The figure 1 shows a very simple timer that will activate the relay for around 45 seconds after pressing the push button S1. The time delay of the circuit can be increase or decrease by increasing or decreasing the value of 470uF capacitor. For example a 1000uF capacitor can give around 2.5 minutes time delay.
For further increasing the time delay with less components can be done by using another 2N3904 transistor as a darlington pair as shown in the figure 2. By using a darlington pair the time delay can be increased to almost double. For example a 1000uF capacitor can give around 5 minutes time delay.
You can also use a single BC517 NPN darlington transistor in the place of two 2N3904 transistors as shown in the figure 3.
The operating voltage of the circuit is 12 volt but it can also be operated with 6 or 9 volts. Make sure to use the same voltage relay also.
A simple timer circuit can be built by using only a single or two transistors. The figures below show different schematics of simple timer circuits, which can be built very easily with few general components. These circuits can be used for variety of timing purposes in electronic projects like time delay relay etc.