Most of the items in this chapter can be found on other pages of this website, but just in case you want to go over the capabilities of the '508A, we have brought them together. 
Quite often when you are programming, the first thing you will run out of is output lines. Many projects need lots of drive lines and if you need more than about 8, you should go to another micro-controller. 
Don't expect an 8-pin chip to perform the impossible. 
The designers of the '508A have done an amazing job providing 5 output lines (and one input line) in an 8-pin chip, but even so, many projects run out of drive-lines. 
On the 5x7 Display we have shown how to expand the drive lines with a counter (Called a Johnson counter). This can increase the lines to 10 however if you want to add just one or two more devices than the chip is directly capable of handling (5), there are clever ways to connect them. 

If the mark-space ratio of this waveform is kept short as shown in fig: 2, the LED will only illuminate very dimly. A short mark-space ratio means the "mark" is very small compared to the "space". A very short on-time (mark) and a long off-time (space) will not affect the tone from the piezo but will deliver very little energy to the LED and this is exactly what we want. 

On the other hand, each time the LED is activated, only a very small click will be heard, and this will hardly be noticeable. In this way the two devices can be combined on the same line. 

The resistor between the switch and micro acts as a safety resistor to prevent the output of the chip being damaged if the switch is pressed when the LED is activated, and it also acts as a dropper resistor for the LED. 
These two items will work in combination because the impedance of the LED is very high when no voltage is across it and when the micro turns the line into an input line, it sees the LED as a high impedance. In other words it is not detected so that when the switch is pressed the micro only sees the switch as a LOW. 

During turn-on, a special program will put a HIGH on each output in turn and the output containing the resistor will determine the program. Combining 5 programs in one chip will reduce inventory costs as the required program can be selected by fitting the resistor to the appropriate place on the board.

The drive lines have a maximum output current of 25mA and this is enough to drive a number of different devices. If a red LED is connected in one direction and a green LED in the opposite direction, they can be turned on and off individually, as shown in fig: 6. 
If the two LEDs are placed near each other or combined in the one LED (called a tri-coloured LED), they will produce a number of colours including orange, depending on the mark-space waveform delivered to each LED.
A single LED containing red and green chips is available in 2 or 3 lead versions. The wiring for a 3-leaded tri-colour LED is shown in fig: 7. The tri-leaded version is shown in fig: 6. 

Tri-coloured LEDs are fairly expensive but if the project can cover the expense, they can be the basis of "running message" displays and simple TV screens. 

If you connect a piezo to two out-of-phase lines as shown in fig: 8b, the sound produced will be slightly louder than the arrangement in fig: 8a. 

When we talk about a piezo we really mean a PIEZO DIAPHRAGM. A piezo diaphragm is a passive device and is very similar to a capacitor as far as the circuit is concerned. Ceramic substrate on a metal diaphragm causes the metal to "dish" and bend to produce a high pitched sound. The size of the voltage (the amplitude) determines the intensity of the sound and the frequency of the waveform determines the tone.
The voltage across the piezo from one drive line is about 5v whereas the voltage seen by the piezo from two reversing lines is about 10v. Unfortunately this doesn't produce twice the sound output but the sound is slightly louder. If you want a louder output you should use a better-quality high-output diaphragm (such as from a Christmas card). 
The loudest output is from a piezo siren and this is an active device containing a transistor oscillator and choke. These units operate from 5v to 15v and produce a very loud output while consuming only about 10mA to 15mA.

For a red LED the characteristic voltage is 1.7v. 
For an orange LED the characteristic voltage is 1.9v. 
For a green LED the characteristic voltage is 2.1v. 

LEDs cannot be connected directly to the output of a drive-line without a voltage-dropping resistor. The reason is very technical but basically a red LED does not turn on AT ALL until exactly 1.7v is placed across it and if the voltage tries to rise above 1.7v, the LED will glow brighter, allow a very high current to flow and will be damaged. 
The only way to prevent damaging the LED is to provide it with a very accurate supply voltage or simply connect a resistor in series. If the value of the resistance is worked out, an accurate current can be delivered to the LED and everything will be ok. The LED will last 100 years! 
Suppose you want to deliver 25mA to a LED. 
If we take a red LED, the value of resistance can be worked out by Ohms law.

To supply them with 25mA, the dropper resistor must be:
R = V/I
   = 1.6/0.025
   = 64 ohms
Use 68R resistor as shown in fig: 10. 

If three LEDs are connected in series, the total characteristic voltage drop will be 1.7v + 1.7v + 1.7v = 5.1v This is higher than the maximum voltage on the output line and in theory, the LEDs will not illuminate AT ALL, no matter what dropper resistor is used! 
This means only two LEDs can be connected in series to an output line. 
LEDs can be connected in parallel AND series as shown in fig: 11. Four LEDs is the maximum that can be driven from a single output line and this will deliver about 12mA to each LED. 

You will notice a separate dropper resistor is required for each column of LEDs because LEDs cannot be operated in parallel due to the 1.7v characteristic voltage required across each for perfect operation. 
For example: one LED may have a characteristic of 1.75v and another may have 1.65v characteristic. The 1.65v LED will rob the other of voltage and prevent it operating.

If the transistor can handle 100mA, four columns can be made, allowing 24 LEDs to be illuminated. 
In this way segments of a large pattern can be illuminated.
Very large displays are simply a repetition of patterns. One of the oldest displays I saw in operation was a set of thousands of globes outside a theater. All the globes were illuminated and one band of globes was switched off in turn to give a “running hole” effect. The mechanism to produce this effect was a geared motor with an output lever moving past 4 micro-switches, turning off one band of globes at a time. A very impressive result from a very simple circuit.

The globe can be dimmed by delivering a variable mark-space waveform. Fig:2 shows the type of waveform with the on-time represented by the "mark" portion of the waveform. 

When the relay is turned off (de-energised) the collapsing magnetic field of the coil generates a very high voltage and this can be passed to the supply rail if it is not "snubbed." The diode absorbs (snubs) this voltage. 

The alarm is required to send out a HIGH on one line and detect the high on the other line. The alarm then sends out a HIGH on the other line and since the diode is reverse biased for this condition, the alarm must sense a low on the first line. 
This "line testing" is done many times per second and if the line is shorted, the program will detect the interference. The only limitation to this system is the micro will not detect a diode fitted across the two lines near the alarm panel. The thief has to know of this limitation and the diode has to be fitted around the correct way to defeat the system. 
How many thieves carry a diode with them and know how to fit it?

Lines A and B are two outputs from a '508A. There is an important point to remember when programming the output lines to prevent a short-circuit occurring with the bridge.
You will notice that if both A and B are HIGH at the same time, transistors L, M and X, Y will be turned on at the same time and a short-circuit will occur on the power rail. 
To prevent this from happening lines A and B must be LOW at the beginning of the program. 
By taking line A HIGH, transistors M and X are turned on and this delivers voltage to the motor to turn it in the clockwise direction. 
To control the RPM of the motor, line A can be given a variable mark-space ratio.
To reverse the motor, line A must be taken low and after a short delay, line B can be taken HIGH. This will deliver voltage to the motor via transistors Y and L and cause the motor to revolve in the opposite direction. Reverse RPM can be adjusted with a variable mark-space pulse.