How Does the Mechanical TV Work?

To understand how the TV works we will split the TV into two main areas; the circuitry that generates the image data, and the mechanical action that turns the image data into a picture. Considering how the TV works, it is best to start with the mechanical theory as the circuitry is designed around that.



The most important mechanical device in the TV is the Nipkow disc, which is a disc that has small holes which spiral inward. Images on TVs typically consists of many horizontal scan lines where images are displayed line by line. On CRT TVs, this scanning technique is rather easy to do with an electron beam which is directed via magnetic fields. In a mechanical TV, this scanline action is done with a Nipkow disc. A small portion of the disc is used as the TV display and as the disc spins there are holes that move across the TV display and this hole scanning action starts at the top of the display and eventually moves down to the bottom of the display. Generating an image is then a matter of using a bright light and turning it on and off at the precise moment. However, timing with a Nipkow disc TV can be tricky which is why early mechanical TVs had issues with synchronizing. Luckily for us, we have microcontrollers and semiconductors which can be used to create all the timing signals for generating images properly!



The TV in this project has three main circuits; the motor control, image generator, and a synchronizer circuit. The first circuit, the motor control, is a 555 PWM astable oscillator that helps to keep the disc spinning at a constant speed (variable speeds distort the image). The PWM adjust is done with the 10K potentiometer and the two diodes D1 and D2. The charge time for C2 is dependent on D2 and the resistance of the potentiometer between C2 and D2 while the discharge time is dependent on D1 and the resistance of the potentiometer between D1 and C2.



The image data is generated by the PIC16F628. Upon receiving a HSYNC signal from the TV (discussed later), the PIC code will take a byte of data image (which represents the entire line), and stream the data out bit by bit on the IMAGE pin (RB3 in our case). The only problem with synchronization that this circuit has is that there is no VSYNC signal which informs that the TV has completed one revolution. This means that imaged formed could be in the wrong place vertically on the screen. This is combatted by providing the viewer with a switch that allows the imaged to be shifted down when pressed. This is continuously pressed until the image appears in the correct vertical position.



The last circuit, the HSYNC generator, has two different circuits; one for generating the HSYNC signal for the microcontroller, and an LED driver for driving the white LED that produces the image itself. The HSYNC generator works by using an IR LED and IR Diode that sits on opposite sides of the Nipkow disc. On the disc, there is a ring of holes close to the center that match up with the scan line holes and so when the IR diode detects the IR LED, a new line of image data needs to be displayed. The IR diode produces pulses which are coupled via C1 (to remove DC offsets), and then amplified by U1A. D3 is very important as negative going signals can make the op-amp behave unexpectedly and so D3 removes negative voltages from input signals. This amplified IR pulse signal is then fed into a Schmitt trigger (U1B) to ensure that the output HSYNC signal is clean and only switches when big changes in signals have been detected. The image driver is just two op-amp buffers connected with U2B providing the current needed to drive the LED and U2A being used as an input buffer.