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Why Class A ? Because , when biased to class A, the transistors are always turned on, always ready to respond instantaneously to an input signal. Class B and Class AB output stages require a microsecond or more to turn on. The Class A operation permits cleaner operation under the high-current slewing conditions that occur when transient audio signal are fed difficult loads. His amplifier is basically simple, as can be seen from the block diagram.

If your letterbox is some distance from your house, you will find a monitoring device useful to indicate when new post has arrived. This can take the form of a US-style visible flag; a more modern alternative uses a 433 MHz radio transceiver. The big advantage of the solution presented here is that is can use an existing two-core bell cable, without requiring any further power source. The arrival of post is signalled by a blinking LED; for added effect, a digital voice recorder can also be connected which will, at regular intervals, announce the fact that the letterbox needs emptying. The device is silenced by a reset button.

The circuit uses one half-cycle of the AC supply to power the bell or buzzer, and the other half-cycle for the post indicator. Suitably-oriented diodes in the device and in the letterbox ensure that the two signals are independent of one another (Figure 1). The bell current flows from K1.A through D3, bell-push S2, D1 and the relay back to K1.B. C1 provides adequate smoothing of the current pulses to ensure that the relay armature does not vibrate. The bell is operated by the normally-open relay contact. If the bell is actually a low-current piezo buzzer, then it can be connected directly and the relay dispensed with.

During the half-cycle for the letterbox monitor current flows from connection K1.B on the bell transformer through current-limiting resistor R1, the LED in the optocoupler, reed contact S1 (a micro-switch can also be used) and D2 and finally back to K1. If the reed contacts are closed, the LED in the optocoupler will light and switch on the phototransistor. A positive voltage will then appear across R3 which will turn the thyristor on via C6. The red LED will indicate that post has arrived. Pressing S3 shorts out the thyristor, reducing the current through it below the holding value. A small extra circuit can be added to provide continuous letterbox monitoring.

This takes the form of a voice recorder whose ‘play’ button is operated by transistor T1. T1 in turn is driven by a 555 timer IC. In the usual 555 timer circuit, where the device is configured as an astable multi-vibrator, the mark-space ratio cannot be set with complete freedom. Here two diodes provide separate charge and discharge paths for capacitor C4. When capacitor C4 is charging, D5 conducts and D4 blocks: the charge rate is determined by R5. When discharging, D4 conducts and R6 and the potentiometer determine the rate. The values shown give a pulse length of approximately 0.5 s with a delay of between 15 s and 32 s.

The short pulse is sufficient to trigger the voice recorder module via transistor T1 connected across its ‘play’ button. The voice recorder module (e.g. Conrad order code 115266) is designed to run from a 6 V supply. The maximum recording time is 20 s and the current consumption is 20 mA when recording and between 40 mA and 60 mA when playing back. Since our supply is at 8 V, the excess voltage must be dropped using between 1 and 3 series-connected 1N4148 diodes (shown as Dx in the circuit diagram). The final voltage should be checked using a multimeter. Alternatively, a 7806 can be used without suffering a significant loss in volume.

If it is desired to use a piezo buzzer to provide an acoustic signal, the pulse length should be increased to at least 2s. In this case, R5 should be increased to 560 kΩ or 680 kΩ: the pulse length, t on, is 0.7.R5.C4, and the interval between pulses, t off, is 0.7. (R6+R7).C4. Suitable buzzers are available with a wide range of rated voltages.

This handy circuit can be used as a speed controller for a 12V motor rated up to 5A (continuous) or as a dimmer for a 12V halogen or standard incandescent lamp rated up to 50W. It varies the power to the load (motor or lamp) using pulse width modulation (PWM) at a pulse frequency of around 220Hz.

SILICON CHIP has produced a number of DC speed controllers over the years, the most recent being our high-power 24V 40A design featured in the March & April 2008 issues. Another very popular design is our 12V/24V 20A design featured in the June 1997 issue and we have also featured a number of reversible 12V designs.

Circuit looks like:

For many applications though, most of these designs are over-kill and a much simpler circuit will suffice. Which is why we are presenting this basic design which uses a 7555 timer IC, a Mosfet and not much else. Being a simple design, it does not monitor motor back-EMF to provide improved speed regulation and nor does it have any fancy overload protection apart from a fuse. However, it is a very efficient circuit and the kit cost is quite low.

Parts layout:

Connection diagram:

There are many applications for this circuit which will all be based on 12V motors, fans or lamps. You can use it in cars, boats, and recreational vehicles, in model boats and model railways and so on. Want to control a 12V fan in a car, caravan or computer? This circuit will do it for you.

Circuit diagram:

The circuit uses a 7555 timer (IC1) to generate variable width pulses at about 210Hz. This drives Mosfet Q3 (via transistors Q1 & Q2) to control the speed of a motor or to dim an incandescent lamp.
Halogen lamps:
While the circuit can dim 12V halogen lamps, we should point out that dimming halogen lamps is very wasteful. In situations where you need dimmable 12V lamps, you will be much better off substituting 12V LED lamps which are now readily available in standard bayonet, miniature Edison screw (MES) and MR16 halogen bases. Not only are these LED replacement lamps much more efficient than halogen lamps, they do not get anywhere near as hot and will also last a great deal longer.

Source: Silicon Chip 15 November 2008

Before you can design an adjustable voltage regulator into your circuit, or do a redesign, you need to calculate the values for two resistors. This is not difficult in itself, but actually finding the right resistors may pose problems. Fortunately a trick is available to make it all much easier. With most adjustable voltage regulators like the LM317 and LM337, the input voltage has to be 1.2 to 1.25 volts above the desired output voltage. This is because the voltage at the ADJ (adjust) input is internally compared to a reference voltage with that value. The reference voltage always exists across R1.
Together with preset R2 it determines the current flowing through the ADJ pin, as follows: Vout = VREF [1+(R2/R1)]+I ADJ R2 If for the sake of convenience we ignore I ADJ, enter the reference voltage (1.2 V) and for R1 select a value of one thousand times that voltage (i.e., 1.2 k?) then the equation is simplified to: R2 = 1000 (Vout – 1.2) In practice, simply determine the voltage drop across R2 (output voltage minus reference voltage) and you get your resistance value directly in kilo-ohms. For example, for 5 V R2 becomes 5–1.2 = 3.8 k? which is easiest made by connecting 3.3k and 470R resistors in series. In the case of relatively low voltages, smaller resistor values are recommended. This is because sufficient current needs to flow to enable the voltage regulator to do its job. A simple solution is to choose, say, 120 ? for R1. R2 then becomes: R2 = 100 (Vout – 1.2)

Fuse Box BMW 318i 1991 Diagram - Here are new post for Fuse Box BMW 318i 1991 Diagram.

Fuse Box BMW 318i 1991 Diagram

Fuse Panel Layout Diagram Parts: seat heating, high beam headlight, auxiliary fan, sliding roof, flashing turn indicator, mirror control, wiper, washer, horn, brake light, cruise control, heated rear window, GLOVEBOX, LUGGAGE COMPARTMENT LIGHT, ON BOARD COMPUTER.instruments, on board computer, reversing light, fuel supply pump, radio, check control and instrument, fog light, fog light, cigar lighter, central locking, door lock heating, hazard warning flasher, parking light, license plate light.