Fire alarm with ionisation chamber - has one threshold level for preliminary warning and other for full scale annunciation

摘要

The fire alarm circuit incorporates one or more ionisation chambers whose resistances will increase above the normal value in the event of a fire. There is a first threshold which will trigger a preliminary warning when this level is exceeded by the signal. The second threshold value will initiate a second type of alarm annunciation. The circuit uses a field effect transistor at the input which will transmit a drop in input voltage to a bipolar transistor for the first warning. This arm of the circuit incorporates a light emitting diode giving a continuous warning. A further increase in transistor current will then trigger an intermittent alarm.