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how can I use an Infrared Sensor to turn on a 2.5v lamp when it detects movment?

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Jan 9, 2013
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Hello all, I signed up to because nothing came up while searching for an answer. And I posted this on another board, but i'v eyet to get any replies, so hopefully I can get some answers here.

I purchased this Pyroelectric Infrared Sensor and followed a sample diagram to see if it was working properly. For the test I used an LED and it worked fine, but now I would like to do the same with a small bulb, but it's not as simple as just replacing the LED.


The output on the sensor is 5v and im trying to light up 2.5v lamp, but i'm guessing there's not enough amps to light it up?

I'm a beginner so if anyone can tell me why it's not working or point me to some sites for examples. I basically just need it to turn on a light when it detects movement, just as it did with the LED.


eariler today I did some searching for examples and came across a circuit using a relay. Could the sensor 5v output be used to trigger the relay, allowing current to pass on another circuit with the light?

ie. The sensor has it's own power supply and the circuit with the light would be running off another supply.

I have no experience in this so be kind. Thanks in advance.

Here is example, you can replace speaker with transistor which drive relay. You can adjust relay on time in code and instead sound_play use pin On/Off.

That relay can drive more power then 2,5A.
First off, thank you for the quick reply.

What I want to accomplish seems easier than what is done in the project you linked. As I said, i'm new to all this and that requires you to program a chip. After a bit of research, i'm sure I can get it to work as shown on there, but is there not a simpler way of getting a light to work?

If I wasn't pressed for time, I would order these parts and give it a shot.

You can use single transistor or small 5V relay on PIR output to drive relay. That small relay if is not enough for needed current he can drive second stronger relay.
Never actually knew what a transistor was for. that's where i'll start, thanks.

Thanks for your help. I ended up using 3 NPN transistors and it worked perfectly. Using one did nothing, adding a second one got it working, but the light dim.

Here's a pic of the hack job I had to rush and get working for my Daughters project. I'm new to this, so don't judge. :)


I put this together realizing that I should have brought a breadboard but continued anyway. I made no calculations, all trial and error - following some basic examples online. The transistors get pretty hot when the light is on, but I have the sensor timer on the lowest possible setting, turning on the light for only 2 seconds. Because of how hot they felt, I can only assume that they would burn out if the light stood on for a longer time.

And if I remember correctly, this is how I wired it.

thanks for pointing my in the right direction.
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Hi Rekaviles,

Do not worry, we'll make it,

This circuit is not good.

Tell me lamp what you want to turn on is on what voltage 220V/110V AC or some other voltage such as 12V 5V DC ?

What output you have from PIR sensing device +5V and Gnd at 2,5A ?

I just edited my post above. It actual worked for what I needed it to do and it's just for a day, as I expect this wouldn't last too long before something burned out.

The circuit is working off of 4 AA batteries, 6v total. My infrared sensor said it can work between 3 - 6v, which it does. The lamp is 2.5v / 120a (I need to double check this, but this is close).

Oh, and I used 2N2222A transistors.

I don't have that info in front of me, but I know the sensor output is 5v.

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That you for that.

Is 5 - 6v considered high voltage for a transistor?

If you want to use relay coil on ~5V then search relay with coil on that voltage, looks as this :

**broken link removed**

- - - Updated - - -

I don't have that info in front of me, but I know the sensor output is 5v.

- - - Updated - - -

That you for that.

Is 5 - 6v considered high voltage for a transistor?

Often transistors have 30V-40V-80V range but this depends from case to case, best is to look in manufacturer datasheet document to see transistor specification. At High Voltage HV i meant mains voltages 220V 50Hz / 110V 60Hz AC.

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From circuit which I post in post #9 you can omit pulldown resistor marked as R. For flyback diode around relay you can use 1N4007, transistor can be 2N2222A, and relay coil should have working voltage of your batteries if you want to run relay on battery source.
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Ah, I thought I wouldn't be able to use a relay with the current I was working with. Thanks again for your help on this.

Ah, I thought I wouldn't be able to use a relay with the current I was working with. Thanks again for your help on this.

When choosing relay watch on coil voltage and look what is current for 120V AC, in your case any current ratings above 2,5A-3A is fine, better 5A-10A. And one very important thing, is to get relay with higher coil resistance around 300-400Ω or higher, this will reduce current consumption from batteries.

Relays can switch 2,5A on HV without problems.

See this relays from DigiKey:
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