Replacing momentary switch with motion sensor?

Thread Starter

huronclean

Joined Aug 12, 2020
12
Hello!
first post here and not an electronics skilled person but having looking and trying to hack together something I’ve still not found any functioning or “off the shelf” solutions to what seems a simple challenge;

Replace a mechanical, momentary switch with a motion sensing switch.
So, instead of being required to activate the normally open,
momentary switch by pressing and releasing- A foot moving past a sensor would create the momentary trigger.

I tired using a PIR unit that is sold to turn led strips on and off - paired with a NO relay...but the PIR kept the relay held closed too long- i need around a 1/2 second close/open.

I’ve found a similar thread on this forum with a circuit that seems it might work but not sure as i’m no electronics pro.
*image attached 495D352E-ACD6-4B64-97C8-E3F595E5BFFA.gif

7EF7D265-1696-4D0D-9583-557AE2818711.gif

hope that is clear and any help appreciated
 

Tonyr1084

Joined Sep 24, 2015
4,918
I have a PIR wall switch in my down stairs hallway. I keep it set to "TEST". When I enter the hallway the light will go on and stay on for around 30 seconds, then shut off. Can you explain the reason for the half second requirement?
 

Thread Starter

huronclean

Joined Aug 12, 2020
12
I have a PIR wall switch in my down stairs hallway. I keep it set to "TEST". When I enter the hallway the light will go on and stay on for around 30 seconds, then shut off. Can you explain the reason for the half second requirement?
Hi,
the PIR i tried using was similar to what you have- the problem was it wouldn’t work because the shortest setting of 10+ seconds was too long.

What i’m hoping to accomplish is to replace the switch which i now have to manually set to activate my equipment- the existing, normally open rocker switch is hooked to a piece of low voltage equipment that requires a contact to close for a short interval (1/2 second approx).

I’d like to replace the manual switch with a sensor circuit.

if i find a circuit with a motion sensor and a pulse output of 5V for a 1/2 second or less- i can then use a 5V relay i have on hand to close the switch terminals and activate the equipment by moving a foot in front of a floor mounted sensor unit i would build.

thx
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
4,918
Just searched Amazon and found THIS. It claims a 2 second delay and 2 second blocking. I don't know what that means. Someone else here will probably know a better solution.

You COULD use this built into a "One Shot" 555 configuration where the One Shot (OS) is triggered for a specific time you set. Regardless of the number of pulses that come in afterwords. The OS has to have a set recovery time. Meaning if the PIR senses motion and triggers the OS, it's held high for 500 mS (half a second) and then can't be retriggered for 10 seconds. I think. I'm not the expert on PIR's OR on 555's.
 
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Thread Starter

huronclean

Joined Aug 12, 2020
12
Just searched Amazon and found THIS. It claims a 2 second delay and 2 second blocking. I don't know what that means. Someone else here will probably know a better solution.

You COULD use this built into a "One Shot" 555 configuration where the One Shot (OS) is triggered for a specific time you set. Regardless of the number of pulses that come in afterwords. The OS has to have a set recovery time. Meaning if the PIR senses motion and triggers the OS, it's held high for 500 mS (half a second) and then can't be retriggered for 10 seconds. I think. I'm not the expert on PIR's OR on 555's.
Thanks Tony!
That sort of what i was trying to do but it needs a custom circuit i’m guessing- beyond my skills-
if anyone can detail this much appreciated :):)
 

sghioto

Joined Dec 31, 2017
1,347
if i find a circuit with a motion sensor and a pulse output of 5V for a 1/2 second or less- i can then use a 5V relay i have on hand to close the switch terminals and activate the equipment by moving a foot in front of a floor mounted sensor unit i would build.
This should be fairly easy. You have a PIR that puts out 12 volts when triggered. A simple capacitor in series with the relay coil might be all you need. What part # relay were you using?
Steve G
EEE Momentary PIR relay.png
 
Last edited:

eetech00

Joined Jun 8, 2013
1,966
Hello!
first post here and not an electronics skilled person but having looking and trying to hack together something I’ve still not found any functioning or “off the shelf” solutions to what seems a simple challenge;

Replace a mechanical, momentary switch with a motion sensing switch.
So, instead of being required to activate the normally open,
momentary switch by pressing and releasing- A foot moving past a sensor would create the momentary trigger.

I tired using a PIR unit that is sold to turn led strips on and off - paired with a NO relay...but the PIR kept the relay held closed too long- i need around a 1/2 second close/open.

I’ve found a similar thread on this forum with a circuit that seems it might work but not sure as i’m no electronics pro.
*image attached View attachment 214591

View attachment 214592

hope that is clear and any help appreciated
Assuming ( I hate that word) the PIR goes high when motion is detected, Just add a leading edge detector circuit to the output of the PIR. So...when the PIR putout goes high, a single short pulse gets sent to the 0.5 second timer.
 

MisterBill2

Joined Jan 23, 2018
6,746
This should be fairly easy. You have a PIR that puts out 12 volts when triggered. A simple capacitor in series with the relay coil might be all you need. What part # relay were you using?
Steve G
View attachment 214629
SG is certainly correct. You will also need to put a high value resistor across the capacitor to eventually discharge it 47K ohms should work,

But what sort of device finds 30 seconds too long for a turn on pulse? That seems a bit strange.
You could also use a beam-break sensor that would deliver an "ON state" only as long as the beam was broken.
 

Thread Starter

huronclean

Joined Aug 12, 2020
12
This should be fairly easy. You have a PIR that puts out 12 volts when triggered. A simple capacitor in series with the relay coil might be all you need. What part # relay were you using?
Steve G
View attachment 214629
thanks SG.
this is the relay i have on hand
C343681D-D218-46CD-A45C-1A345E1F2F64.jpeg
i tried paring it with an LED motion sensor- it tripped the relay but it kept it closed for 10 seconds and the device i need to use it with requires CLOSED>OPEN of less than a second to turn on.
here’s is the sensor i tried and failed with
548055FC-3DB4-4D5C-8CD6-57FCE1DE1EC6.jpeg
PS: i tried replacing the pot that was included in the PCB of the PIR unit but it didn’t seem to shorten the length of the “ON” duration-
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
21,257
What is the nature of the load, If high voltage and/or highly inductive, I prefer the vented version of these type relays, this one show sealed.
The other optional vented have a small 'pip' on top that can be removed.
Otherwise you could see premature failure.
Max.
 

Thread Starter

huronclean

Joined Aug 12, 2020
12
Thanks G
I see that there is a diode on the relay module across the coil so you won't need to add D2. Wasn't sure if you were aware of that.
Steve G
I was not aware of that!
Thanks for adding that detail Steve... i have pretty basic skills so it is appreciate you thought of that!
 
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