Magnetic card reader

In a reader for a coded card having a plurality of magnetized spots of predetermined polarity, the combination of:

• a housing including guide means for receiving a card;
• magnetodiode means carried in said housing for motion relative to the magnetized spots of the card;
• a voltage source connected across said magnetodiode means whereby a change in magnetic field at said magnetodiode means produces a voltage signal;
• a first amplifier having said voltage signal connected as one input thereof;
• a second amplifier having the first amplifier output connected as one input thereof;
• a time delay circuit connected between the second amplifier output and the second input of said first amplifier providing a variable reference voltage input to said first amplifier forming a self balancing nulling circuit so that the voltage at the second input of said first amplifier tracks the voltage at the first input of said first amplifier with a delay provided by said time delay circuit;
• third and fourth amplifiers, each having the first amplifier output connected as one input thereof;
• a voltage divider circuit connected across said voltage source and providing separate reference voltages as second inputs to each of said second, third and fourth amplifiers, with said second amplifier intermediate third and fourth amplifiers;
• first and second resistors connected in series across said voltage source;
• and
• first and second diodes connecting the outputs of said third and fourth amplifiers respectively to the junction of said first and second resistors, with one diode connected in opposite polarity to the other, with said junction providing a three level voltage pulse stream corresponding to the code of the card with a voltage pulse for each spot and with the voltage level returning to an intermediate level after each pulse.