A battery-powered fire alarm including a smoke detector circuit, a controllable horn circuit and a battery monitoring circuit. The smoke detector employs a pair of complementary field-effect transistor switches with gates respectively connected to an ionization chamber and a potentiometer of a Wheatstone bridge circuit connected across the battery. The field-effect transistors are biased off to minimize standby power consumption and are connected such that the threshold voltages thereof are offsetting to minimize supply voltage sensitivity of the detector. When the voltage from the ionization chamber assumes a value approximately equal to a preselected alarm voltage at the potentiometer, both field-effect transistors turn on to energize an alarm circuit to sound an alarm. Hysteresis circuitry is provided to ensure that the complementary switches, once turned on, will not turn off and thereby terminate the alarm until after the alarm condition has terminated. In one embodiment, a feedback signal causes the preselected alarm voltage to increase in response to alarm activation. In other embodiment, hysteresis is achieved by means of a feedback signal proportionately lowering the sensing voltage from the ionization chamber in response to alarm activation. A test switch and associated circuitry is also provided to manually lower the sensing voltage below the preselected alarm voltage to test the detection circuitry in addition to the battery and alarm horn.

Improvements in battery operated smoke detector electronics including micro power astable multi-vibrator circuitry which has fast reliable switching of the inverters and electronic circuitry for single or dual ionization chamber smoke detector which maintains constant smoke detector sensitivity throughout the usable life of the battery.

A water detector alarm is disclosed. The device includes a power supply, an alarm unit, moisture detecting electrodes and circuitry for activating the alarm unit in response thereto, and a power monitoring unit for monitoring the power supply and activating the alarm unit when the power supply grows weak, with a frequency noticeably distinct from the water presence alarm frequency.

In an electronic horn for alarm systems of the type in which an alarm condition is indicated by a reversal of polarity in the d.c. power supply, an improvement provides a pair of terminals for connection to said d.c. power supply, a series circuit including a current limiting element and a capacitor connected across said terminals, a speaker operable to produce sound in response to current flow through the speaker, the speaker being constructed so that it will reverberate for a period after the current flow through the speaker has stopped and switch means for connecting the speaker in parallel to said capacitor for the discharge of energy stored in the capacitor through the speaker when the switch means is in its conductive state with means for continuously cycling the switch means between its conductive and non-conductive states at a predetermined frequency and for a predetermined duration of "on" time for each switch cycle such that the sound produced by the pulses combines with that produced by reverberation of the speaker to provide a continuous tone as long as said pair of terminals is connected to the power source in the polarity indicative of an alarm condition.

An ionization fire-signal device with an ionized measuring chamber, a reference element connected in series with the reference chamber, an amplifier stage controlled by the electric potential of the connection point of the measuring chamber and the reference element and comprising a field effect transistor, and a signal circuit controlled by the amplifier stage and comprising another transistor. A bistable behavior of the ionization fire-signal device is achieved by a positive signal feedback from the signal circuit to the substrate of the field effect transistor.