Working Principle and Production of TV Scanning Demonstrator

In the figure below, the light-emitting diode LED, transistor V1, and potentiometer W1 constitute a charging constant current source. Since the forward voltage drop on the LED is very stable, it has a voltage stabilizing effect. This voltage is added to the emitter junction of V1 and W1 and R2. When the resistance value of W1 is changed, the base current of V1 can be changed. After being amplified by V1, this current charges the capacitor C1 through the diode D1. The smaller the resistance value of W1 is adjusted, the faster the charging speed is, that is, the faster the sawtooth voltage output at both ends of C1 rises. When the voltage at both ends of C1 rises to a certain value, the output state of the output terminal 3 of the 555 integrated circuit IC is reversed from a high level to a low level, corresponding to the saturation conduction of the transistor V2. The discharge constant current source composed of diodes D2, D3, transistor V3, and potentiometer W2 starts to work. Similarly, since the forward voltage drop on D2 and D3 is very stable, it has a voltage stabilizing effect. This voltage is added to the emitter junction of V3 and W2 and R5. When the resistance value of W2 is changed, the base current of V3 can be changed. At this time, capacitor C1 discharges. When the voltage across C1 drops to a certain value, the output of pin 3 of the 555 integrated circuit changes from high level to low level, and the corresponding pin 7 is also low level. However, due to the effect of diode D1, C1 cannot discharge through pin 7, but can only discharge through the V3 constant current source. The smaller the resistance value of W2 is adjusted, the faster the discharge speed, that is, the faster the sawtooth voltage output across C1 drops. Since the output signal of this TV scanning demonstrator is sent to the oscilloscope, the input impedance of the oscilloscope is very high, so the effect on the charging and discharging of capacitor C1 is very small. In the figure, W1 corresponds to the field forward scanning speed potentiometer; W2 corresponds to the field reverse scanning speed potentiometer, and this potentiometer has a power switch K. The red terminal is the signal output terminal, and the black terminal is the output ground terminal.

Component list: 1 555 time base integrated circuit (1C in the figure below); 1 470K potentiometer with switch (W2 in the figure below, adjusts the sawtooth wave falling speed); 1 100K potentiometer (W1 in the figure below, adjusts the sawtooth wave rising speed), 1 LED light-emitting diode (for power indication and voltage stabilization of charging constant current source); 2 9012 or A1015 transistors (V1 and V2 in the figure below), 1 9013 transistor (V3 in the figure below), 1N41 1 48 diode (D1 in the figure below, blocking the oscillation capacitor from discharging to pin 7 of the timing integrated circuit); 2 ordinary diodes (D2 and D3 in the figure below, acting as voltage regulators for the discharge constant current source); 2 low-power 5.6K resistors (R1 and R4 in the figure below); 1 low-power 560Ω resistor (R2 in the figure below); 1 low-power 15K resistor (R3 in the figure below); 1 low-power 2K resistor (R5 in the figure below); several red and black connecting wires; 1 9V battery.

Demonstration steps:

1. Choose an oscilloscope with a relatively large screen and a relatively low X-axis scanning frequency, such as the J2458 teaching oscilloscope.
2. Install a 9V battery in the TV scanning demonstrator.
3. Use a signal line to connect the red terminal on the output end of the TV scanning demonstrator to the "Y input" end of the oscilloscope, and the black terminal to the "ground" end of the Y axis of the oscilloscope. Set the Y-axis AC/DC input selection to "DC".
4. Adjust the "scan range" on the oscilloscope's × axis to the lowest "10-100", and turn the "scan fine adjustment" counterclockwise to the minimum.
5. Adjust the field reverse scanning speed potentiometer W2 on the TV scanning demonstrator counterclockwise to the bottom, so that the field reverse scanning speed is the slowest.
6. Turn on the oscilloscope power supply, adjust the oscilloscope's "X-axis gain" and X-axis shift knob, so that the bright spot sweeps the entire screen in the left and right directions.
7. By rotating the field forward scanning speed potentiometer W1 on the demonstrator clockwise, and adjusting the "Y-axis gain" and Y-axis shift knob on the oscilloscope, the bright spot can simultaneously sweep the entire screen in the up and down directions. At this point, you can see that the bright spot is slowly moving left and right, and is also slowly moving up and down. Carefully observe the rules of electronic scanning, especially the field forward and field reverse scanning.
8. Rotate the field forward scanning speed potentiometer and field reverse scanning speed potentiometer on the demonstrator clockwise respectively, increase the two scanning speeds respectively, and observe the changes in electronic scanning. 9.
Adjust the scanning speed of the X-axis of the oscilloscope. Adjust the "scan range" of the X-axis of the oscilloscope to "1K-10K", and at the same time adjust the resistance values ​​of w1 and w2 on the demonstrator to the minimum, and you can see the full-screen grating. At this time, increase the Y-axis or reduce the Y-axis gain, and you can find that the grating is compressed up and down. Stop the horizontal scanning of the oscilloscope, and there will be only the scanning signal of the demonstrator, and a vertical bright line will appear on the fluorescent screen.
10. Fine-tune the "scan fine-tuning" of the oscilloscope's × axis, and more than ten stable field retrace bright lines will appear on the grating. Through the demonstration, it can be seen that the number of retrace bright lines is related to the field retrace scanning speed of the demonstrator and the × axis scanning speed of the oscilloscope.