Storing information through magnetic patterns was basically demonstrated to record audio. Ever since then, this concept continues to be applied for different items like floppy disks, audio/video tapes, hard disks, and magnetic stripe cards. This post is focused on Magnetic stripe cards used extensively for financial transactions and access control across the world.
Reading magnetic stripe cards requires significant analog circuitry besides digital logic to decode data. Recording of data on the magnetic cards is digital and is also done by magnetizing particles along the size of the stripe. Reading the magnetic card successfully is actually a challenge due to the fact the amplitude of sensor signal varies with all the speed in which card is swiped, the quality of the credit card, and the sensitivity of magnetic read head. Moreover, frequency also varies using the swipe speed. This involves card dispenser to evolve to those changes and process the sensor signal without distortion. This post explains mechanisms for handling variations inside the sensor signal.
So that you can comprehend the outcomes of card swipe speed, the caliber of the card, and sensitivity of your sensor, it is very important know how details are stored over a card and also the way it is sensed through the read head. In magnetic-based storage systems, information is represented by pole patterns over a magnetizing material like iron oxide. Figure 1 shows a magnetic stripe coated with magnetizing material. The particles within a magnetizing material probably have some specific alignment or could be in random directions if it has not been previously exposed to a magnetic field by using a particular orientation. However, when put through another magnetic field, particles about the stripe are aligned together with the external applied field.
In practical systems, a magnetic write head is commonly used which can be nothing but a coil wound around a core. The magnetic field orientation can be simply programmed by controlling the current direction from the coil. This helps to make north-south pole patterns around the card. The narrower the air gaps between your poles, the greater the density of data, which can be programmed in the card.
In F2F encoding, if a pole transition occurs in between the bit period, it really is logic 1 else it is logic . By way of example, as shown in Figure 3, when the bit period is ? and when a transition takes place at ?/2, then its logic 1, else it is logic . See that the length occupied by logic 1 and logic around the card is same. However, the bit period ? varies with all the swipe speed and that should be included when reading the credit card.
The reading process is exactly reverse. It will require a read head which is a lot like the passport scanner arrangement shown in Figure 2. Remember that you will have one sensor for each and every track. If the card is swiped, the magnetic field through the stripe induces voltage from the read head coil. Figure 5 shows the waveform from the read head.
The signal peaks at each and every flux transition. This is because of the high density of magnetic flux in the pole edges. As you have seen, information and facts are represented from the location of signal peaks. A peak detector circuit can decode this signal or even a hysteresis comparator with the thresholds kept not far from the signal peak. However, additional processing is needed before we can easily give this signal for the detector circuit for that following reasons:
Swipe speed: Swipe speed is specified in inches/sec (IPS). Generally, a magnetic card reader is needed to function properly within the swipe speed selection of 5 IPS to 50 IPS. The amplitude in the sensor signal varies together with the swipe speed: a rise in swipe speed results in an increased rate of change of flux cut through the coil inside the 89dexlpky head, resulting in increased amplitude from the signal. In contrast, when the swipe speed is slow, the signal amplitude is less which could result in difficulty in reading the data.
Expertise of the card: As time passes and based on the usage, card quality degrades with decreased magnetic field strength and distortion due to dust and scratches on the card. Together, these lower the amplitude from the sensor signal.
As a result of every one of these parameters, TTL magnetic card reader can be anywhere between several 100s of uV to 10s of mV. This range may be compensated employing an amplifier. However, it can not be a set gain amplifier. As soon as the swipe speed is high as well as the card quality is great, the amplifier output can saturate on the rails. So when the signal saturates, information, the time distinction between two successive peaks, is lost. Thus, it is important to faithfully amplify the sensor signal without saturating or altering the wave shape. This requires a configurable gain amplifier to ensure that we are able to tune the gain around the fly. To get this done, the device must have the ability to sense once the signal is weak.