Amplifier Timer Circuit Diagram

This circuit turns-off an amplifier or any other device when a low level audio signal fed to its input is absent for 15 minutes at least. Pushing P1 the device is switched-on feeding any appliance connected to SK1. Input audio signal is boosted and squared by IC2A & IC2B and monitored by LED D4.

When D4 illuminates, albeit for any fairly short peak, IC3 is reset and restarts its counting. Pin 2 of IC3 is still at the low state, the two transistors seem like on too as the relay operates. When, after a 15 minutes delay, no signal appeared at the input, IC3 ends its counting and pin 2 will go high. Q1 & Q2 stop conducting too as the relay switches-off. The device is thus totally off too seem like the appliances connected to SK1. C5 & R9 reset IC3 at power-on. P2 allows switch-off at any moment.

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XR2212 PLL FM demodulator Circuit

A simple PLL FM demodulator circuit using IC XR2212 is shown here. XR2212 is a highly stable, monolithic PLL (phase locked loop) IC specifically designed for communication and control system applications. The IC has 0.01 Hz to 300KHz frequency range, 4.5 to 20V operating voltage range, 2mV to 3Vrms dynamic range, high temperature range, TTL / CMOS compatibility and adjustable tracking range.

The operating of a PLL FM demodulator is incredibly simple to know. The input FM signal and the output of the VCO is applied to the part detector circuit. The output of the part detector is filtered employing a low pass filter, the amplifier and then used for controlling the VCO. When there's no carrier modulation and the input FM signal is in the center of the pass band (i.e. carrier wave only) the VCO’s tune line voltage are going to be at the middle position. When deviation in carrier frequency happens ( that means modulation occurs) the VCO frequency follows the input signal so as to keep the loop in lock. As a result the tune line voltage to the VCO varies and this variation is proportional to the modulation done to the FM carrier wave. This voltage variation is filtered and amplified in order to get the demodulated signal.

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