Looking for help in designing a simple circuit that allows a PIR sensor to drive a motor

[DanSteely]

Hi All,

I realise that this is not the right forum but am looking for recomendations for an electronics forum that would help me design a small single transistor circuit.

The background is I have a PIR sensor module that runs on 6V and I'd like to control a small 12V 250mA DC motor with it.

The HC-SR501 sensor outputs approx 2.5V when triggered and I'd like to control the motor with something like a TIP120 (as I have a few to hand)

I'm looking for an electronics forum that could design this circuit for me.

Any recomendations?

Many thanks.

Dan

[sunray]

Dan.

 

As to a forum for such a project I don't know but simply typing  'HC-SR501 sensor' into google comes up with lots of snippets and ideas.

[kgallen]

Search for "arduino npn motor 12V" for ideas and explanations.

For example https://www.instructables.com/12V-Motor-Control-With-5V-Arduino-and-NPN-Transist/

[themadhippy]

something like this should work

 

 

[DanSteely]

Many thanks for all your suggestions and advise.

As the PIR has a 3.5V regulator on board it will work at 12V without issue so I've omitted the 7805

Taking inspiration from TMH's circuit, I'm driving the base directly with the PIR O/P and it seems to work well.

I've added 1N7001 across the motor.

[themadhippy]

Quote

I'm driving the base directly with the PIR O/P and it seems to work well.

you might want to add a resistor from the sensor to the base to limit the base current to help avoid the magic smoke appearing.

[DanSteely]

It doesn't work relyably with the 4.7K but does with a 1K2 and draws just under a 1mA on that leg.

 

[kgallen]

You might want to build a darlington pair with your TIP120's to give you more current gain from your sensor to the motor.

From a search of the HC-SR501 it looks like it has an internal 1k resistor on the output line, so maybe just use something small (470 ohms to 1k) in your external circuit, especially if it's struggling to drive the motor - and use a darlington pair.

https://circuitdigest.com/tutorial/darlington-transistor-pair

 

Edited July 12, 2023 by kgallen

[themadhippy]

Quote

It doesn't work relyably with the 4.7K but does with a 1K2 and draws just under a 1mA on that leg

Either the gain of the tranny is lower than the datasheet claims or my sums are wrong,but  either way you got it working

Quote

You might want to build a darlington pair with your TIP120 's to give you more current gain from your sensor to the motor.

wouldn't that be a darlington quad as the tip120 is already a darlington pair

Edited July 12, 2023 by themadhippy

[DrV]

If the pir is only outputting 3.3V then there's not much headroom over the Vbe of the transistor. A Darlington will be at least 1.2V (datasheet says max of 2.5 but at a higher current) so directly connecting with an internal 1k will only give about 1mA base current. I'd just connect directly.

 

[boatman]

You might find that adding a couple of general purpose transistors improves the performance.

[themadhippy]

Quote

or my sums are wrong

 

Quote

A Darlington will be at least 1.2V

there me first mistake, guess who just  thought, silicon tranny yea that'll be 0.6v then.

[sunray]

10 hours ago, themadhippy said:

 

there me first mistake, guess who just  thought, silicon tranny yea that'll be 0.6v then.

Been there, done that. Had a detector powered by a 2032  cell, Open circuit output PD being ~ 1.5V and short circuit being ~2mA.

My job involved installing several commercial units from different manufacturers supposedly expected talk to each other

There was no way this was going to work:

I had no way of knowing what was in the sealed detector, that on the left is my guess.