The NE555 bistable circuit

Figure 1: Circuit diagram of the NE555 bistableWe now have now seen two circuits that can be built with the 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.

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

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 bistable 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.

See also: The NE555 astable circuit for an explanation of the NE555 timer IC

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

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

Click here for details

 

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. A mechanism reaches the end of its 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 Maplins', 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

See also: Rotary switch motor controllers , Motor control with a Darlington Pair

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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 (for DC and purely resistive AC loads only) 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.

The Power Law can be re-arranged to find out the maximum current required by a 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.

Using mains power in models

Mains power should not be used to power mains voltage motors in Meccano models, even if the model chassis has been earthed.  Earthing is not enough to ensure the safety of a model. Even though the earthing on a model may function correctly, the power point used to power the model may have a faulty earth connection. This means that if a fault develops in the model and the model chassis becomes live, the earth wire will become live also. If someone touches the model they would therefore receive an electric shock.  To avoid the possibility of this situation occurring, it is clearly best not to use mains power directly in Meccano models unless an isolating transformer is used.

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More about power supplies

Continued from Building a power supply in issue 4

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 ICs (especially logic ICs, 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.

See also: Practical Matters: Connectors for single-way connectors

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.

The NE555 bistable circuit
 
Parts needed for the NE555 bistable circuit in Figure 2 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
1 x 1kW Resistor M1K 7p 221
1 x Push button FH59P 59p 314
Stripboard (about 24 holes x 12 rows) JP47B £1.79 204
Simple motor control
Maplin Order Code Price Page
1N4001 diode QL73Q 5p 246
SPST centre-off toggle switch FH01B £1.29 308
Practical Matters: Multi-way connectors
Maplin Order Code Price Page
0.1" PCB pin strip (36 pins) JW59P 79p 129
0.1" polarised locking plug assembly (10-way) RK66W 79p 130
Socket housing (10-way) FY94C 25p 130
Terminal pins (10-way) YW25C 59p 130
2.1mm standard power plug HH60Q 49p 137
2.1mm plastic socket FT96E 49p 137
2.1mm PCB mounting socket RK37S 49p 137
3.5mm stereo audio plug HF98G 79p 104
3.5mm stereo panel mounting audio socket FK03D 79p 105
D-type plug (25-way) YQ48C 99p 122
D-type socket (25-way) YQ49D £1.49 122
DIN plug (5-way) HH27E 49p 100
DIN socket (5-way) HH34M 49p 101

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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www.eleinmec.freeserve.co.uk

Electronics in Meccano September 1999 -- Issue 5

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


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