It is nice to hear from you. Now, let me guide you how to create 1-sec interrupt using TC1 of the MCU of UNO Board. I assume that your external interrupt works and your L at DPin-13 (built-in LED of UNO) is ON. Now the MCU has been waiting for Timer1 overflow interrupt.
Working Principle: TC1 (a 16-bit Timer/Counter) is configured to generate an interrupt request signal when there occurs an overflow (roll-over). Roll-over event occurs when the TC1 reaches to its full count (all 1s) and then the arrival of the next pulse causes it counts to be all 0s. TC1 will be counting the incoming clock pulses (clkTC1) (statrting from a pre-set value) for 1-sec time. After counting these pulses, the TC1 will encounter roll-over. As a result, the 'Timer-1 Overflow' interrupt will occur; the MCU will enter into ISRTOV1 (Interrupt Sub Routine due to TC1 Overflow), and it will turn OFF the Led.
1. Let us drive the TC1 (16-bit) by clkTC1 = clkSYS/1024 = 16MHZ/1024 = 15625 Hz
2. Compute pre-set count ffor TCNT1 of TC1 for 1-sec time delay. It is:
Overflow Count - counts for 1 sec
==> 0x10000 - 15625 = 0xC2F7.
3. Keep TC1 OFF; pre-load 0xC2F7 into TCNT1.
4. Prepare ISRTOV1 -- the Interrupt Sub Routine.
5. Enable TC1 Overflow interrupt logic.
6. Bring TC1 into RUN (ON) condition.
7. Convert the above Text Codes (consult data sheets for TC1 Registers) into Programming Codes and post here. I will check for you.
8. Also, post the codes of your external interrupt process.