Whistling Kettle

Most electric kettles do
not produce a whistle and just switch off when they have boiled.
Fitting a box of electronics directly onto an electric kettle (or even
inside!) to detect when the kettle has boiled is obviously out of the
question. The circuit shown here detects when the kettle switches off,
which virtually all kettles do when the water has boiled. In this way,
the electronics can be housed in a separate box so that no modification
is required to the kettle. The box is preferably a type incorporating a
mains plug and socket. In this application, the current flowing in coil
L1 provides a magnetic field that actuates reed switch S1. Since the
current drawn by the kettle element is relatively large (typically 6 to 8
amps), the coil may consist of a few turns of wire around the reed

The reed switch is so fast it will actually follow the AC current
flow through L1 and produce a 100-Hz buzz. The switching circuit driven
by the reed switch must, therefore, disregard these short periods when
the contacts open, and respond only when they remain open for a
relatively long period when the kettle has switched off. The circuit is
based on a simple voltage controlled oscillator formed around T2 and T3.
Its operation is best understood by considering the circuit with
junction R4/R5 at 0 V and C4 discharged. T2 will receive base current
through R5 and turn on, causing T3 to turn on as well. The falling
collector voltage of T3 is transmitted to the base of T2 by C4 causing
this transistor to conduct harder.

Since the action is regenerative, both transistors will turn on
quickly and conduct heavily. C4 will therefore charge quickly through
T2’s base-emitter junction and T3. Once the voltage across C4 exceeds
about 8.5 V (leaving less than 0.5 V across T2’s b-e junction), T2 will
begin to turn off. This action is also regenerative so that soon both
transistors are switched off and the collector voltage of T3 rises
rapidly to +9 V. With C4 still charged to 8.5 V, the base of T2 will
rise to about 17.5 V holding T2 (and thus T3) off. C4 will now discharge
relatively slowly via R5 until T2 again begins to conduct whereupon the
cycle will repeat. The voltage at the collector of T3 will therefore be
a series of short negative going pulses whose basic frequency will
depend on the value of C4 and R5.

The pulses will be reproduced in the piezo sounder as a tone. The
oscillation frequency of the regenerative circuit is heavily dependent
on the voltage at junction R4/R5. As this voltage increases, the
frequency will fall until a point is reached when the oscillation stops
altogether. With this in mind, the operation of the circuit around T1
can be considered. In the standby condition, when the kettle is off, S1
will be open so that C1 and C2 will be discharged and T1 will remain off
so that the circuit will draw no current. When the kettle is switched
on, S1 is closed, causing C1 and C2 to be discharged and T1 will remain
off. C3 will remain discharged so that T2 and T3 will be off and only a
small current will be drawn by R1.

Circuit diagram:

Whistling Kettle Circuit

Whistling Kettle Circuit Diagram

Although S1 will open periodically (at 100 Hz), the time constant of
R1/C1 is such that C1 will have essentially no voltage on it as the S1
contacts continue to close. When the kettle switches off, S1 will be
permanently open and C1/C2 will begin to charge via R1, causing T1 to
switch on. C3 will then begin to charge via R4 and the falling voltage
at junction R4/R5 will cause T2/T3 to start oscillating with a rising
frequency. However, once T1 has switched off, C3 will no longer be
charged via R4 and will begin to discharge via R3 and R5 causing the
voltage at R4/R5 to rise again. The result is a falling frequency until
the oscillator switches off, returning the circuit to its original

As well as reducing the current drawn by the circuit to zero, this
mimics the action of a conventional whistling kettle, where the
frequency rises as more steam is produced and then falls when it is
taken off the boil. The circuit is powered directly by the mains using a
‘lossless’ capacitive mains dropper, C6, and zener a diode, D2, to
provide a nominal 8 V dc supply for the circuit. A 1-inch reed switch
used in the prototype required about 9 turns of wire to operate with a
2-kW kettle element. Larger switches or lower current may require more
turns. In general, the more turns you can fit on the reed switch, the
better, but do remember that the wire has to be thick enough to carry
the current. It is strongly recommended to test the circuit using a
9-volt battery instead of the mains-derived supply voltage shown in the
circuit diagram. A magnet may be used to operate S1 and so simulate the
switching of the kettle.

This circuit is connected directly to the 230-V mains and none of the
components must be touched when the circuit is in use. The circuit must
be housed in an approved ABS case and carry
the earth connection to the load as indicated. Connections and solder
joints to components with a voltage greater than 200 volts across them
(ac or dc) must have an insulating clearance of least 6 mm. An X2 class
capacitor must be used in position C6.

Author: Bart Trepak – Copyright: Elektor 2004

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