What happens when only a few pins of 555 is used? Still we can make circuits. Here ,is a simple Fire alarm based on IC555. The trigger pin2 is used to achieve the result in addition the VCC and Ground pins. In this design, the IC555 act like a switch to give high output. When the temperature in the Room or a device exceeds the set limit, alarm sounds.
The output of IC 555 goes high when its Trigger pin 2 is below 1/3 Vcc. An NTC Thermister is used to set this voltage. Resistance of NTC Thermister reduces when the temperature increases. So let us see what happens. VR1 is adjusted to give more than 1/3 Vcc to the trigger pin2. NTC Thermister also has resistance of 4.7K at normal temperature. So the output of IC remains low. When the temperature increases, resistance of Thermister drops, the trigger pin goes below 1/3 Vcc , output of IC turns high and buzzer beeps.
Want more? See how IC 555 Works