Before we start, a few words on what led me to make this instructable.

I live in Israel, it's a fairly hot country and June-August are a kind of an endless heatwave.
Every year, in Israel alone, approximately 10 babies die of Heatstroke as they are left stranded in the back seat of a car. Israel is a small country, just short of 8 million people. Yen in this case 1 in a million is way too much!

2 years ago i became a father, a responsible one, at least i do my best....
When my baby girl was just 4 month old i came home from a family visit. when i went home with my sleeping baby in one had and packs of whatever-you-take-with-you-when-you-have-a-bably in the other, opened the door, and my dog didn't jump at me. i forgot it in the car. it was a cool night, there was no danger. 3 min later the whole family, dog included, was safely home.
What i did learn is that this could happen to anyone, and you can never be too safe or too alert.

Wonder what happens when you leave an infant in the car, at any time, not just a summer day?
It's very simple: a horrible death, there is no easy way to say it.

As humans we get distracted, it's a fact, but it helps no one.
There are many ways to fight this, here is a link with some information, please consider looking into it.

This experience led me to think of a very simple and robust solution to the problem.
While most recommendations attempt to mitigate the forgetfulness with passive reminders, i think an active one is probably better.
I wanted to create a device that can be plugged into any vehicle without complex installation.
Nothing that requires a continuous power supply such as batteries or solar panel.
it had to address at least 2 senses, i chose the obvious sight and hearing, but feel free to modify for touch, smell or whatever gets your attention.

To build this you will need:

Basic soldering skills.
Basic stitching ability.
A soldering iron and some solder.
Jumper Wire - to connect things on the prototyping board.
Prototype Board - I find this one very convenient to work with.
10mF Capacitor - this will be the power supply.
20mF capacitor - this is another option.
Potentiometer - to control the capacitor charging time
Diod - to prevent power loss on the relay
Red LED - to install behind the car windows - this is optional and will not be shown in the instructable
Buzzer - to sound the alarm
Relay - To connect the buzzer when the car is off
Terminals - To easily attach the sensor and power cord
Concoctive Fiber - part of the pressure sensor.
Graphite sheet - part of the pressure sensor

Some final thoughts:

This is not an approved safety system, you should never put the safety of your children solely in the hands of a home made reminder system!
Use other ideas in combination with this one.
Not all cars power down as soon as you turn you key. if your car doesn't, this solution probably isn't for you!
I am planing to make a system that works in this situation, but it will take some time....
Test it before you use it, and test it every once in a while, just to be sure.

OK, lets get to it!

Step 1: How It Works

You don't have to read this step, if you don't want to know the inner work, just look at the scheme and wire it if you like.

The basic idea is to alert the driver about the presence of a child before leaving the car.
Since people tend to turn off the car when they leave, that's the input i want to use for the system.
The problem is, when the car is off there is no power.

Building a project to be placed in a car on a hot summer day, poses some interesting challenges.
One of these challenges is the power source.
You cannot use the cars battery - that will require some tricky installation.
You don't want to use rechargeable batteries - the heat will destroy them very quickly.
You shouldn't use alkaline batteries - those will run out very quickly.

My solution is to use a large capacitor - it's usable at fairly high temperatures and lasts for a very long time. the down side is the time it can run the system is very limited, but i believe it's enough.

A quick look at the wiring scheme shows all the vital parts for the system.

I'll explain how it all plays:

I planned the system to run on a 5V power supply, so you can plug it into any USB port in you car, or use a cellphone charger.
The Diod (D1) prevents wasting power on the relay when the power is off
The Capacitor (C1) will store some energy to power the system after the car is turned off.
The Relay (RL1) will be active as long as the car powering the system (thus disconnecting the buzzer disabling the sound until the car is powered off)
The Pressurepad (SW1) will be used to detect the baby and sound the alarm if he is in the car as it is turned off.
The Buzzer and LED (LED1) will be used to draw the drivers attention to the baby.
The Resistor (R1) will drop the 5V down to 3.3V for the LED to function properly

BabySafe.fzz
Download

Step 2: Assembly

In the attaches pictures you will see the parts you need and how to assemble them on a matrix.

i recommend doing this before soldering, to make sure everything is working properly, and most importantly, that you got the Relay terminals right, if you connect it as NO, it will buzz when the car is on, and stop when turned off, which is the opposite to our intention.

To help you follow the leads, the Blue leads are the positive side and the Green ones are the common ground.

you will notice 2 leads Yellow and Black, used to set the Relay active, those do not appear in the wiring scheme, but must be connected.

In addition there is a video to show how this works.

Step 3: Soldering

In the attached photos you will find the final product.
I made some effort to keep it small so i could fit it in a compact box under the car dashboard.

i am not very good at soldering, so i am sorry for the messy result.

As you can see in the video it is fully functional.

Step 4: The Pressure Sensor and Final Product

I am still waiting for materials to build the pressure sensor, but the summer is not waiting for anyone.

Take a look at this instructable on how to make a pressure sensor.

I will add my version as soon as i can.

https://www.instructables.com/id/Flexible-Fabric-P...

The sensor leads should be connected to the Green terminal.

The powerline (a cut USB wire in my case) should be connected to the Blue terminals.

The installation itself is very simple:

Please the pressure sensor under the safety seat fabric.
connect the USB cable to an available USB port in the car.

I will be happy to get comments and improvement recommendations!
Feel free to send me a message with your idea!

Have a good summer and STAY SAFE!

Step 5: 12V Modification

This step is one of the recommendations i revived, there is no need to do this if you don't want to modify the original project.

I didn't make one. so if you make one, please consider sharing your photos.

12V directly connected to the car battery:

Why do this?
- This option is great for some one with some car electricity knowledge
- It will remove the short power supply the capacitor allows and keep running as long as the car battery is working, which should be a very long time.
What you will need?
- Buzzer - this is a stronger 12V version
- Relay - 12V version
- Fuse - You should never plug into the car battery without a fuse
- Power adapter - For the 12v via switch power
- 470ohm resistor - assuming you use the 20ma led.

In the wiring scheme you will find:

adding a fuse
2 power sources
- 12V direct battery power - this will power the buzzer + LED
- 12V via car switch - this will control the Relay

The 12V direct battery power can be connected directly to the battery or to the car fuse box, removing the need for an external fuse.

The 12V via switch can be connected to the regular 12V socket

Schematic marked "3" (and properly drawn) shows another option, this will function exactly like the original one, removing the need for a relay and replacing the USB connector with the 12V Power adapter.

Step 6: Buzzer With Slow Discharge Modification

This step is one of the recommendations i revived, there is no need to do this if you don't want to modify the original project.
I didn't make one. so if you make one, please consider sharing your photos.

This version will allow you to control the buzzer volume, the lower it is the longer it will buzz.

To do this you will have to add a potentiometer before the buzzer, as shown in the scheme.
Try different levels to find the suitable sound level and length.

Step 7: Flash/Beep Modification

IC1A and IC1C are both used as inverters i.e. a logic “0” on the input, gives a logic “1” on the output
and v.v. A logic “0” is 0V and a logic “1”, depends on the supply voltage, here it's 12V.

IC1A is connected to the ACCessories switch on the ignition lock switch, and goes to 12V at the first position of the ignition switch and is kept there, until the key is removed, whether the motor is started/running or not. This line can also be tapped from e.g. the supply to the car stereo.

R2, which pulls the input to IC1A low is probably not needed, as the load from car stereo and other accessories will most likely be quite enough to hold it low, but if testing out of the car, it's needed and as it has no negative impact...

Next gate is IC1B, which (N)ANDs the input from IC1A and the Baby Seat Sensor (BSS). R1 is needed to pull pin 6 low, when the baby is absent. A baby in the seat makes this output go high (“1”) and when ACC goes low (“0”), pin 5 goes high as well, making the output of the gate (pin 4) goes low, saying “Remember The Rug Rat”.

This signal is inverted in IC1C , as we need a high (“1”) gating signal to IC1D.

IC1D is the core of an oscillator that oscillates (swings back and forth) between “0” and “1”. Only when both inputs (pins 12 and 13) of this gate is high, its output (pin 11) can go low, so it will be high until an alarm signal is issued. With a high output on pin 11, Q1 (the PNP transistor) will be “off” i.e. not conducting.

When pin 12 goes high (assuming that pin 13 is already high), the output of IC1D (pin 11) goes low and two things happen: Q1 conducts turning on the LED/buzzer combo and C1 (which was at “high” potential) starts to discharge through D2 and R4 into pin 11.

When the voltage on C1 reaches the lower threshold voltage of IC1D (very roughly 4V here) and pin 13 sees this as a “low”, shifting pin 11 high once more, shutting off Q1 and C1 now charges through D1 and R3 from pin 11 until the voltage on C1 reaches the upper threshold voltage (very roughly 8V here), which again makes pin 11 low and this back and forth goes on as long as pin 12 is high (i.e. alarm signal issued).

Q1 is thus driven on/off/on/off/etc. with a timing for the off-periods determined by R3 and C1 and the on-periods determined by R4 and C1. Both R3 and R4 could be variable resistors, but in an automotive environment, one should never use trimmers unless absolutely necessary – much better to determine the value of the resistors and be done with it. Suitably timing would be from 1 to maybe 5 flashes/beeps a second.

With the values chosen (33k and 18k) T ON is ~230ms and T OFF is 125ms~, giving a period of ~355ms, which equates to ~2.8 flashes/s
Approximate formula for T ON = 0.7 x R4 x C1 and T OFF = 0.7 x R3 x C1

Example: 47k chosen from R4 T ON = 0.7 x 47000 Ohm x 0.00001 Farad = 329 ms

A big thanks to Omnivent for this design, explanation and other suggestions/improvements to the project!