Working Principle and Production of TV Scanning Demonstrator

In the figure below, the light-emitting diode (LED), transistor V1, and potentiometer W1 form a charging constant-current source. Because the forward voltage drop across the LED is very stable, it provides a voltage regulator. This voltage is applied to the emitter junction of V1, W1, and R2. Changing the resistance of W1 changes the base current of V1. This current, amplified by V1, charges capacitor C1 through diode D1. The smaller the resistance of W1, the faster the charging speed, which means the sawtooth voltage across C1 rises faster. When the voltage across C1 reaches a certain value, the output state of pin 3 of the NE555 integrated circuit (IC) flips from high to low, corresponding to transistor V2 saturating and conducting. The discharge constant-current source formed by diodes D2 and D3, transistor V3, and potentiometer W2 begins operation. Similarly, because the forward voltage drops across D2 and D3 are very stable, they provide a voltage regulator. This voltage is applied to the emitter junction of V3, W2, and R5. Changing the resistance value of W2 changes the base current of V3, discharging capacitor C1. When the voltage across C1 drops to a certain level, the output of pin 3 of the NE555 integrated circuit flips from high to low, and pin 7 also goes low. However, due to the action of diode D1, C1 cannot discharge through pin 7 and can only discharge through the constant current source V3. The smaller the resistance value of W2, the faster the discharge rate, which means that the sawtooth voltage output across C1 decreases faster. Since the output signal of this TV scanning demonstrator is fed to an oscilloscope with very high input impedance, the effect on the charging and discharging of capacitor C1 is minimal. In the figure, W1 corresponds to the field forward scan speed potentiometer; W2 corresponds to the field reverse scan speed potentiometer. This potentiometer has a power switch K. The red terminal is the signal output terminal, and the black terminal is the output ground terminal.

Components: 1 NE555 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 (serves as power indicator and charging constant current source voltage regulator); 2 9012 or A1015 transistors (V1 and V2 in the figure below), 1 9013 transistor (V3 in the figure below), 1N4148 One diode (D1 in the figure below, blocking the oscillation capacitor from discharging to pin 7 of the time-base integrated circuit); two ordinary diodes (D2 and D3 in the figure below, acting as voltage stabilizers for the discharge constant current source); two low-power 5.6K resistors (R1 and R4 in the figure below); one low-power 560Ω resistor (R2 in the figure below); one low-power 15K resistor (R3 in the figure below); one low-power 2K resistor (R5 in the figure below); several red and black connecting wires; and one 9V battery.

Demonstration steps:
1. Select an oscilloscope with a relatively large screen and a relatively low X-axis scan frequency, such as the J2458 teaching oscilloscope.
2. Install a 9V battery in the TV scan demonstrator.
3. Use a signal cable to connect the red terminal on the TV scan demonstrator output to the "Y input" terminal of the oscilloscope, and the black terminal to the "ground" terminal of the oscilloscope's Y axis. Set the Y-axis AC/DC input selector to "DC."
4. Adjust the "Scan Range" on the oscilloscope's X axis to the lowest setting of "10-100" and turn the "Scan Fine Adjustment" counterclockwise to the minimum.
5. Turn the field reverse scan speed potentiometer W2 on the TV scan demonstrator counterclockwise to the lowest setting, for the slowest field reverse scan speed.
6. Turn on the oscilloscope power and adjust the oscilloscope's "X-axis Gain" and X-axis Shift knobs so that the bright spot sweeps across the entire screen in both left and right directions.
7. Turn the forward scan speed potentiometer W1 on the demonstrator clockwise and adjust the Y-axis gain and Y-axis shift knobs on the oscilloscope so that the bright spot simultaneously sweeps across the entire screen vertically. You should see the bright spot slowly moving left and right while also slowly moving up and down. Carefully observe the electronic scanning patterns, especially the forward and reverse scans.
8. Turn the forward and reverse scan speed potentiometers on the demonstrator clockwise to increase both scan speeds and observe the changes in the electronic scanning.
9. Adjust the oscilloscope's X-axis scan speed. Set the X-axis "Scan Range" to "1K-10K" and minimize the resistances on w1 and w2 on the demonstrator. A full-screen raster should now be visible. Increasing or decreasing the Y-axis gain should reveal compression of the raster vertically. Stop the oscilloscope's horizontal scan. Only the demonstrator's scan signal will be present, and a bright vertical line will appear on the screen.
10. Fine-tune the oscilloscope's x-axis "scan fine-tuning" to produce more than ten stable field retrace bright lines on the raster. The demonstration shows that the number of retrace bright lines is related to the demonstrator's field retrace scan speed and the oscilloscope's x-axis scan speed.