power requirements for a motion sensing system

goddey's picture

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Joined: Oct 12, 2019

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power requirements for a motion sensing system
November 10, 2019 - 12:38pm

Hello all

i'm working on a security system that involves up to twenty (20) PIR sensors (HC-SR501) with a microcontroller, a buzzer and a few LEDs.  what will be the power requirements if i want this system to run on a single rechargeable power source?

can a 12v  made up of three (3) 3.7v batteries connected in series work effectively?

pls  I need your help to make this device as economical as possible

Bradyen's picture

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Calculating battery requirements for project
November 11, 2019 - 1:20pm

Hi goddey,

i'm working on a security system that involves up to twenty (20) PIR sensors (HC-SR501) with a microcontroller, a buzzer and a few LEDs.  what will be the power requirements if i want this system to run on a single rechargeable power source?

If the sensor you are using is HC-SR501 PIR sensor, then according to this page https://components101.com/hc-sr501-pir-sensor your current consumption per sensor will be 65mA. Lets round it off to 100mA so for 20 PIR sensor a source current of 2A will be good. Again, the buzzer and LED will also consume power, I cant help with the exact numbers without knowing the rating of LED. But if they are the normal ones then still a current of 2A should be enough. Now to decide the battery rating you have to know for how long you have to keep this thing powered. Let's say you have a 2Ah battery then it means that it can provide 2A continuous current for 1 hour. Also you have to take voltage in consideration.

can a 12v  made up of three (3) 3.7v batteries connected in series work effectively?

Yes if you connect batteries in series their output voltage will add up. So having three 3.7V batteries will give you 11.1V as output voltage. But, why do you need 12V your system works on 5V right? 

goddey's picture

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Thank you
November 11, 2019 - 2:12pm

Thank you

I'm thinking of the 12v because of the distance the sensors will be spread across (50ft x 100ft fence )

goddey's picture

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Thank you
November 11, 2019 - 3:58pm

Thank you

I'm thinking of the 12v because of the distance the sensors will be spread across (50ft x 100ft fence )

Aswinth Raj's picture

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it is good idea to power from AC mains
November 12, 2019 - 7:15am

Hi goddey,

Your project sounds too big to be powered by a battery. I would suggest using an SMPS to convert your AC mains to 12V DC with a current rating of 3-5A. If you want inspiration you can look at how CCTV cameras are installed, you just have to replace the cameras with sensors and DVR with MCU, you can even use the same type of wires to power your sensors and get you output back to the MCU

goddey's picture

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Thanks a lot Aswinth Raj
November 13, 2019 - 11:42pm

Thanks a lot Aswinth Raj

What if I get a 12v battery and make a charger for it so that I can use but AC then there power and then switch to battery when power is out?

Aswinth Raj's picture

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A battery back up will work!
November 14, 2019 - 12:00pm

Hi goddey, yes a 12V battery will work. As Brayden pointed out your system will consume around 2A so with a 12V 2Ah battery you can expect your system to work for around 60 mins with battery power.

A good way to proceed from here would be to first set-up thins to work directly from AC mains, then introduce a 12V battery with a charging and protection circuits as required.