Simple timer

Sometime is not necessary to go sophisticated for some applications but at the same time, "easy is not always easy".

Here an interesting problem.
An old puppet contained a timer to turn on a LED for few minutes.
The goal is to replace the broken circuit.

The original circuit was presumably just a timer.
I said presumably because it was based on a chip glued directly on the PCB, so is impossible to determine the type of circuit.

Anyway, at this point the choice was to use a small MCU or a traditional circuit.

Advantages of MCU :

• precise timing
• easy to set up
• expandable

Cons :

• overheading
• extra cost
• take time to develop 

So I opted for a more easy and traditional approach : a transistor and capacitor.

The circuit is basic and simple, let see it 

The circuit is quite simple as said.
When the pushbutton is pressed, the capacitor C1 is charged and thus Q1 start to conduce as well as Q2, turning ON the LED.

Then when the pushbutton is removed the C1 start to discharge thru R1 and Q1.
When the threshold is reached between R2 and R3, Q2 can't conduce anymore and the LED turn OFF.

With the value used the LED remains lit for about 30 seconds then start to dim quickly and turn it OFF after ~35 seconds.

Also in this case a quick PCB was prepared