Step 1: Basic Modules

The basic modules of this project are:

LCD 7 segments modules TM1637
Relay module
Arduino Nano
18650 battery and DC to DC 5V USB (or other suitable 5V supply)

optional:

2 toggles switchs
blinking led circuit (with a 555 chip)
mp3 speaker

Step 2: Connections

Connect the LCD module to Arduino as follow:

Ground to Arduino Ground
D2 to LCD CLK
D3 to LCD DIO
VCC to Arduino 3.3V

Connect Relay module:

Ground to Ground
VCC to 5V
Relay signal to D5 of Arduino

Connect the optional blinking circuit to the ground and 5V

Step 3: The Code

The code is an evolution of a project of Alastair Aitchison

https://www.patreon.com/posts/everything-you-16313...

You need to add the TM1637 library, check the previous link for more details.

The original code is a countdown from 10000 to zero, I have modified in order to add the relay.

The simple code is a countdown from 90 seconds and at the end it switches on the relay.

The reverse code 90 is more complicated and is the one in the video: is a countdown from 90 seconds, at the beginning of the routine the arduino switches on the relay then counts to zero and finally switches off the relay.

The reverse code 183 is identical to reverse code 90 but the time is 3:03 minutes, the same of the mp3 song attached: the song at the end it has an explosion, you can start with the arduino at the same time.

I suggest to use code 183 to challenge someone with knowledge of arduino but the code 90 for a party,family event, etc.

You can change the time in the code here:

void countdown() {
for(int i=900; i>0; i--) {

900 correspond to 90 seconds (900 hundredths of a second)

Step 4: Pull the Yellow Cable

The timer is ON and the time is running out....you can pull only one yellow cable...chose wisely but fast.

This apply to Reverse code.

so if you pull the cable:

disconnect the connection from the relay ON position, if disconnected noting happens, and when the time runs out explodes.
the correct cable, disconnect the connection from the relay OFF position, if disconnected noting happens, and when the time runs does not explodes.
onlythe blinking circuit switches off but everything keeps going
positive supply of relay if disconnect explodes immediately
negative supply of relay if disconnect explodes immediately
ony the LCD switches off but everything keeps going