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  • Dark Activated Switch Circuit

  • Created: Sep 08, 2014

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The circuit utilizes the light sensing property of a light dependent resistor (LDR) to activate the circuit and Microchip Technology’s MCP6401T-H/OT to provide a single supply voltage source. It features low quiescent current and rail-to-rail input and output operation. It is well suited for single-supply, battery-powered, automotive and industrial applications.

The variable resistor (VR) and LDR form a potential divider that gives a variable voltage to the inverting input (pin2) of IC1. The non-inverting input (pin3) of IC1 gets a fixed voltage (half supply voltage) from the junction of the potential divider R1 and R2. The feedback resistor R3 gives some hysteresis so that the relay turns on when the light level falls to a particular value and does not turns off again until the light level rises above this value. This prevents spurious switching by sensing mild changes in the light level. This also prevents relay clicking when the light level gradually falls at sunset. During daytime, LDR gets sunlight and it conducts. This reduces the voltage at pin2 of IC1. Since this voltage level is lesser than the voltage at pin3, output of IC1 goes high to make the PNP transistor T1 off. Thus relay remains off during daytime. When the light level decreases at sunset, LDR cease to conduct and the voltage level at pin 2 increases above the voltage level at pin3. Output of IC1 then turns low and T1 conducts. This activates the relay and the AC load connected to the NO (Normally Open) contacts of the relay turns on. This condition remains until LDR illuminates in the morning and then the lamp turns off. Diode D1 protects T1 from back E.M.F when T1 switches off. VR can be used to adjust the sensitivity of LDR at the particular light level, say at 6 pm to turns on the lamp.

The Microchip MCP6401/1R/1U family of operational amplifiers (op amps) has low quiescent current (45 µA, typical) and rail-to-rail input and output operation. This family is unity gain stable and has a gain bandwidth product of 1 MHz (typical). The MCP6401/1R/1U family is designed with Microchip’s advanced CMOS process. It is offered in SC70 and SOT23 packages for -40°C to +125°C temperature range and in SOT23 for -40°C to +150°C temperature range with a power supply range of 1.8V to 6.0V. These features make the family of op amps well suited for single-supply, battery-powered, automotive and industrial applications.



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