Never Forget to Lock Your Car Again! (V1)

Hello there! This is the Instructable for a little device I made after my girlfriend's car got stolen from when she forgot to lock it because she was preoccupied with a bunch of other things. The point of this device is to put the idea of locking your car on your mind every time that you turn the car off, and it does so by beeping at you and flashing a little LED light every time that it detects you've turned your car off. It's also got a button on it so that you can shut its beeping up when it goes off.

However, this V1 does have one minor flaw: it does go off a second time for about 20 seconds, and it can't be turned off for these 20 seconds - so there's a V2 coming! But, if you're interested in this project and its design, here's the tutorial. Or, if you want to give me any feedback on this first version while I work on V2, let me know through the comments here! I'm open to any suggestions or ideas for the circuit!

I did my best to keep this project light on the supplies, and if you've done any electronics projects in the past and have selections of resistors, capacitors, and diodes, you probably won't have to buy much of anything for this build.

Note: apart from the PCBWay links, none of these links are affiliate links.

Resistors:

1x 1 Ohm

2x 100 Ohm

1x 220 Ohm

3x 470 Ohm

1x 1k Ohm

10x 10k Ohm

1x 100k Ohm

1x 220k Ohm

1x 1M ohm

This resistor kit should cover all of these values

Capacitors:

1x 100nf (0.1uF) Ceramic

2x 1uF Ceramic

1x 10uF Electrolytic

2x 3F 2.7V Supercapacitor (I've linked to some 3.3F ones, the fact that they're slightly higher capacitance than spec'd won't matter in this circuit)

Other components:

1x IRLZ44N MOSFET

2x IRF9540 MOSFET

1x 2N3906 BJT

2x 2N3904 BJT

1x 1N5818 Diode (or 1N5817 or 1N5819)

1x 1N4148 Diode

1x LM358 Op Amp

2x NE555P Timer

1x 12mm Tactile Button

1x Active Buzzer

1x 10MM LED (feel free to pick a different color)

1x USB A Cable (Any basic USB A cable will work, we'll be cutting whatever other end it has on it off, so it will become a USB A to bare wires cable)

The only other part needed for this project is the custom PCB that all of the components are mounted onto. I can't host the zip folder of Gerber files here, so head over to my PCBWay Shared Project where you can download the Gerbers or just buy them directly from the shared project page. Note that if you purchase them directly through the page, I will get a small cut of the purchase to support my channel and also note that PCBWay is my YouTube sponsor.

Recommended board settings (most of these should already be the defaults):

1. Board type: Single pieces
2. Different design in panel: 1
3. Size: Should be automatically detected
4. Quantity: Up to you
5. Layers: 2 layers
6. Material: FR-4
7. FR4-TG: TG 150-160
8. Thickness: 1.6
9. Min Track/Spacing: 6/6mil
10. Min Hole Size: 0.3mm
11. Solder Mask: Up to you, I chose purple because it looks really cool
12. Silkscreen: Whatever has good contrast with the solder mask color of your choice
13. UV Printing Multicolor: None
14. Edge Connector: No
15. Surface Finish: Either HASL with Lead or HASL Lead Free - Note that the leaded version, while containing lead, will only contain a very small amount. This setting basically determines the kind of tinning that is done on the solder pads of the PCB from the factory, so a very small surface area overall. If you're concerned about the lead, the lead-free option is available, while costing more.
16. Via Process: Tenting vias
17. Finished Copper: 1 oz Cu

Solder all of the components to the PCB! It should be relatively self-explanatory, just ensure that when installing components like diodes and supercapacitors, that you pay attention to the markings on the board that specify which orientation they go in. Then, be similarly careful with the IC's and the transistors/MOSFETs, but other than that, the assembly of the PCB should be relatively simple. Provided is the schematic so that you can put the right value component in the right place.

The only note that should be needed apart from that is that the square solder pad is the LED's negative terminal. Have fun soldering!

Take the USB cable that you have set aside for this project and chop off the end of it that isn't a USB-A connector. Ideally, use a USB 2.0 cable for this purpose, as it will have fewer conductors inside it to differentiate between and it's likely to also not have any shielding that can get in the way.

After cutting off the end, strip the outer insulation off, and you should be left with four separate conductors: One red, one black, and two others, likely some combination of blue/white/green. The red and black cables are +5V and GND respectively, and the two others should be the D+ and D- connections for USB 2.0. We don't need those cables, so cut them off so that we've only got the red and black conductors left. Then, solder the red wire to the USB power input positive terminal on the PCB, and the black to the negative terminal. Then, I'd recommend using some hot glue around where the wires get soldered to the board for some strain relief, and we're done with all of the soldering for this project!

Yep, we're already at the place where it can be installed into the car!

To install it in your car, I would recommend using something like a command strip or some regular double-sided tape to tape the PCB somewhere on your dash. But be careful not to put it somewhere where there's an airbag for safety reasons. Definitely secure it somewhere, though, so that if you stop suddenly, it doesn't go flying around your car.

Then, plug it into a USB port that turns on when your car turns on, and is off when your car is off - you wouldn't want this thing constantly powered and draining your battery. Most cigarette lighter ports should be toggled with the car's ignition, and therefore any USB adapters you put in there should be as well. If your car has built in USB ports, those should also be perfect, just ensure that they're also toggled with the car's ignition.

That's it, you've done it!

Go ahead and go for some short drive/errand, and when you turn your car off, make sure that the device goes off, reminding you to lock your car.

If it doesn't go off, here are some troubleshooting steps:

Is the USB port toggled with your car's ignition? If it's not, it won't go dead when the car is turned off, and the device will never trigger. You can test this by unplugging the device from the USB port; if it goes off when unplugged, the port isn't toggled, and if it still doesn't go off when unplugged, there's either an issue on the PCB or the USB port hasn't ever provided power to the PCB to charge its supercapacitors. You can determine if it's the USB port by plugging another device (i.e. a phone) into the port and seeing if it starts charging or not.

If you rule it out to be a PCB error, check all of your solder joints and component placements. If that fails, ensure that you don't have a faulty component, which may require probing with a multimeter and swapping out components.