50Hz strobo/timebase for turntables

A speed variation of just six percent can alter pitch by a full sharp or flat, and the effect is broad-band, changing every frequency reproduced. The broadcast industry has set a standard of 0.3% variation as its tolerance.

Strobes work on the principal of "freeze motion" illusion by using a precise flashing light (glim-lamp or LED). A disc with incremental markings, evenly spaced around a circle is illuminated while it is revolving. When the rate of the flashing light matches the passing of the markings beneath it, the motion of the disc will appear stationary. The mathematic formula that describes this condition is:

   The Ratio of F ÷ RPM = N × (1 ÷ 60)
   With Strobe Frequency and Platter RPM known, solving for N:
   N = F ÷ RPM × 60
   Where F = Strobe Light Repetition Rate or Frequency
   RPM = RPM of Turntable
   N = Number of incremental dots, lines or other icons, evenly spaced around the disc.

Example for 33.33 RPM 60 Hz: N = 60hz ÷ 33.33 = 1.800 × 60 = 108 markings.
Example for 33.33 RPM 50 Hz: N = 50hz ÷ 33.33 = 1.500 × 60 = 90 markings.

Note that older strobe discs that used neon or overhead fluorescent light will have double those markings because gas lamps flash on every half cycle and thus produce flashes at twice the line frequency.

The circuit drives an ultrabright white LED, with bezel. The base frequency are generated with a 3.2768MHz crystal, using a CD4060 (14 stage binary counter and oscillator), and a CD4013 (dual D flip-flop, as 2 binary counters) as dividers, and one transistor to drive the LED. There is a trimming capacitor (C1) for accurate frequency setting - can be adjusted using reference timebase and an oscilloscope. The accuracy will depend by used crystal type, but are better than 15 ppm. The supply voltage can be between 6-18 Volts, all capacitors are 25 V, C3 and C4 are Ta type. R4 and R5 can be adjusted in function of used supply voltage: R4=R5=(U-3.4)/(2*0.01) (in case of 10mA's white LED), but their value are not critical. The strobe pulse-width and frequency (100Hz), was optimised for 50Hz turntable strobe illumination. These values makes a good turntable strobe.

Figure 1 This is the schematic diagram of 50Hz timebase.

An example of pcb pattern are presented in (Figure 2).

Figure 2 The pcb drawing - as example

Figure 3 The first trial - and are working! :-)

If your turntable haven't marking for stroboscope, here you can download one for 33 rpm and 50Hz - the drawing are in pdf format.