The high-frequency effects of U

A combustion products detector of the ionization chamber type comprises a reference ionization chamber and a signal chamber arranged in series for connection across a direct current source such as a dry battery. An insulated gate field effect transistor is arranged with its drain electrode for connection to the high side of the source, and with its source electrode to the law side; its gate is connected to the common junction of the chambers; a high resistance is connected across the chambers in parallel. A control amplifier supplies the signal to a circuit for producing a preliminary alarm or proximity signal upon a predetermined increase in combustion product density in the signal chamber and a full alarm upon an additional increase. The alarm may be produced as a throbbing or undulating sound.