Converting a variable frequency to TTL HIGH and LOW levels, based on a fixed (possible non-fixed?) frequency
One way to solve this problem is to use a re-triggerable one-shot component. You said that absolute tolerance is not super critical and so the typical tolerances of a one-shot may be a suitable and simple means to detect the frequency shift of your signal. 

The one-shot needs to be setup with a R/C timing delay just at the timing period of your signal at 50Hz. This would be 20msec. As long as the input trigger signal at the one-shot stays above the 50Hz rate the output of the one-shot will stay re-triggered at a high level. When the input frequency drops below 50Hz the one-shot will start to timeout on each input pulse and its output will start to pulse with each cycle of the lower frequency input.

You could possibly use this as is depending upon what you are using to detect the frequency changes or you could add some additional simple logic to convert the lower frequency pulsing output of the one-shot to a static signal envelope. Below I show an example of using a second re-triggerable one-shot to convert the output pulses of the first one shot to the envelope pulse. In this example the venerable 555 chip is shown being used for the re-triggerable one-shots. 





In this simulation the first 555 has its timing components (R3 and C4) set to detect just at 50 Hz. An input below 50Hz causes the output to start pulsing. The input shown here has switched to 25Hz. The second 555 chip has to have its timing components (R1 and C6) set for a delay time a little longer than the period of the lowest frequency expected for the input signal.

If you decide to use this circuit concept it would be a good application of the NE556 dual 555 chip. The NE556 is only marginally more expensive than the NE555 so would be more economical to use in the build out of this circuit.