# Wireless Motion Sensor modification question

#### jvmh

Joined Feb 1, 2021
15
I am trying to modify an inexpensive PIR wireless motion sensor to bypass the PIR and add a simple on/off tilt switch to activate the transmitter. The unit works very well with long range and reliability. It's a Bunker Hill (sold through Harbor Freight) Model RL-9816B. I will attach a photo of the board. I have already attached an external antenna which works well. Now just need to figure out how to just use the transmitter, activated by an add-on tilt switch (remote to the unit) to send signal to the receiver. Any ideas appreciated! Thanks.

#### Dodgydave

Joined Jun 22, 2012
11,349
I think the signal to the transmitter is via R2 from the pir chip, what happens when you measure the voltage on R2 when the pir is detected and rested?

What is the number on the pir chip?

#### jvmh

Joined Feb 1, 2021
15
I think the signal to the transmitter is via R2 from the pir chip, what happens when you measure the voltage on R2 when the pir is detected and rested?

What is the number on the pir chip?
Thanks for your reply, Dodgydave! I'm reading 3.32 volts on R2. There is no number on the PIR itself. The number on R2 is 474. Does that help?

#### Dodgydave

Joined Jun 22, 2012
11,349
Thanks for your reply, Dodgydave! I'm reading 3.32 volts on R2. There is no number on the PIR itself. The number on R2 is 474. Does that help?
Read the voltage on one side of the resistor R2 ideally on the pir chip side in both cases when the pir is at rest and when it's detected your hands.

#### jvmh

Joined Feb 1, 2021
15
Read the voltage on one side of the resistor R2 ideally on the pir chip side in both cases when the pir is at rest and when it's detected your hands.
Got it. The voltage at rest is 3.32 and when motion is detected it's 1.29. Unit works off a 9 volt battery. Thanks!

#### Dodgydave

Joined Jun 22, 2012
11,349
Got it. The voltage at rest is 3.32 and when motion is detected it's 1.29. Unit works off a 9 volt battery. Thanks!
Ok so it pulls the pin down to ground when detected, so R2 is pulled low to enable the transmitter..

#### jvmh

Joined Feb 1, 2021
15
Ok so it pulls the pin down to ground when detected, so R2 is pulled low to enable the transmitter..
That's how it appears. I guess my question is is it possible to wire in an on/off switch to the circuit so that switch when activated will let the transmitter send its signal?

#### Dodgydave

Joined Jun 22, 2012
11,349
It's probably possible to add the switch using a resistor in series to pull the Tx pin down to ground, try it with a 10K resistor on the input to the U2 chip.

#### jvmh

Joined Feb 1, 2021
15
It's probably possible to add the switch using a resistor in series to pull the Tx pin down to ground, try it with a 10K resistor on the input to the U2 chip.
Good idea, thanks! Can you be more specific on setup for the switch and resistor? Looking forward to giving it a try and just don't want to mess it up ... Many thanks!

#### Dodgydave

Joined Jun 22, 2012
11,349
Try this circuit, connect the tilt switch to the resistor R2 as shown, see if the transmitter is enabled when the pin is pulled low with a resistor between 10K to 100K.

#### MisterBill2

Joined Jan 23, 2018
19,424
I just read thru this thread, and now one question is about the LED. My guess is that it functions like the LED on most motion sensors, which is to indicate motion detected. If that is the case, then tracing back will locate the signal from the detector., which also activates the radio transmitter.
I see many comments about measuring the voltage but not one single word about the reference point that the measurement uses.
What is the reference vor all of those voltage readings?? Or do some folks have a one-terminal voltmeter??

Considering how the system operates, there is a power supply portion that is constantly feeding the motion sensor section, and then a switch section that operates both the transmitter and the LED.
So tracing back from the LED to the power supply will show where the switch command that powers both the LED and the transmitter is connected back to the power supply feed.

#### jvmh

Joined Feb 1, 2021
15
Try this circuit, connect the tilt switch to the resistor R2 as shown, see if the transmitter is enabled when the pin is pulled low with a resistor between 10K to 100K.View attachment 287983
Awesome, thanks so much. Will set this up and try it out and let you know how it works. May be a day our two before I have results. Your assistance is so appreciated!

#### jvmh

Joined Feb 1, 2021
15
I just read thru this thread, and now one question is about the LED. My guess is that it functions like the LED on most motion sensors, which is to indicate motion detected. If that is the case, then tracing back will locate the signal from the detector., which also activates the radio transmitter.
I see many comments about measuring the voltage but not one single word about the reference point that the measurement uses.
What is the reference vor all of those voltage readings?? Or do some folks have a one-terminal voltmeter??

Considering how the system operates, there is a power supply portion that is constantly feeding the motion sensor section, and then a switch section that operates both the transmitter and the LED.
So tracing back from the LED to the power supply will show where the switch command that powers both the LED and the transmitter is connected back to the power supply feed.
Thank you! Will try to trace that, seems like a good path to follow!

#### Dodgydave

Joined Jun 22, 2012
11,349
I think the tx U2 chip is a PT4450.?

#### jvmh

Joined Feb 1, 2021
15
The only marking I see on that U2 chip is "OF".

#### jvmh

Joined Feb 1, 2021
15
Try this circuit, connect the tilt switch to the resistor R2 as shown, see if the transmitter is enabled when the pin is pulled low with a resistor between 10K to 100K.View attachment 287983
Well, I finally got around to trying this approach but unfortunately it didn't work. Kind of back to square one!

#### MisterBill2

Joined Jan 23, 2018
19,424
Is this a single-sided board or a double sided one? And what sort of scheme is the wireless link? Bluetooth? Ethernet? or possibly in the 330 megahertz realm? Or none of the above?? I think that the RF circuit is in the upper right section of the PCB photo. If the transmission is simply a power on/power off control then tracing the power feed control makes more sense.

#### Dodgydave

Joined Jun 22, 2012
11,349
I came across this circuit for a similar device , a driveway alarm using 433 Mhz pir sensor, see if it's works on your pcb.

On this design output from the pir chip is pin 7, and it powers the TX chip on pin 6 via a transistor, yours has an 8 pin chip probably just two opamps inside.

#### MisterBill2

Joined Jan 23, 2018
19,424
Interesting that in this circuit there is a "data" feed to the transmitter IC as well as a power connection.

So now I am asking if the tilt switch is to be IN ADDITION to the motion sensing function, or is the tilt switch A REPLACEMENT for the motion sensing function. That makes quite a difference.
Observe that this motion sensor circuit has two stages of AC coupled amplification prior to the DC gain section feeding the "data" transistor. So the TS photo may be showing something that is not as simple as it seems. I suggest a detailed tracing of the circuit between Diode D1 and transistor Q1. I think there is a power switch in that section.

Another question is about the desired output from the receiver: Does it just need to give a momentary event signal? Or change state with the tilt switch? That makes a very big difference.

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#### jvmh

Joined Feb 1, 2021
15
Interesting that in this circuit there is a "data" feed to the transmitter IC as well as a power connection.

So now I am asking if the tilt switch is to be IN ADDITION to the motion sensing function, or is the tilt switch A REPLACEMENT for the motion sensing function. That makes quite a difference.
Observe that this motion sensor circuit has two stages of AC coupled amplification prior to the DC gain section feeding the "data" transistor. So the TS photo may be showing something that is not as simple as it seems. I suggest a detailed tracing of the circuit between Diode D1 and transistor Q1. I think there is a power switch in that section.

Another question is about the desired output from the receiver: Does it just need to give a momentary event signal? Or change state with the tilt switch? That makes a very big difference.
The tilt switch will be ideally a replacement for the motion sensor -hopefully it can just be eliminated from the circuit. And I believe a momentary event signal would work to activate the chime/light receiver.