trick battery

The phase-locked loop (PLL) in IC1 is used as a frequency discriminator. The mid-fre- quency of the internal oscillator can be preset by means of P1, R8, and C5, and is, of course, around 40 kHz! Capacitor C6, in conjunction with the internal resistance of the IC, forms the smoothing filter be- tween the phase detector and the oscillator. As usual, in a PLL, the f.m. signal at pin 3 is compared with the oscillator output by the phase detector. The resulting error signal at the output of the detector (pin 2) is used to tune the oscillator until the error has disappeared. The demodulated signal of around 50 Hz is then available across C6. At the same time, the internal compara- tor switches its output (pin 8) to logic 0 which causes LED D3 to light, indicating that the 40 kHz carrier has been received. The 50 Hz signal is amplified in T3 and applied to tone decoder IC3. On receipt of this 50 Hz signal, the output at pin 8 of IC3 is low and causes LED D4 to light. At the same time, t...