Schematic Reference

Temporarily Silencing A Smoke Detector

It provides a means of temporarily silencing a battery-powered smoke detector after you’ve burnt the toast, scorched the baked beans – or whatever! Unlike the earlier design, this more sophisticated version does not cause strange chirps and whistles to emanate from the smoke detector towards the end of the silenced period. It also flashes a LED and produces a series of short, unobtrusive tones from its inbuilt buzzer while it is active. A separate 9V (or 6V) battery is required to power the circuit, which is mounted remotely from the smoke alarm. Connection to the alarm is made via a 3-core data cable terminated in a 3.5mm stereo plug, while a matching switched socket is fitted to the alarm’s casing. In addition to the socket, only three other components are installed inside the smoke alarm.

These are Mosfet Q3, its 100W gate resistor and 15V zener diode ZD1. These parts can all be mounted on a small section of prototyping board or soldered point to point from the socket terminals. The Mosfet is wired in series with the smoke alarm’s negative battery lead and acts as a switch. As shown, the contacts of the socket must be wired so that the Mosfet drain-source connections are shorted out when the plug is removed, thus allowing immediate restoration of the smoke alarm to normal operation. When the silencer circuit is inactive, the reed relay (RLY1) is off, so battery power is disconnected from the circuit. An exception to this is Q3’s 4.7kO gate pull-up resistor, which is powered directly from the battery. This holds the Mosfet switch on, powering the smoke alarm from its on-board 9V battery.

Now consider what happens when the "silence" switch (S1) is pressed. This action applies battery power to the entire circuit through the switch contacts. At the same time, IC1 (which is wired as a monostable) is triggered by a brief pulse on its reset input (pin 2). This initiates the 555’s timing sequence, so its output (pin 3) immediately swings high, switching on Q1 and activating the relay. A second transistor (Q2) wired to IC1’s output also conducts, pulling Q3’s gate low and switching it off. As a result, the smoke alarm is disconnected from its 9V battery and all of the noise ceases instantly! When the relay is closed, an additional path exists from battery positive to the circuit’s power rail – so that when the switch is released, the circuit keeps running.

The circuit then continues to run for the duration of IC1’s timing period (over 8 minutes). The remaining two 555 timers (IC2 & IC3) are configured as astable multivibrators. IC2 is used exclusively to flash an indicator LED at a rate of about once per second. IC3 has a longer period, sounding a piezo buzzer briefly about once every 10.5 seconds. Use a 5V reed relay when the circuit is powered from a 6V battery and a 12V version when powered from 9V. Because of the high impedance and low leakage of the Mosfet’s gate, the silencer’s battery can be expected to last almost its shelf life – assuming that you don’t burn the toast too often!

Warning:

1. This circuit must not be used with mains-connected smoke detectors
2. Test your smoke detector and this silencer circuit regularly.