Fullscreen HTML5Med Res (??MB)HTML4

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