Trigger a circuit with square wave
Many ways to do this. You intend to do a logical function (turn on - turn off) which suggests a comparator. First circuit uses a transistor. The square wave must be larger than 0.6V peak-to-peak. If it is, the transistor turns on - its collector current pulls R1 down near zero-volts. Capacitor C1 is pulled down as well, but starts to charge back to the supply voltage during the last half of the 800 Hz. square wave. \$ R1 * C1 \$ time constant should be significantly longer than the 800 Hz. period.







simulate this circuit &ndash; Schematic created using CircuitLab
You could use a proper comparator chip to do a similar function.
When the square-wave amplitude exceeds the threshold voltage set by R2, its output goes low, otherwise output is high. The output stays low because of the RC time constant of R1 &amp; C1 at its output. The time constant should be longer than the 800 Hz. period (1.25 milliseconds).
At the input, a diode clamp prevents a large amplitude square wave from going too far negative. R3 establishes a zero-volt reference:






simulate this circuit

A disadvantage of these circuits is that any frequency (not just 800 Hz) will cause the output to go low. And a low frequency will output multiple pulses - use this circuit if 800 Hz. (or higher frequency) is expected.

If you want to detect only 800 Hz., then a more complex circuit is required. A LM567 can provide a frequency-selective output indicator.
A 4046 phase-locked-loop can perform a similar function: CMOS Phase-Locked-Loop Applications