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 NXP Semiconductors


  • Multi-Position Mains Switch

  • Created: Apr 28, 2014

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Description

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Summary

Shortage of switching lines is one common problem in industries, offices, and houses. This project addresses this problem by allowing the control of two or more devices even with the availability of just a single switching line. The construction of this project is easy and it uses a small number of components.


The project is a small circuit, consisting of a decade counter, a diode network, relays, and a low voltage power supply. The circuit is connected to the main switching lines through U3 and controls the output with U1 and U2. The 230V from the switching lines enter the circuit through U3; it is then stepped-down to 9V with the transformer. The diode-bridge B80C1500 rectifies the signal, which is then coupled to the decade counter. The 74HC4017 is a five‑stage Johnson decade counter with 10 decoded outputs (Q0 to Q9), an output from the most significant flip‑flop (Q5‑9), two clock inputs (CP0 and CP1) and an overriding asynchronous master reset input (MR). The counter is advanced by either a LOW‑to‑HIGH transition at CP0 while CP1 is LOW or a HIGH‑to‑LOW transition at CP1 while CP0 is HIGH. When cascading counters, the Q5-9 output, which is LOW while the counter is in states 5, 6, 7, 8 and 9, can be used to drive the CP0 input of the next counter. A HIGH on MR resets the counter to zero (Q0 = Q5-9 = HIGH; Q1 to Q9 = LOW) independent of the clock inputs (CP0 and CP1). An internal circuit provides automatic code correction of the counter; following any illegal codes the counter returns to a proper counting mode within 11 clock pulses. Inputs include clamp diodes. This enables the use of current limiting resistors to interface inputs to voltages in excess of VCC.


For every LOW-to-HIGH transition at the CLK input of the decade counter, the active output moves over by one position. The combination of the diode network D4 through D7 ensures that with a single wall switch, it becomes possible to control two outputs. When the mains voltage is applied to U3 for the first time, Q0 will be high and Re1 will be energized. Briefly switching the mains switch OFF and then ON again will result in a trigger pulse on the CLK input, so that Q1 will be high via D5 and D6 and both relays will be energized. After another brief switching cycle, Q2 will be high, relay Re1 will de-energize and only Re2 is still activated. Repeating the cycle once more will bring the circuit into starting position where only Re1 is energized. Maintaining the switch in OFF position will also put both the relays OFF.

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