This keeps the circuit simple and prevents possible overcharging of any of the cells, if you are alternately charging AA and C cells.
There are currently two types of AA nicad cells on the market.

The ampere-hour capacity of the old-style cell is 450mAHr. Some of them are 500mAHr, 550mAHr or 600mAHr.
Newer AA cells are 1.1AHr or 1.2AHr. Fortunately, most cells have the amp hour rating on them and this will help you set the charge-rate and/or time.
The first thing you cannot do is combine old-style cells with the new style. If you do, you will either over-charge the old-style ones or only partially charge the new ones. Stick to one type of cell at a time and select the time and charge-rate you require.
If you intend to charge the 1.2Ahr cells, we have added two components on the circuit diagram to change the charging current to 90mA for 14 hours to cater for this type of cell. This consists of a 12R resistor and slide switch that can be mounted on the top of the box and labelled 600mAHr when in one position and 1.2AHr in the other position. We have not provided a quick charge facility for the 1.2AHr cells.
Instead of adding this switch, you can simply give them three cycles at the 5 hour-rate or two cycles at the 14 hour rate.
We have lumped all the old-style cells in to one category as the difference between a 450mAHr cell and 600mAHr will be very small as far as the charging is concerned and in use you will hardly detect the difference as you will rarely be able to use all the energy in a cell.
The full capacity of a 450mAHr or 600mAHr cell is only obtained when the cell is discharged under fairly low laboratory conditions such as 10 - 15mA. Most toys and radios take 2 - 30 times more than this and the amp-hour capacity you get from the cell falls considerably as the current-demand increases.
Every cell and every condition is different but you can easily reduce the Amp-hour capacity to 200 - 300mAHr if you use it in something that takes 200 - 300mA.
That's why you only get one or two hour's life (or even less) out of a set of cells when you are using them in a toy such as a remote control car.
However the 1.1 and 1.2AHr cells will provide twice the energy of the old-style cells, under the same conditions.
No matter which type of cell you choose, the cost of powering high-performance toys will drop considerably by using rechargeable types.
And with this charger you can keep them fully charged without over or under charging.
To get the best performance out of rechargeable cells you must remember four things:

1. Cells must not be left in an uncharged state,
2. They must not be over­charged,
3. They must not be totally discharged (cell voltage must remain above 1v) and
4. They must not be discharged at more than 500mA (for more than a few seconds).

Any abuse of these rules will reduce the capacity of the cell. Nicads (and all their closely allied variations) also have another unusual characteristic that reduces the capacity. It's called "memory." 
If you recharge a cell that is only say 25% discharged, the cell will think its new capacity is only 25% of the true value. Thus you will only be able to get 25% of its energy before the cell starts to falter.
This means it is very important to almost fully discharge a cell before starting the charging process.
Some of the new cells do not have this "memory" problem but their cost at the moment is more than twice the older style cells - so, it's the old saying: you get what you pay for. This project will enable you to care for AA cells and even rejuvenate some of them that have generated a memory. If a cell with a memory is discharged fully and charged completely, over a number of cycles, its poor performance can be improved and some can be restored to near full capacity. There is another device, called a nicad Zapper, that will rejuvenate some of the cells that have developed an internal fault, such as a short, that prevents it from receiving a charge. Now back to our circuit.

HOW THE CIRCUIT WORKS
The heart of the circuit is a 4541 timing chip. It has a built-in oscillator that requires only two external components for timing. A chain of sixteen binary dividers inside the chip is capable of producing a division of 65,536. With this we can produce very long time delays by making the internal clock of the chip a low-frequency oscillator and using it to clock the divider chain.
The other feature of this project is the constant-current charging circuit that allows you to fit any number of cells, up to 8, in the charging holder. Some chargers only accept 1,2 or 4 cells at a time while ours will accept any number providing the unused parts of the holder are connected with a jumper lead to make a complete charging path. 
The BC 547 and BC 557 in the Automatic charger detect when a battery has been fitted to the circuit and turn on the 4541 timing chip. The green "Charge" LED comes on to show the battery is charging.
The orange LED shows you have selected the 14 hour rate and the red LED indicates you have selected the 5 hour rate.

CHARGING
During charging, no gases are generated. However during the latter part of the charge-cycle, during over­charge and during heavy discharge, hydrogen and oxygen gases are generated as well as electrolyte fumes. These gases can normally react with each other and not increase the pressure inside the cell however if they are produced at a fast rate, this equilibrium condition cannot be maintained and the pressure inside the cell builds up.
A pressure release valve is built into the cell but any gases that escape will include some of the electrolyte in the fumes and the cell will eventually dry out. For this reason it is important to limit the charging cycle.

ADAPTING THE CHARGER
The charger can be adapted to charge AA, C, or D cells or any combination of these by modifying the circuit slightly or by adding extra constant-current sections as shown in the circuit diagram below.

It will even charge 1.2AHr or 1.9AHr Gel cells of the 12v type as used in alarm systems, simply by making sure the rail voltage is above 18v, (but not more than 22v) to take into account the 3v minimum voltage drop across the BD 679 transistor.
If you want to charge only C or D cells, the circuit can be modified by changing the current-determining resistors R1 and R2 as follows:

The wattage of the resistors in the above table will have to be taken into account due to the high current that will be flowing through them The following table shows you how to "make-up" the resistors:

If you want to charge AA, C and D cells, you can add extra circuitry consisting of the constant-current section shown above to cater for the C and D cells.
To charge these cells, the BD 679 transistor will have to be heat-sinked with a larger heat-sink than is provided in the kit and the plug pack will have to be upgraded.
For a C cell, the 14 hour rate is 150mA and the 5 hour rate is 300mA. For a D cell, the 14 hour rate is 350mA and 700mA for the 5 hour rate.
Most plug packs have very poor regulation and the output voltage will drop considerably when full current is delivered. You will need a 500mA plug pack to deliver 350mA and a 1amp plug pack to deliver 700mA. Otherwise a 300mA or 400mA plug pack will be sufficient for the AA version.
Alternatively you can charge C and D cells with the original circuit by giving these cells two or three charge-cycles as show in the following table:

When choosing a plug pack, you must be careful not to supply the circuit with more than 15v AC as this becomes 22v DC when rectified and will create extra heat loss in the transistor.

The PCB, showing the overlay and the track work

Automatic AA Nicad Charger kit, showing all the components