The 850LM is going to be used to detect button clicks from my car and convert those clicks to MQTT messages that will be sent to my home-assistant controller via an MQTT broker. I expect to interface this on a Raspberry Pi Zero W (RPi0W) using its gpio pins. However, there are a couple of issues that need to be resolved. The gpio pins on the RPi take 3.3v and the lowest voltage DC that the 850LM can handle is 9v. Fortunately, I noticed previously that the Adafruit bi-directional level shifter can take a high side voltage up to 10vDC. That means that I can use the level shifter between the 850LM and the RPi0W. The main reason that I want to use the RPi0W for this interface is that I can run the software under a Docker container, and allow for another container to House a Bluetooth proximity sensor / MQTT setup. Note that the RPi0W needs a 5v DC power source, but generates its own 3.3v power for the gpio.
The 850LM has a power port as well as a screw connection marked plus and minus. It has a total of three relay connections; the first two having NO / GND screw terminals and the third having NO / GND / NC screw terminals. So my first question was how do the power port and power screw terminals relate. The second question was when the relays switch is it momentary (governed by how long you press the button on the car), is it momentary (fixed close then open time), or is it more of an on / off switch (changes as you click the car button). Fortunately, I should be able to use a remote control to do the testing.
Test 1: I powered up the 850LM using a 5v to 9v usb cable that I had lying around by plugging it into the power port. I had to use an adapter because the center pin on the power port was thick (white size). Once powered up, I had no problem setting up the remote control by following the 850LM instructions (click yellow button on 850LM, led next to button flashes, click button on remote, led goes out). Once I did this, a click on the remote caused the relay to close and then open on its own. So I answered the second question. It makes sense because most garage door controllers have wall switches that momentarily close a connection which starts the garage door motor running.
Test 2: I then checked the power terminal pins and discovered that there was on voltage in the millivolt range. This means that I cannot power the 850LM and draw power from these terminals when the 9v supply is plugged into the power connection.
Test 3: while clicking on the remote, I checked the voltage levels appearing between the NO / GND terminals. This did not measure any change from zero volts. This means that I will need to use a pull-up resistor for transitioning on the level shifter. It also means that I will need to deal with negative logic; high showing up on the gpio means False / 0 / not clicked, and low means True / 1 / clicked. I might end up using interrupts instead of running everything through a loop in order to not interfere with other container programs.
Test 4: I connected my 9v power source to the power screw terminals. I checked the on / off with my remote and everything worked out just fine. So I will be connecting the 9v source (via a barrel to screw terminal adapter) to these terminals, along with the input side of a Buck converter (to give 5v from the 9v source) and some connection wires to power up the high side of the level shifter. What I may do instead is put everything on a breadboard connected to the RPi0W gpio and minimize the connections needed (use the breadboard as the connection points for all of it).
I wonder if I can mount the Buck converter on the breadboard as well, that would save some space for me. I am also concerned that the Buck converter might interfere with the functioning of the RPi, but I will have to test that out as everything is built.
I wonder if I can mount the Buck converter on the breadboard as well, that would save some space for me. I am also concerned that the Buck converter might interfere with the functioning of the RPi, but I will have to test that out as everything is built.
