Introduction: Light on Time

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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 ...

https://www.aliexpress.com/item/32948972482.html?s...

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 :-)