Security Light & Switch with PIR Sensor Update

Posted on Feb 4, 2014

This is a simple update to Mr. Hareendran`s PIR Sensor Security Light circuit. It has a shortcoming limits the relay voltage to approximately 3. 3V. While this may function with some 5V relays, it will not function with all. The nature of an emitter follower Darlington forces the output voltage to be two junction drops below the base drive voltage

Security Light & Switch with PIR Sensor Update
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plus a small voltage drop across R1. In keeping with the spirit of his design, I retained the Darlington arrangement, and the transistor type number however, I changed the transistors from common collector to common emitter configuration, and connected the relay between the collector output and the positive rail. The relay now utilizes a 12V coil to use an existing 5V relay, simply add a resistor in series with the coil whose value is scaled up from the coil resistance. Also, I connected the LED across the relay coil rather than powering it via the 2nd relay contact this way, only one form C  relay contact is required thus making the relay less expensive of course it can be done either way. I eliminated some additional, unneeded components. When motion is detected, the output (pin 2) goes to 5V and current flows through R2 to the base of Q1. The emitter current of Q1 becomes the base current of Q2 the current gain of a Darlington is the hFE squared or about 10, 000 to 50, 000 (very high). This easily provides adequate drive current for the relay. R1 provides a means of conducting any potential leakage in Q1 to common and also helps Q2 to turn off faster. D1 is the back diode that circulates the relay coil current when Q1 & Q2 turns off thus preventing a damaging voltage transient. No, I did not build and test this one I just happen to know from experience that this circuit will work OK if the circuit was new to me, I would...

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