01_elektor_electronics_UK_january_2006

In all mains-operated equipment certain important safety requirements must be met. The relevant standard for most sound equipment is Safety of Information Technology Equipment, including Electrical Business Equipment (European Harmonized British Standard BS EN 60950:1992). Electrical safety under this standard relates to protection from

•a hazardous voltage, that is, a voltage greater than 42.4 V peak or 60 V d.c.;

•a hazardous energy level, which is defined as a stored energy level of 20 Joules or more or an available continuous power level of 240 VA or more at a potential of 2 V or more;

•a single insulation fault which would cause a conductive part to become hazardous;

•the source of a hazardous voltage or energy level from primary power;

•secondary power (derived from internal circuitry which is supplied and isolated from any power source, including d.c.)

Protection against electric shock is achieved by two classes of equipment.

Class I equipment uses basic insulation ; its conductive parts, which may become hazardous if this insulation fails, must be connected to the supply protective earth.

Class II equipment uses double or reinforced insulation for use where there is no provision for supply protective earth (rare in electronics – mainly applicable to power tools).

The use of a a Class II insulated transformer is preferred, but note that when this is fitted in a Class I equipment, this does not, by itself, confer Class II status on the equipment.

Electrically conductive enclosures that are used to isolate and protect a hazardous supply voltage or energy level from user access must be protectively earthed regardless of whether the mains transformer is Class I or Class II.

Always keep the distance between mains-carrying parts and other parts as large as possible, but never less than required.

If at all possible, use an approved mains entry with integrated fuse holder and on/off switch. If this is not available, use a strain relief (Figure, note 2) on the mains cable at the point of entry. In this case, the mains fuse should be placed after the double-pole on/off switch unless it is a Touchproof® type or similar. Close to each and every fuse must be affixed a label stating the fuse rating and type.

The separate on/off switch (Figure, note 4), which is really a ‘disconnect device’, should be an approved doublepole type (to switch the phase and neutral conductors of a single-phase mains supply). In case of a three-phase supply, all phases and neutral (where used) must be switched simultaneously. A pluggable mains cable may be considered as a disconnect device. In an

approved switch, the contact gap in the off position is not smaller than 3 mm.

The on/off switch must be fitted by as shor t a cable as possible to the mains entry point. All components in the primary transformer circuit, including a separate mains fuse and separate mains filtering components, must be placed in the switched section of the primary circuit. Placing them before the on/off switch will leave them at a hazardous voltage level when the equipment is switched off.

If the equipment uses an open-con- struction power supply which is not separately protected by an ear thed metal screen or insulated enclosure or otherwise guarded, all the conductive parts of the enclosure must be protectively earthed using green/yellow wire (green with a narrow yellow stripe – do not use yellow wire with a green stripe). The ear th wire must not be daisychained from one part of the enclosure to another. Each conductive part must be protectively earthed by direct and separate wiring to the primary ear th point which should be as close as possible to the mains connector or mains cable entry. This ensures that removal of the protective earth from a conductive part does not also remove the protective ear th from other conductive parts.

Pay particular attention to the metal spindles of switches and potentiometers: if touchable, these must be protectively earthed. Note, however, that such components fitted with metal spindles and/or levers constructed to the relevant British Standard fully meet all insulation requirements.

The temperature of touchable parts must not be so high as to cause injury or to create a fire risk.

Most risks can be eliminated by the use of correct fuses, a sufficiently firm construction, correct choice and use of insulating materials and adequate cooling through heat sinks and by extractor fans.

The equipment must be sturdy: repeatedly dropping it on to a hard surface from a height of 50 mm must not cause damage. Greater impacts must not loosen the mains transformer, electrolytic capacitors and other important components.

Do not use dubious or flammable materials that emit poisonous gases.

Shor ten screws that come too close to other components.

Keep mains-carrying par ts and wires well away from ventilation holes, so that an intruding screwdriver or inward falling metal object cannot touch such parts.

As soon as you open an equipment, there are many potential dangers. Most of these can be eliminated by disconnecting the equipment from the mains before the unit is opened. But, since testing requires that it is plugged in again, it is good practice (and safe) to fit a residual current device (RCD)*, rated at not more than 30 mA to the

Stop! Don’t throw away your old phone! You can give it a second life using the circuit described in this article. That makes a nice Christmas present (maybe a bit belated) or gimmick, although it’s naturally a bit too heavy to carry around wherever you go. However, it’s perfect on your desk at work or in the pub. You’re bound to draw a second glance or be accused of being off your head, but that will change when the phone rings – despite not having the usual cable – and you start having a lively conversation.

What’s inside

Old phones are primarily based on mechanical systems, because there weren’t any PICs or microcontrollers fifty years ago. The mechanism behind the dial is no longer used, but that doesn’t mean it wasn’t a well-consid- ered design, far from it.

Figure 1 shows how it works. When you dial a digit, the small, grey oval wheel (arrow 1) rotates once for every two digit positions while the dial is returning to its starting point. As a result, the contacts (arrow 2) generate a pulse for each digit position on the dial. The microcontroller uses these pulses to determine which number you dial.

Figure 2 shows the mechanism with the dial in the rest state. The other set of contacts (marked by the arrow) is open in this state. That tells the microcontroller that the dial has stopped rotating.

We use a 16LF88 microcontroller here. Figure 3 shows how it is connected to the rest of the circuit. The firmware can be downloaded free of charge from our website.

A 3.7-V lithium polymer battery provides the supply voltage. Although the minimum supply voltage is specified as 4 V in the 16LF88 data sheet, it still

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Figure 1. Good news: the mechanical construction of a 50-year-old phone is compatible with modern microcontrollers. The small grey wheel generates a series of pulses after you turn and release the dial.

Figure 2. The lower set of contacts (see arrow) provides an electrical indication that the dial is at rest.

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Figure 3. The schematic diagram of the control unit for the circuit. Most of the circuitry is contained in the GM82 and 16LF88, so only a few external components are necessary.

10V

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works at 3.7 V (actually 3.8 V). The GSM module draws a peak current of around 2 A, which is more than most voltage regulator ICs like to provide, but a LiPo battery simply shrugs its shoulders.

The GM862 module comes from Spark Fun Electronics. It is a GSM module that can handle three bands: 900 MHz, 1800 MHz and 1900 MHz. The microphone and speaker of the telephone are connected to it via K2. The module can drive most types of speakers directly. The sensitivity of the microphone input is 50 mV, so you can probably manage without amplifying the microphone signal. A coupling capacitor and a resistor connected to the supply voltage to power the electret microphone should be sufficient. Connect the dial contacts to K3 and the ‘offhook’ contact to K5.

The GSM module can be controlled by the PIC using AT commands. CON1 and CON2 provide convenient debug access. K6 and K7 were added for connecting an optional keyboard, and they can be omitted.

The ringer circuit is energised via K4 (see Figure 4). It has the difficult task of converting a 3.8-V DC signal to an AC voltage at around 60 V. A boost converter followed by an H bridge is used for this purpose. The relatively high voltage is necessary to make the clappers strike the bells with sufficient force. The frequency of the AC voltage determines how fast the bell rings. The ringer coil and C2 form a resonant circuit with a frequency of approximately 22 Hz. Of course, the value of C2 can be adjusted to suit the electromagnetic ringer used in your phone.

If all of this is a bit too much work for you, you can visit the Spark Fun website to order a ready-made telephone.

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This electronic timer switch starts the washing machine by bridging the pushbutton for the start button for a few seconds using a relay. This does, however, require a small change to the washing machine. It is necessary to solder a wire to each side of the start pushbutton on the washing machine control PCB and to connect these wires to the relay contacts.

The timer switch is built on two circuit boards. The power supply PCB with the relay will go inside the washing machine. The PCB with the microcontroller is mounted in a separate enclosure and is attached to the outside of the washing machine.

The connection between the two PCBs (switch signal and power supply) is

1/2006 - elektor electronics

achieved with a 3.5-mm stereo jack plug and mating socket.

The design

Figure 1 shows the schematic for the complete timer switch. The top section is the control part with a PIC16F84 and four seven-segment displays. The bottom section is the power supply and switching part that will be built into the washing machine.

Power supply PCB

The power supply PCB is a straightforward design. A small mains transformer provides an AC voltage of about 9 V. The direct voltage resulting from rectification and filtering is regulated

by IC1 into a supply voltage of 5 V for the entire circuit.

The relay, whose contacts bridge the start button, is energised with the aid of switching transistor T1. The purpose of diodes D2 and D4 is to protect the base of T1 when the jack plug is either connected or disconnected.

If the washing machine has an on/offswitch instead of a start button, then this switch can be bridged with a bigger relay instead (however, take note of the maximum power rating of the washing machine).

This also requires a small change to the software: after the washing machine has been switched on by the timer switch, the relay needs to remain energised until the clock is reset.

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Control PCB

In this part of the timer switch, the objective was to use as few electronic parts as feasible and integrate as much functionality as possible into the microcontroller.

Diodes D3, D5 and D6 have the same purpose as D2 and D4, that is, protection for the ports when the jack plug is connected or disconnected.

X1 is a 32.768 kHz quartz crystal, the same type as those used in watches. This frequency is a power of two, which makes it easy to divide it down to a half second (half the time period of the colon).

The pins from the microcontroller are used as follows. Pins RB0 through to RB6 are used to drive the seven segments of the displays in multiplexed fashion. Resistors R4 to R13 limit the current through the LED-segments. Pin RB7 is for the colon (LEDs D7 and D8). Pins RA0 through RA3, via driver transistors T2 to T5, generate the multiplex drive signals for the displays. In addition, RA2 and RA3 are read once per cycle of the ‘main loop’ to test the state of the two pushbuttons S1 and S2.

In case you would like to modify the software yourself, it is possible to use RA0 and RA1 in the same manner as RA2 and RA3 and allow for two more pushbuttons.

RA4 is used to switch the relay via T1. Since this is an open-collector output, the purpose of R3 is to supply the drive current for T1.

Construction and Installation

Figure 2 shows the PCB layout that has been designed for this timer switch. Here, the two parts are joined together, but normally the two PCBs will be mounted in different locations. The assembly of the PCBs is mostly routine, but don’t forget the three wire links on the control PCB.

The construction and installation of this circuit has already been mentioned at the start of this article. You can choose to keep the two parts completely separate. That is, the power supply PCB inside the washing machine and the control PCB outside. The front of the washing machine could be fitted with a jack socket for the interconnection with the control PCB. The control enclosure can then be fastened on the washing machine somewhere (for example with mag-