Light on Time

Introduction: Light on Time

About: Born in London in 1957. 1st job at age 16 was a TV engineer (valves !) Worked at a computer company repairing hardware (when 1k of ram was on a board the size of an A4 sheet of paper). Self taught in programmi…

My wife purcahsed this LED illumintaed sign and hung it in our small hallway.

She was very good at switching it on, but very forgetful re switching it off ! The obvious result being the fact we were changing the batteries far too often!

I decided the best solution to have it switch on by a sensor, but my main concern was drain on the batteries caused by the sensor.

Then I came across a 'little gem' PIR MH-SR602. It only draws 20 micro amps, (yes micro amps) while sensing and it has an adjustable 'on' time too!

Problem solved!

Step 1: Component List

MH-SR602 - eBay

100 uF capacitor (essential for stability an false trigger prevention !)

100 K timing resistor (this can be any value between 0 ohms & 316 M)

4.7K resistor

BC337 or pretty much any NPN transistor

Step 2: About the MH-SR602

There is some good information on the aliexpress site ...

The photo of the postion of the timing resistor is far better than my sketch!

I didnt bother with the sensitivity resistor, I actually removed it in error, nor did I bother with a photo resistor. I could not find much information pertaining to these 2 functions but I did not need them !

I did change the timing resistor.

****NOTE **** The table on the aliexpress site for the timing resistor is different to the one I used ! I'm not sure why it is different, but the 100K resistor I used gives me about 1 minute 'on' time !! ( Google MH-SR602 and look at 'images' !)

The default 'on' time is 2.5 seconds.

The 100uF capacitor is essential, as without it, all sorts of false triggering will occur!

Step 3: The Circuit & Fitting

The circuit itself it pretty straight forward.

When the MH-SR602 is triggered the output pin goes high for a duration dependant on the timing resistor and turns the transistor on, which in turn allows current to flow through the LEDs.

I removed the battery compartment and put the circuit between the negative battery wire and the battery connector. I drilled the bottom of the case and inserted the small circuit board.

Job done :-)

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