Wiper pulser
for rear windows
Does your car or van have a wiper attached
to the rear windscreen? If so, this device will
be useful. It allows you to push a switch and
have the wiper automatically operate for one
or two wipes before parking itself off.
By JOHN CLARKE
Rear screen wipers are a good idea
for vehicles which do not have a protruding boot to protect the rear windscreen from road grime when the
roads are wet. Vehicles such as hatchbacks, station wagons and vans all
suffer from road grime on the rear
windscreen and a wiper is virtually a
necessity for maintaining clear vision.
However, many rear screen wiper
controls have only a single on/off
switch and this can be a nuisance in
light drizzle conditions. Essentially,
you've got two choices: either you
leave the wiper running continuously
whether it is needed or not, or you
manually switch the wipers on and
off to clear the windscreen as required. The first option causes excessive wear of the wiper blade, while
the second is distracting for the driver.
This simple wiper controller solves
both those problems. To activate the
wipers, all the driver has to do is
press a pushbutton switch. The controller then takes over and automati-
All the parts, including the relay, are installed on a small PC board which can
be mounted out of sight under the dashboard. The external wiring leads connect
to power, to the existing wiper switch & to the new wipe switch.
54
SILICON CHIP
cally switches off the wiper after one
or two wipes. That way, the driver
doesn't have to worry about switching the wiper off again and can concentrate on the task at hand.
Unlike many automotive projects,
this project is easy to install. All you
have to do is assemble a small PC
board and fit a pushbutton switch to
your dashboard. There are just six
wires to run: two for the power supply (+12V & GND), two to the contacts
of the existing wiper switch, and two
to the new pushbutton switch ..
You don't need to run any wiring
to the rear wiper itself; instead, all
wiring is concentrated at the front of
the vehicle.
How it works
Refer now to Fig.1 which shows all
the circuit details.
ICl is a 555 timer which is connected as a monostable, with Cl
(22µF) and Rl (lOOkQ) as the timing
components. Initially, Cl is held discharged by a transistor inside ICl,
pin 3 is low, and Ql and the relay are
off.
Assume now that the wipe button
(S1) is pressed. When this happens,
ICl's pin 2 trigger input is pulled low
and this both releases the internal
short circuit across Cl and drives the
output (pin 3) high. This high turns
on Ql which in turn switches on the
relay and closes the relay contacts.
Because the relay contacts are wired
in parallel with the rear wiper switch,
power is now applied to the wiper
motor.
Capacitor Cl now charges via Rl
towards the positive supply rail (Vee).
After about 2.4 seconds, the voltage
across Cl reaches 2/3 Vee (ie, 2/3 of
the supply rail) and this switches pin
3 low again and turns off the relay. At
the same time, the internal transistor
in ICl switches on and discharges Cl
so that the timer is ready for the next
cycle.
+12V FROM
IGNITION
SWITCH
I
01
1N4002
PARTS LIST
TO EXISTING
REAR WIPER
SWITCH
10!l
v---
R1
100k
15k
mJ
100
16VW
1W
c-
4
0.1
+
IC1
555
-
B
WIPEI
S1
C1
22
16VW
EOc
VIEWED FROM
BELOW
+
-
TO
CHASSIS
SINGLE WIPE FOR AUTOMOTIVE REAR SCREEN WIPER
Fig.1: the circuit uses a 555 timer IC which is wired for monostable operation.
When the wipe switch (S1) is pressed, pin 3 goes high & turns on Ql & the relay
to activate the wiper. Timing capacitor Cl then charges via Rl and, when it
reaches 2/3 of the supply voltage, the relay turns off again.
The 2.4 second monostable period
will be quite sufficient to pulse the
rear screen wiper to give one or two
wipes, depending on the speed of the
wiper. At the end of the monostable
period (ie, when the relay contacts
open), the wiper will auto-park in the
conventional manner.
The 15kQ resistor pulls pin 2 of
ICl high and thus disables the timer
circuit when Sl is open. The parallel
0. lµF capacitor is included to prevent noise from triggering the 555,
while DZ protects Ql by quenching
the back EMF that is generated when
the relay turns off.
Power for the circuit is derived from
the car battery via the ignition switch.
Dl and its associated lOQ resistor and
lO0µF capacitor provide supply decoupling to prevent ICl from false
triggering on voltage transients, while
zener diode ZDl protects ICl against
any high voltage spikes that might be
present on the supply line.
As a further precaution against false
triggering, the relay is separately
powered from the 12V supply to isolate it from the supply to ICl.
Construction
There's nothing at all complicated
about the construction. All the parts
(including the relay) are mounted on
a small PC board coded SC 05211901
and measuring just 78 x 23mm. This
in turn slots into a small plastic zippy
box to give a compact arrangement
that can be hidden out of sight under
the dashboard.
Note that the case specified is the
Dick Smith Electronics HL-2855 ,
since this is the one that contains the
internal mounting slots. Alternatively,
you could use a case from one of the
other retailers and simply wedge the
board in position using foam rubber.
Before installing any parts on the
board, first check that it slides neatly
into the case and that the lid can be
fitted. If you strike problems here,
file the edges of the board carefully
until you do get a neat fit. This done,
fit PC stakes to the five external wir-
1 plastic case, 28 x 54 x 83mm
(DSE H-2855)
1 PC board, code SC05211901 ,
23 x 78mm
1 DPDT 12V PCB-mounting
relay
1 momentary contact push-on
switch
7 PC stakes
2 rubber grommets
Semiconductors
1 555 timer IC (IC1)
1 BC337 NPN transistor (01)
2 1N4002 1A diodes (D1 ,D2)
1 16V 1W zener diode (ZD1)
Capacitors
1 100µF 16VW PC electrolytic
1 22µF 16VW PC electrolytic
1 0.1µF metallised polyester
1 .01 µF metallised polyester
Resistors (0.25W, 5%)
1 100kQ
1 15kQ
1 1kn
1 10n
Miscellaneous
Solder, hookup wire, etc.
ing points (see Fig.2).
Refer to Fig.2 when mounting the
parts on the PCB. Install all the low
profile parts first and push them down
onto the board as far as they will go
before soldering their leads. Be sure
to orient the polarised parts correctly.
These include the IC, transistor, diodes and electrolytic capacitors.
The resistor values can all be
checked using a digital multimeter
before installing them into circuit.
Alternatively, refer to Table 1 for the
colour codes.
SC05211901
Fig.3: here is a full-size artwork for the PC board.
+12V
FROM
IGNITION
SWITCH
S1
Fig.2 (left): you can install the parts on the PC board in any
order, although it's best to leave the relay until last. Use
automotive connectors to terminate the external wiring leads.
DECEMBER1990
55
the case to accept rubber grommets
(see photo). These two holes accept
the external wiring leads.
Testing
The PC board is designed to slot neatly into a small plastic zippy case as shown
here. Alternatively, you could use a case without slots & secure the board using
foam rubber. Use rubber grommets at the wiring exit points.
To test the unit, first connect the
external wiring leads and temporarily wire in the pushbutton switch (S1).
Now apply power to the circuit and
check that the relay switches on for
about 2.4 seconds when the switch is
pressed (note: the relay will remain
on if the switch is held down).
If you strike problems, switch off
immediately and carefully check the
wiring. Does pin 3 of IC1 go high (ie,
to +12V) when S1 is pressed? If not,
the 555 circuit isn't working correctly.
If pin 3 does go high but the relay
doesn't turn on, check the circuit
around Q1.
In particular, check the back of the
board carefully for shorts or missed
solder joints. Most project problems
are caused by faulty soldering.
Installation
Be sure to disconnect the battery before installing the wiper controller in the
vehicle. The wipe switch can be mounted at any convenient point on the
dashboard.
Finally, install the relay on the
board. The relay used in the prototype was a DSE P-8012 but equivalent relays from other retailers will
do the job just as well (eg, Jaycar Cat.
SY-4052; Altronics Cat. S-4190). Push
the relay right down onto the board
before soldering the leads.
Once the board has been completed,
drill a couple of holes in the side of
Assuming everything checks out
OK, the unit can now be installed in
the vehicle. Be sure to disconnect the
battery before connecting the wiring
to prevent short circuits. Also, make
sure that the +12V supply from the
ignition switch is derived from the
fused side of the fuse box. The ground
lead can be connected to a suitable
chassis location using a self-tapping
screw and solder lug.
The leads to the existing rear wiper
switch can either be soldered directly
to the switch lugs or spliced to the
existing leads using automotive crimp
connectors. Be sure to install the wiring in a professional manner and
cover any exposed connections with
insulation tape.
Finally, re-connect the battery and
check that the circuit operates the
wiper correctly. If you want to increase the number of wipes, just increase the value of Cl. Similarly, reduce the value of Cl if you want to
reduce the number of wipes.
~
TABLE 1: RESISTOR COLOUR CODES
0
0
0
0
0
56
No
Value
4-Band Code (5%)
5-Band Code (1%)
1
100kO
15kO
1kO
100
brown
brown
brown
brown
brown black black orange gold
brown green black red gold
brown black black brown gold
brown black black gold gold
1
SILICON CHIP
black yellow gold
green orange gold
black red gold
black black gold
