How to design an electric circuit that makes an LED light flash at 50 Hz
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A simple LED powered from an unsmoothed DC signal.

Here's an almost free fix that can be tried with very little effort.


Use a wall-wart style transformer power supply. (You may have to rummage through your junk as most are switched mode power supplies now. Go by weight!)
If it is a DC version then strip out everything except the bridge rectifier and wire that up as shown.
Measure the rectified DC voltage, \$ V_{DC} \$.
Ignoring D5 for now, calculate R1 to give 20 mA through the LED. Assuming a 1.8 V drop on D1 we get \$ R1 = \frac {V_{DC} - 1.8}{20m} \$.
With the addition of D5, a 5.1 V Zener diode the LED won't turn on until the half-wave reaches 5.1 + 1.8 V. This may narrow the pulse enough to give a sharper stroboscopic image.




Figure 2. Here's a graph of UK 50 Hz grid from live operating data.

Figure 1 shows that as I write this the UK mains frequency has dipped to 49.9 which is -0.2%. If synching the record player to this frequency everything will be a little over 3 cents (3% of a semitone) flat. Close enough for jazz?
See Pitch-to-Frequency Calculator at Fluteopedia.

Mains frequency is allowed to drift a little with load but will go overspeed when load reduces to pull any mains powered clocks back into synch within a day.