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By NICHOLAS VINEN
Timer for
Fans or Lights
This simple circuit provides a turn-off delay for a 230VAC light or
a fan. It can be used to make a bathroom fan run for a set period
after the switch has been turned off or it can be used with a
pushbutton to turn a light on for a specific time. The timer circuit
consumes no standby power when the load is off.
B
ATHROOMS AND toilets need an
exhaust fan to vent humid air or
odours outside. It’s a good idea to have
the fan running while you shower and
then for a little while afterwards, to
prevent condensation and mould. This
unit makes it easy, by automatically
running the fan for a preset period
after the wall switch has been turned
off and then switching itself off.
And while this timer was designed
specifically with bathroom or toilet
fans in mind, it is equally applicable to
exhaust fans in kitchens where cooking odours need to vented outside. Of
course, cooking also produces large
amounts of water vapour so a fan is
64 Silicon Chip
desirable to avoid condensation on
the walls which can lead to mould.
It has other applications too. For
example, many apartment buildings
have lights in the foyer or stairwell
with pushbuttons to turn them on.
This allows people on any level to
turn the lights on for long enough to
get into or out of the building without
the possibility of them being left on for
long periods. This unit can perform
that task too, when combined with
mains-rated momentary pushbuttons
or spring-loaded switches.
Or do you forget to turn off outdoor
lights after visitors have departed?
This timer will avoid that problem.
You can easily set the time-out from
five seconds to one hour by changing an on-board link and possibly a
capacitor. The whole thing fits in a
standard junction box (Arlec 9071 or
equivalent) for ease of installation.
And as noted above, it has no standby
power so it’s quite “green” (well, the
PCB is anyway).
Commercial units to do these jobs
are available but can be hard to get
and expensive. This SILICON CHIP
design has relatively few parts and it
can handle loads of up to 5A/1250VA.
Improvements
We published a similar mains timer
siliconchip.com.au
SWITCH
A
A
A
Aperm
SWITCH
Asw
A
Aload
FAN
TIMER
Asw
A
FAN
N
N
Aperm
SWITCH
N
LAMP
FAN
FAN
TIMER
Aload
A
FAN
N
N
N
N
N
(a)
(b)
(c)
Fig.1(a) at left shows how a fan (or light) is normally connected, while Fig.1(b) shows how the wiring is changed so the
Timer controls the fan, in conjunction with the switch. Fig.1(c) shows the circuit with the ceiling light in place, where
the light and the fan share a common switch.
for fans in the October 2005 issue.
That design used a PIC16F88 microcontroller and a Triac to control the
fan. While it had some fancy features,
its standby power was several watts
which is something of a drawback
these days. As well, its use of a Triac
makes it incompatible with some compact fluorescent lamps (CFLs).
This new design dispenses with the
need for a micro, so there is no need
for programming. Instead, it is based
on a cheap and commonly available
CMOS oscillator/counter IC. There is
no Triac either, as the load switching
is done by a mains-rated relay.
Connections
The Mains Timer is designed to be
added to an existing fan or light installation with minimal fuss. Fig.1(a)
shows how a typical fan is wired up
(this also applies to lights). The 3-core
mains cable is normally run in the ceiling cavity with the Active line splitting
out to run down to the power switch,
mounted on the architrave or wall
below. The switched Active line then
runs back up into the ceiling to connect to the fan. Neutral and possibly
Earth are permanently connected to it.
Fig.1(b) shows how the Mains Timer
would be connected into the circuit.
As before, Neutral and Earth wires
run straight to the fan. The timer is
connected in-line with the switched
Active, with the wire from the switch
going to its “Asw” terminal and the
wire to the fan coming from its “Aload”
terminal.
Two additional wires, “Aperm”
and “N”, are run back to the Active
and Neutral supply. For the sake of
convenience, the junction of the two
Neutral lines may be made on the
siliconchip.com.au
Mains Timer PCB if desired.
The additional Active line (Aperm)
is necessary to power the fan or light
after the mains switch has been turned
off. The Neutral line is used to power
the timer circuitry without affecting
the voltage delivered to the load.
Finally, Fig.1(c) shows how you can
add an exhaust fan to a room which
only has a light switch, using the one
switch to turn on the both the fan and
the light. When the switch is turned
off, the light goes off immediately but
the fan runs for the preset time before
it too goes off. All you have to do is
run the switched Active line from the
light to the Asw terminal on the timer
and then run the Active, Neutral and
load (fan) wires as shown.
Circuit description
Fig.2 is a simplified diagram of the
S1
(OFF
BOARD)
K
Timer, showing how its power supply
works. This configuration allows it
to have zero standby power. Initially,
the mains switch (S1) is off and so is
D1
D3
A
10M 1W
K
K
D5
A
A
K
K
D4
330nF X2
K
K
A
NEUTRAL
This circuit is directly connected
to the mains and all parts operate
at 230VAC. As such, contact with
ANY part of the circuit could be fatal!
DO NOT operate this circuit unless
it is fully enclosed in the specified
junction box and DO NOT touch any
part of the circuit while it is connected to the mains.
Note that, in most states, a licensed electrician must be used to
connect this unit to fixed electrical
wiring.
RELAY 1
MAINS
ACTIVE
ACTIVE
TO LOAD
Warning!
ZD1
24V
220F
TIMER
(IC1)
A
A
A
D6
D2
470 1W
Fig.2: simplified circuit of the timer power supply. The mains is rectified
by diodes D3-D6, filtered with a 220µF capacitor and regulated by 24V
zener diode ZD1. A 220nF X2 capacitor in the Neutral leg limits the mains
current. Diodes D1-D2 work in conjunction with Relay1 to supply power for
the circuit after switch S1 is turned off, until the off-timer period expires.
August 2012 65
+24V
+24V
INSTALL
ONE LINK
ONLY
16
Vdd
100nF*
9
Ctc
O14
O13
O12
1M
10
O10
Rtc
3.3M
11
12
LK1
2
LK2
1
LK3
15
LK4
3.3M
20min
RS
O6
O5
Vss
8
O4
B
Q1
BC557
Q2
BC557
K
C
D5
C
5min
K
K
A
A
D1
D3
A
RELAY1
K
1min
D7
CON1
A
6
A
4
D9
5
7
K
K
1
A
2
K
470
A
+12V
22nF
X2
22k
220F
35V
ZD1
24V
1W
K
K
330nF
X2
A
A
K
10M 1W
D2
D6
SC
2012
* VALUE MAY BE CHANGED FOR
DIFFERENT DELAYS (SEE TEXT)
D4
A
BC557
B
D1-D8: 1N4004
A
MAINS TIMER FOR LIGHTS & FANS
K
A
K
N
K
A
D9: 1N4148
Asw
4
1W
0V
WARNING: ALL PARTS AND WIRING
IN THIS CIRCUIT MAY BE AT 230V AC
POTENTIAL WHEN OPERATING.
CONTACT COULD BE FATAL!
Aload
3
D8
1nF
Aperm
0V
ZD2
12V
1W
220nF
10k
100k
E
10min
13
E
B
O9
IC1
4060B O8 14
O7
MR
3
1M
E
C
ZD1, ZD2
A
K
Fig.3: the full circuit of the Mains Timer. IC1 is a 4060 CMOS oscillator/counter which provides the time delay. It
controls Relay1 to switch power to the load as well as the circuit’s power supply (refer to Fig.2). Diode D8 allows the
timer to sense when the mains switch is turned off; while it is on, the timer is held in reset as IC1’s MR input (master
reset, pin 12) is held high.
Relay1. So the circuit has no Active
connection until switch S1 is closed.
When S1 is closed, the mains voltage is applied across the bridge rectifier formed by diodes D3-D6. The output
is limited to 24V DC by zener diode
ZD1 and filtered by a 220µF capacitor.
The mains input current is limited in
the Neutral leg by a series 330nF X2
capacitor with a parallel 10MΩ bleed
resistor and a series 470Ω resistor for
in-rush current limiting.
Once the 220µF capacitor has
charged up, the timer circuitry energises the coil of Relay1 and its contacts
switch over. The incoming mains Active is then connected to the junction
of diodes D1 and D2 via the relay and
these are effectively in parallel with
diodes D3 and D4 in the bridge rectifier. But when switch S1 is turned off,
the circuit remains powered, via the
relay contacts and the bridge rectifier
formed by D1, D2, D5 & D6.
The circuit remains powered until
the timer runs its course, at which
point Relay1 is switched off and the
24V supply collapses, bringing it back
66 Silicon Chip
to the initial state, where it is not consuming any power.
Note that the circuit is powered
directly from the 230VAC mains and
floats at or near mains Active potential
so it must be considered as hazardous
(lethal) once it has been connected.
Also note that zener diode ZD1
dissipates little power as the 330nF
X2 capacitor value has been chosen
to limit the mains current to a value
very close to that drawn by the relay.
Details
Now take a look at Fig.3 which
shows the full circuit diagram. Besides showing the details of the timing
circuitry (at left), this also reveals an
additional diode (D8) which is connected to mains Active via switch
S1 (off board). This diode allows the
timer to sense when S1 is turned off
and this is the reason we didn’t simply
arrange for Relay1’s contacts to short
out the switch when it turns on. If we
had, there would have been no way to
sense when S1 is switched off.
While switch S1 is on, D8 is for-
ward-biased and so at the peak of each
mains cycle, current can flow through
it and its series 10kΩ current-limiting
resistor to charge the 1nF capacitor
between the MR (master reset, pin
12) and Vss (negative supply, pin 8)
terminals of timer IC1. While S1 is
on, MR is kept high and this holds the
timer in its reset state, with its oscillator inhibited and its 13-bit counter
reset to zero. While the counter is zero,
all its outputs (O4-O10 and O12-O14)
remain low.
Depending on how the timer is
configured, one of the four outputs
O10 or O12-O14 is connected to the
base of PNP transistor Q1 via a 3.3MΩ
resistor. That output being low, it sinks
current from the base of Q1, turning it
on. It in turn drives PNP transistor Q2,
which energises Relay1’s coil, turning
it on. One of its set of contacts supplies
mains power to the load and the other
connects the mains to this circuit, as
described earlier.
Note that Q1 and Q2 are in a PNP
Darlington configuration. The 1MΩ
resistor between Q2’s base and emitter
siliconchip.com.au
shunts any leakage current from Q1,
preventing a false turn-on.
When switch S1 is turned off,
current can no longer flow through
D8 and so the 1nF capacitor is discharged by its parallel 100kΩ bleeder
resistor. The 22nF X2 capacitor at the
anode of D8 is necessary to suppress
capacitively-coupled electrical noise
and leakage current through S1 from
keeping the MR pin high even when
S1 is off. When MR goes low, IC1’s
internal oscillator starts running and
incrementing the counter.
Oscillator frequency
The oscillator’s frequency is set by
the combination of the 100nF capacitor and 1MΩ resistor between pins 9
& 10 of IC1. The formula in the 4060
data sheet gives us 4Hz for these values
but we measured 7Hz on two different
prototypes so we use this measured
value and assume that the formula
must be inaccurate when such a high
resistor value is used (even though it
is within the specified range). So IC1’s
internal counter is incremented seven
times per second.
The 3.3MΩ resistor minimises frequency variation with supply voltage
by isolating the input capacitance of
pin 11.
IC1’s O10 output goes high after
512 (29) oscillations or 512 ÷ 7Hz =
73 seconds. Similarly, the O12 output
goes high after 5 minutes, O13 after 10
minutes and O14 after 20 minutes. So
depending on which of links LK1-LK4
is installed, after the selected delay, Q1
and Q2 switch off. This de-energises
the coil of Relay1 and diode D7 absorbs
the resulting back-EMF.
This cuts power to the load and the
Timer also powers down as the 220µF
capacitor discharges. If the mains
switch is turned back on before the
time-out (ie, while the load is still energised), the MR input of IC1 is pulled
high and so the timer is reset. When
the switch is turned off, the timer again
starts counting from zero.
We need 24V to drive the relay coil
but IC1 has a maximum rating of 15V
so the supply from the 220µF capacitor
is fed via a 22kΩ resistor to the 12V
zener diode, ZD2. This limits the supply for IC2 to +12V.
So while it might not be immediately apparent from the circuit of
Fig.3, the Mains Timer has two supply
rails: +24V and +12V. Note, however,
that IC1 (4060B) is actually connected
siliconchip.com.au
between the +24V and +12V rails.
Don’t be fooled by those low DC
voltages though – as stated, this whole
circuit “floats” at mains potential
(230V AC) and is potentially lethal.
The 220nF capacitor and 22kΩ
resistor also form a low-pass RC filter
to remove much of the 100Hz ripple
from IC1’s supply.
You might be wondering about the
purpose of diode D9. It stops the timer
from running once the relay switches
off. Normally, this isn’t an issue since
the power supply then collapses. But
without D9, if the delay was set short
enough, it’s possible the relay could
come back on while the mains switch
remained off.
Other uses
Up to now we have been describing
how the timer circuit is used with a
standard wall switch and in that case,
the timer provides an off-delay, ie, the
load is powered while ever the switch
is on as well as for the preset period
after it is switched off.
But this is no good if you want to
use the Timer to prevent the load from
being accidentally left on after use,
which can be a concern for both fans
and lights. If that’s your aim, you simply need to change the mains switch
to either a momentary push-button or
a spring-loaded momentary switch.
These are available from electrical
suppliers in the standard Keystone format to clip into a wall-plate. They may
be sold as a bell-press button or similar.
Parts List
1 PCB, code 10108121, 60 x
76mm
1 4-way PCB-mount (screw fix)
terminal barrier (CON1) (Jaycar
HM3162, Altronics P2103)
1 5A 24V DC coil DPDT or
DPST relay (Altronics S4195D
or equivalent)
1 junction box (eg, Arlec 9071)
2 M3 x 15mm machine screws
and nuts
4 M3 shakeproof washers
4 No.4 x 9mm self-tapping screws
Semiconductors
1 4060B oscillator/counter (IC1)
2 BC557 PNP transistors (Q1,Q2)
1 24V 1W zener diode (ZD1)
1 12V 1W zener diode (ZD2)
8 1N4004 1A diodes (D1-D8)
1 1N4148 small signal diode (D9)
Capacitors
1 220µF 35V/50V electrolytic
1 330nF 250VAC X2 MKT/MKP
(Element14 Part No. 1215460;
Altronics Cat. R3129)
1 220nF MKT polyester
1 100nF MKT polyester (see
panel below)
1 22nF 250VAC X2 MKT/MKP
1 1nF MKT polyester
Resistors (1%, 0.25W unless stated)
1 10MΩ 1W 5% 1 22kΩ
2 3.3MΩ
1 10kΩ
2 1MΩ
1 470Ω 1W 5%
1 100kΩ
1 0Ω
Changing The Switch-Off Time Delay
Four time delay options are available by default: one minute, five minutes,
10 minutes and 20 minutes. These are selected by installing one of links LK1,
LK2, LK3 or LK4 respectively. If none of these suit, you can change the value
of the 100nF MKT capacitor to give other time delays as shown in Table 1
below. Simply select the appropriate value and then install the corresponding link
to give the desired delay.
Note that these times are approximate and can vary by about ±20%, due to
component tolerances and rounding errors.
Table 1: Setting The Timing
C1
330nF
220nF
150nF
100nF
22nF
15nF
4.7nF
LK1
1 hour
45 minutes
30 minutes
20 minutes
4 minutes
3 minutes
1 minute
LK2
30 minutes
20 minutes
15 minutes
10 minutes
2 minutes
1.5 minutes
30 seconds
LK3
15 minutes
10 minutes
7.5 minutes
5 minutes
1 minute
45 seconds
15 seconds
LK4
4 minutes
2.5 minutes
2 minutes
1 minute
15 seconds
10 seconds
5 seconds
August 2012 67
D6
N
4004
IC1 4060B
100nF*
1nF
(LK4)
3 .3M
100k
10k
(LK3)
LK1
+
12V
1M
3 .3M
D5
C 2012
10M 1W
470 1W
4004
MAINS
NEUTRAL
D8
35V
D9
Mains
Timer
4004
1W ZD2
D4
22k
4004
22nF
X2
N
220F
4004
24V
SW
D2
1W ZD1
L
LOAD
(FAN OR
LIGHT)
N
E
(if present)
D3
Asw
D1
4004
4004
A
220nF
1M
A
Aperm
Aload
MAINS
EARTH
Q1
BC557
4148
(S4195D)
SWITCH
BC557
Q2
RELAY1
(LK2)
MAINS
ACTIVE
D7
4004
remiT
1210108121
180101
330nF X2
WARNING: ALL PARTS ON THIS PCB OPERATE AT MAINS
POTENTIAL (230VAC). CONTACT COULD BE FATAL.
Fig.4: follow this layout and wiring diagram to assemble the timer board. Take
care with the orientation of the diodes, the 220µF capacitor and IC1. Note that
only one link (LK1-LK4) is installed, giving four time options (see text and panel
for details on selecting the appropriate link).
The completed PCB fits into a junction box.
It’s shown here mounted on the base.
So if you swap the switch over to a
momentary pushbutton and wire in
the timer as shown above, the load
will then turn on for the chosen period
when the button is pressed and then
automatically turn off again. The button can also be pressed any time the
load is on, to reset the timer and keep
it on for the preset period.
Construction
The Mains Timer is built on a PCB
68 Silicon Chip
coded 10108121 (60 x 76mm). This fits
in a standard junction box (eg, Arlec
9071). But note that not all junction
boxes are the same and you will need
to check that the one you are purchasing has mounting holes in the same
positions as those of the PCB.
The PCB is available from the
SILICON CHIP Partshop. While it is
notionally a single-sided design, we
have made it double-sided and added
parallel tracks on the top to improve
its mains current-carrying capability.
In the absence of a kit being available,
we recommend you build the timer using one of our boards since they have
a solder mask which greatly reduces
the chance of leakage paths developing
and causing flash-over.
Referring to the PCB overlay diagram (Fig.4), start by installing all the
small resistors. Use a DMM to check
each as you go, since the colour codes
can be hard to read accurately. The
0Ω resistor is used for one of LK1LK4 and you must only install one of
these. Refer to Table 1 and select your
desired time-out, then fit the link (0Ω
resistor or tinned copper wire) in the
appropriate position.
Follow with the diodes, being careful with the orientation, and make
sure the smaller 1N4148 diode goes
in the top-right corner. Note that the
orientation of diodes D1-D6 and D8
alternates as you go down the board.
Install the two zener diodes (ZD1 &
ZD2) also. These are in a larger glassencapsulated package and both are
orientated with the cathode stripe
towards the top of the PCB.
Solder IC1 in place next, with the
pin 1 notch or dot towards the top
of the board as shown. Follow with
the two 1W resistors – don’t get them
mixed up. You can then fit the smaller
MKT capacitors. The 100nF capacitor
can be a different value if you want to
adjust the timing – see Table 1.
Now install the two transistors,
bending their pins with a small pair of
pliers to fit the pads provided. The flat
faces are orientated as shown on the
overlay diagram. You can then solder
the electrolytic capacitor in place, with
the longer (+) lead towards the top of
the board.
Follow with the two X2 capacitors.
Note that the larger X2 capacitor can
have one of several lead pitches so
multiple pads have been provided to
suit them all; its left-most lead should
go in the left-most hole provided and
the other into the best fitting position.
After that, solder the relay in place.
The terminal barrier is attached to
the PCB using two 15mm M3 machine
screws with a star washer under each
screw head and nut. Check that the
connector is straight and do the screws
up tight before soldering the four pins.
Use a hot iron to ensure that the solder
joints form proper fillets.
Finally, attach the PCB to the junction box baseplate using four small
siliconchip.com.au
self-tapping screws and you are ready
to test it.
Testing
If you have a bench supply and
would like to test the PCB before
it is installed and connected to the
mains, you can do so. Connect a DC
supply, set to slightly less than 24V,
across ZD1, with the positive lead to
its cathode (striped end). The circuit
should draw about 30mA so if it draws
much more than this, switch off and
check for faults.
The relay may or may not switch
on initially; if it does not, apply 24V
to the SW terminal of CON1 and it
should turn on. After the delay you
have selected, it should turn off again.
Assuming it does, the unit is working
correctly and you can power it down.
Otherwise, carefully check the component orientation, component values
and solder joints.
Installation
Note that, in most states, this unit
should be connected to the house
wiring by a licensed electrician only.
Note also that all parts on this circuit
operate at mains potential (230VAC),
so do not touch any part of the circuit
when power is applied.
It’s a matter of following the wiring
diagram (Fig.4) to make the connections. You must switch off the circuit
before you start working on it and
check that it really is off before starting work.
Ensure that the junction box baseplate is securely anchored to a joist
or ceiling batten using the supplied
screws before doing the wiring. Note
that you will need to knock out one or
two panels in the junction box housing
to allow the wiring to pass through.
The mains cables must be clipped or
clamped to convenient beams or joists
once you have finished. This keeps the
ceiling space (or wherever the unit
SILICON
CHIP
Fig.5: front panel label
for the Mains Timer. Print
this out, laminate it and
glue it to the lid of the
junction box (eg, using
silicone sealant) for future
reference.
MAINS TIMER
MAINS
ACTIVE
SWITCH
Aperm
Aload
MAINS
EARTH
Asw
A
E
(if present)
N
LOAD
(FAN OR
LIGHT)
PC BOARD
N
MAINS
NEUTRAL
WARNING: ALL PARTS INSIDE OPERATE AT 230VAC.
DISCONNECT FROM MAINS BEFORE SERVICING.
is installed) neat and prevents wires
from being tripped over, accidentally
yanked, etc. It also makes it easier to
trace the wires to see where they go.
In some cases, you may wish to use
a single switch to control both a light
and a fan – see Fig.1(c) for wiring
details. Now, both the light and the
fan will come on when the switch is
turned on but when it is turned off,
the light will go off immediately while
the fan will continue to run for the
programmed period before turning off.
If the fan has an existing earth connection, this should be left intact. Fans
with a metal housing will tend to have
an earth wire while those with a plastic
housing may not. If the earth wire has
to be cut, it can be re-joined using a
double-screw BP connector.
Once everything is hooked up,
check that all the terminal barrier
screws are tight and there are no stray
strands of copper from any of the wires
that might short to something else.
You can then clip the terminal barrier
covering in place, fit the junction box
cover, turn the power circuit back on
and check that everything is working
as expected.
Fans with 3-pin plugs
Many existing ceiling fans and all
new fans these days come fitted with a
lead complete with 3-pin mains plug.
This simply plugs into an adjacent
mains socket in the roof space.
In that case, a better idea may be to
ditch the junction box and install the
Mains Timer PCB in an IP65 sealed
box. This can then be fitted with a
socket, so that the fan can be plugged
into it.
Short delay
Finally, note that in operation, you
may notice a short delay between flicking the switch and the load coming
on. This is usually only a couple of
hundred milliseconds and is due to the
power supply capacitors charging to
the relay’s operating voltage. It’s short
enough that it should not present a
problem, especially when used with
fans, which take some time to spin
SC
up anyway.
Table 3: Capacitor Codes
Value
330nF
220nF
100nF
22nF
1nF
µF Value IEC Code EIA Code
0.33µF 330n
334
0.22µF 220n
224
0.1µF
100n
104
.022µF 22n
223
.001µF 1n
102
Table 2: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
siliconchip.com.au
No.
1
2
2
1
1
1
1
1
Value
10MΩ
3.3MΩ
1MΩ
100kΩ
22kΩ
10kΩ
470Ω
0Ω
4-Band Code (1%)
brown black blue brown
orange orange green brown
brown black green brown
brown black yellow brown
red red orange brown
brown black orange brown
yellow violet brown brown
single black stripe
5-Band Code (1%)
brown black black green brown
orange orange black yellow brown
brown black black yellow brown
brown black black orange brown
red red black red brown
brown black black red brown
yellow violet black black brown
single black stripe
August 2012 69
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