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AT89C2051 Night light saver

AT89C2051 Night light saver

The Saver V5.0 runs simple clock emulation program, turns a night light on and off with preset time, say 1900hrs to 2200hrs everyday. The design features low cost, easy installation, no battery backup and no EMI. The AT89C2051 uses external oscillator generated by schmitt trigger gate CD4093, 680kHz. Reference frequency was derived from 50Hz main line.
Explanation of the circuit operation for each parts is as follows;
1)Transformerless direct supply: R1 and C1 forms a simple current limitting AC source. D1 and D2 converts AC to DC with C2 performing a reservoir. D3 limits voltage across the circuit to ~5V.

2)External oscillator: A reliable schmitt gate oscillator with R8 and C5 produces clock approx. 680kHZ. One machine cycle is ~18us.

3)89C2051 circuits: C4, 33uF and R4, 10k forms a reset circuit that need time for riasing logic at least two machines cycle. S1 is a momentary button for setting time to 18:00 by pulling P3.0 to logic low. The functioning LED is connected to P3.7. Q2 is a small triac MAC97A6, and is drived by Q1, PNP transistor sinking through P1.7.

4)Reference frequency: R3 and C3 integrates a 50Hz main frequency and unwanted transients producing triangle-like waveform which is fed to U2 schmitt gate. The output pin4 is clean 50Hz square wave tied to int0.

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