Cassette Tape Speed Control Circuit

In this instructables I am going to create an electronic circuit to control the speed of a reproducing motor in a tape recorder. Let's get started!

TAPE SPEED: 1 7/8 IPS

The tape speed is determined by the voltage across the motor. The purpose of the Motion Control is to set the tape speed by adjusting the motor voltage, to keep the motor voltage constant as the battery voltage drops. To do this, I am going to use a motor control IC(AN6650). The IC is manufactured by Panasonic, which is used to control the speed of the reproducing motor and contains the following major parts:

1. Reference Voltage - This circuit together with the current source feeding it maintains a constant 1.3V between pins 2 and 1 (and thus across the voltage divider) until the supply voltage to the IC drops to approximately 1.6V.

2. Op-amp - The op-amp amplifies the voltage difference between the (+) and (–) inputs. If the difference is positive (+ input more positive than – input) the output goes positive. If the difference is negative (+ input more negative than – input) the output goes negative. The gain of the opamp is high. A small difference at the inputs produces a large change in the output.

The op-amp circuit consisting of the op-amp, the transistor and resistors RA and RB employs negative feedback. This means that the op-amp output changes so as to reduce any voltage difference at the inputs. For example, if the (–)

input goes negative, creating a positive difference at the inputs, the output goes positive. The transistor then turns on harder and the collector voltage drops. This voltage drop is fed back via RA to the (+) input which tends to remove the original difference between the inputs. Since the gain of the op-amp is very high, the two inputs are kept at virtually the same voltage.

The motor voltage is adjusted by VR2. Turning the wiper of VR2 toward R8 lowers the voltage at the (–) input of the op-amp. This, as in the example above, lowers the transistor collector voltage and increases the voltage on the motor.

Once the motor voltage is set, the motion control section keeps the voltage constant as the battery voltage drops. If, for example, the battery voltage drops by 0.5V, point A will drop by 0.5V. Due to the constant 1.3V across the voltage divider, point C, and the (–) op-amp input also drop by 0.5V. As explained above, this drops the transistor collector voltage (point B). In our example, the collector voltage must drop 0.5V to make the (+) op-amp input equal to the (–) input. Since points A and B both drop 0.5V, the motor voltage remains constant.

List of Electronic Components---- 1. AN6650 integrated circuit(I.C.) 2. 0.5 ohm Resistor 3. 1.0 ohm Resistor 4. 1K potentiometer 5. 0.1 UF Capacitor(ceramic) 6. 0.001 UF Capacitor(ceramic) 7. 8 pin IC holder 8. 5mm RED LED 9. 1K Resistor 10. PCB (printed circuit board) 11. 2-PIN Screw Terminal 12. Bridge Wires

PIN Description of AN6650

PIN Description of AN6650