Bypass wireless motion sensor, add tilt switch to activate transmitter? - Page 1
Forum section: electronics / beginners

author: jacksf
author_rank: Newbie
timestamp: February 22, 2023, 12:54:43 am
content: Our mail is being stolen so we have installed a heavy steel locking mailbox. In our previous mailbox I had installed an inexpensive Bunker Hill Wireless PIR Alarm (model 9816B) to which I added an external antenna. This setup worked well to send an alert to the receiver inside the house whenever the mailbox was opened. It does not work in the new mailbox because its design does not allow the alarm to detect motion.I would like to add a simple tilt switch to the alarm which could bypass the PIR sensor and just send a signal directly to the transmitter through the external antenna. I have not been able to determine how to make this work yet! It's a good transmitter with more than adequate power to cover the 150 feet to the house. I need to know if this is possible and if so how to wire it in to the PCB. I will attach a photo of the PCB below. Many thanks for any ideas/input.

author: ledtester
author_rank: Super Contributor
timestamp: February 22, 2023, 02:45:23 pm
content: If you search for "PIR schematic" you'll see that many PIR sensor modules have a digital or open collector type output.Can you identify the markings on the U3 chip? It is likely the "brains" of the device and knowing what it is would greatly help in figuring out how it works.Also, my current guess is that one of its pins is connected to the PIR sensor output and bringing that pin high or low will trigger the alarm. To be safe I would connect that pin to Vcc / GND through a 1K resistor.

author: jacksf
author_rank: Newbie
timestamp: February 22, 2023, 04:36:32 pm
content: Unfortunately the markings on the U3 chip are illegible, even under USB microscope. There's something there but far too faint. Should I try the pins one by one...and would a 10K resistor be ok (don't have 1K at hand). Thank you!

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 22, 2023, 04:50:16 pm
content: You can try with a digital multi meter to see what happens on R4. It might be the output of the PIR module.It can also be that on R2 the output signal to the transmitter is to be found. U2 most likely is the transmitter.It depends if these are controlled by open collector outputs if you can override them, When the other side of R4 is on the plus rail it could be. If it is ground it probably is not.With a bigger more clear picture it might be possible to reverse the schematic to some extend.

author: ledtester
author_rank: Super Contributor
timestamp: February 22, 2023, 07:10:36 pm
content: Quote from: jacksf on February 22, 2023, 04:36:32 pmUsing a 10K resistor might work, but a lower value might be required. You can create a 3.3K resistor by placing 3 10K resistors in parallel. Or 4 in parallel to create a 2.5K resistor which might be low enough. I don't think it would hurt to try with the 10K.I would first trace out as much of the circuit around U3 as you can. There should be a pin which is directly connected to the battery negative terminal, i.e. GND. Also, U4 looks like a voltage regulator and so there should be a pin which is directly connected to the output of U4 -- that would be Vcc. U2 is likely the IC which generates the radio signal and so a connection from U3 to U2 is likely how U3 tells U2 to start broadcasting.It appears the motion sensor was once sold at Harbor Freight stores in the US. An alternative which might work for you would be to modify this Bunker Hill wireless doorbell:https://www.harborfreight.com/wireless-doorbell-57225.htmlReplacing the push button with a tilt switch should be easy.

author: jacksf
author_rank: Newbie
timestamp: February 22, 2023, 07:17:52 pm
content: Thanks for all your ideas! I actually had tried the Bunker Hill Wireless Doorbell but its range was nowhere near as long as advertised. The range of the motion detector is excellent which is why I'm trying to just make that transmitter work with a switch. I would love to have provided a larger clearer photo of the PCB but this one was the best the site would upload. I'll try that again this evening and will try your other suggestions.