or crimp a wire.
  • Cheap, not robust, and can be awkward to assemble!

Power connectors

  • These allow a two wire power connection, with the outside part of the barrel commonly used of the 0V connection.
  • The standard size is 2.1mm which is likely to be found on small multi-jack PSU’s, but other sizes are available.
  • The plug normally has a strain relief sleeve to protect the wires, which need to be soldered to tags inside the plug.
  • The sockets are available with solder tags or for mounting on a PCB. An inline socket can be mounted on a panel.

Audio connectors

  • These have the same characteristics as the power connectors above, but the ‘stereo’ version provides three connections.
  • Not necessarily suited to power connections since short-circuiting can occur when the plug is inserted and removed.

D-type connectors

  • D-type connectors are the standard connectors used for the serial and parallel ports on the back of PC’s, and are called ‘D’ types because of their ‘D’ shape when viewed from the front which stops the plug being inserted into the socket the wrong way around.
  • Connection of wires to both the plug and the socket normally requires soldering, although crimp types are available.
  • The rear of both the plug and the socket can be protected from damage by an optional plastic or metal case which also incorporates two fixing screws that stop the plug from accidentally being removed from the socket.
  • Available in sizes from 9-way to 50-way, and in high density versions, as well a PCB mounting types.
  • Useful if you require lots of connections and a robust connector.

DIN connectors

  • Round connectors available in sizes from 3-way to 14-way.
  • Sockets available for panel mounting or PCB mounting.
  • Connections of wires to both the plug and the socket requires soldering.
  • The metal case of the plug can provide a ground connection and a larger solder tag is provided for this in the plug. The plastic sheath on the plug includes a strain relief sleeve.
PCB pins, Socket housing and Terminal pins
Power plug (2.1mm short type) and socket
Various sizes of D-type plug and socket
DIN plug
DIN socket

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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.

Description

Maplin Order Code

Price

Page

The NE555 bistable circuit

Parts needed for the NE555 bistable circuit in Figure 2 to be constructed on stripboard

1 x NE555 Timer IC
1 x 8-pin IC Holder
1 x 220
mF Capacitor
1 x 0.01
mF Capacitor
2 x 1k
W Resistor
2 x Push button
Stripboard (about 24 holes x 12 rows)






QH66W
BL17T
VH41U
BX70M
MF1K
FH59P
JP47B






32p
13p
22p
15p
4p
59p
£1.79






1177
1200
236
232
910
1296
742

Simple motor control

1N4001 diode
SPST centre-off toggle switch



QL73Q
FH01B



7p
£1.29


1030
1248
Practical Matters: Multi-way connectors

0.1" PCB pin strip (36 pins)
0.1" polarised locking plug assembly (10-way)
Socket housing (10-way)
Terminal pins (10-way)

2.1mm standard power plug
2.1mm plastic socket
2.1mm PCB mounting socket

3.5mm stereo audio plug
3.5mm stereo panel mounting audio socket

D-type plug (25-way)
D-type socket (25-way)

DIN plug (5-way)
DIN socket (5-way)


JW59P
RK66W
FY94C
YW25C

HH60Q
FT96E
RK37S

HF98G
FK03D

YQ48C
YQ49D

HH27E
HH34M


80p
70p
20p
55p

39p
39p
49p

79p
69p

90p
£1.35

49p
50p


432
432
433
433

447
447
447

396
396

416
416

390
390

Maplin charge £3.95 for delivery on orders under £25.00 ex VAT.
Prices are taken from the March 1999 - August 1999 Maplin catalogue, and include VAT at 17.5%

Contact their order line on 01702 554000 or visit one of their shops.
Their customer service line is 01702 554002 and they have a website at www.maplin.co.uk where on-line ordering is available.


EiM Logo

www.eleinmec.freeserve.co.uk

Electronics in Meccano September 1999 -- Issue 5

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