To make LED works, you need a voltage source that is higher than the LED’s forward bias voltage, and it’s higher that 1.5V (around 2V for red LEDs). Fortunately, now you can make a LED flasher circuit that needs only 1.5V or a single cell battery to work. Here is the schematic diagram of the circuit:
The core of this LED flasher scircuit is a capacitor charge pump, to make a higher voltage.  You can see the final transistor driver is connected to a 330 ohm resistor. When this transistor is switched off, the 220uF capacitor is charged to 1.5V via two 330 ohm resistor. After the capacitor is full of charges, then when the transistor is switched on, the voltage acrros the LED is the supply voltage plus the capacitor voltage, or about 3V, and now the current will flow from the supply through the LED, capacitor, and transistor.