Fullscreen HTML5Med Res (??MB)HTML4

Don’t get caught with a flat battery! A courtesy light switch-off timer for cars Have you ever left a car door slightly ajar & returned later to find a flat battery? Or maybe your kids have been play­ing in the car & left the interior light on. This simple circuit will automatically switch the light(s) off after two minutes to save the battery. er saves your battery by switching off the power to the interior lamps after about two minutes. If necessary, the lamps can then be relit for another two minutes simply by closing and reopening the door to restart the timer. The timer connects in series with the positive supply to the interior light circuit, so that it can control the power supplied to the lamps. Apart from that, the timer circuit does not interfere with the operation of the courtesy lights. The lights continue to come on immediately a door is opened and will go out as soon as the door is closed in the normal manner. By JOHN CLARKE Unfortunately, it’s all too easy to leave the interior lights in your car on. In most cars, the lights remain on if a door is not properly closed (ie, is on the first catch) and that can easily occur if you’re in a hurry or struggling with shopping bags. If the lights are left on for long enough, the result is a flat battery and loads of frustration. This problem is particularly prevalent in later model cars which have a number of interior lights; eg, one on each door sill, one in the roof and one for the ignition lock. If all these lamps are alight, it doesn’t take long for the battery to discharge. This Courtesy Light Switch-Off Tim- +12V (VIA INTERIOR LAMP FUSE) INSERT INTERIOR CAR LAMP TIMER HERE LAMP LAMP DOOR SWITCH DOOR SWITCH E 10k D1 1N4148 470k 4 8 Q1 BD650 C IC1 7555 6 1 3 5 D2 1N4148 LAMP 470  1W 1k DOOR SWITCH .01 4.7k Q2 BC337 B C 0.1 BCE E .01 COURTESY LIGHT SWITCH-OFF TIMER 30  Silicon Chip TO LAMPS 100k 2 7 220 16VW DOOR SWITCH B 10 100 16VW Fig.1 shows the standard circuit for the interior lamps. In some cars, all the lamps and switches are in parallel. This means that as soon as one switch closes, all the lamps turn on. In other cases, each door switch only activates some of the lamps (eg, the lamp associated with that door plus the main interior lamp). In order to make sure that the power to all the interior lamps can be inter- Fig.1: the timer is installed between the courtesy light circuit & the fuse. Note that the door switches are usually on the negative side of the lamps but this is not always the case. +12V (VIA INTERIOR LAMP FUSE) ZD1 16V 1W Interior light circuit SEPARATE COURTESY LAMP B E VIEWED FROM BELOW C Fig.2: the circuit is based on a CMOS 7555 timer (IC1), wired as a monostable. When the door switch closes, IC1 is triggered via the .01µF capacitor on pin 2 & switches its pin 3 output high. This turns on Q2 & Q1 to light the lamp. IC1 then times out two minutes later & switches Q2, Q1 and the lamp off. The circuit can be retriggered for a further two minutes simply by closing and re-opening the door. PARTS LIST 1 plastic case, 54 x 82 x 30mm 1 PC board, code 05209931, 46 x 61mm 1 U-shaped heatsink, 18 x 19 x 10mm 1 3mm screw and nut 1 10mm rubber grommet 1 20mm length of 0.8mm tinned copper wire 3 PC stakes The circuit is assembled on a small PC board which is then clipped into a plastic utility case so that the parts cannot short against other wiring in the car. Note the small heatsink fitted to Darlington transistor Q1. rupted, regardless of the door switch arrange­ment, the timer circuit must be installed into the positive supply line as shown. If the timer were to be installed between the lamps and switches, we would not be able to switch off a separately switched courtesy lamp. As shown in the photograph, the circuit is built on a small PC board and this is clipped into a plastic utility case. This will provide a good insulating barrier to prevent the board from shorting onto any part the car body. Circuit description Fig.2 shows the circuit for the interior car lamp timer. It’s based on a CMOS 7555 timer (IC1) and this drives transistors Q2 & Q1 to switch the power to the lamps. IC1 is connected as a standard monostable timer, the duration of which is set by the 470kΩ resistor and 220µF capaci­tor on pins 6 and 7. When IC1 is triggered, either by a low signal to its pin 2 trigger input or when power is first applied, pin 3 goes high. This turns on Q2 which then turns on Darlington transistor Q1 via a 470Ω resistor to supply power to the lamp circuit. At the same time, the 220µF capacitor charges toward the +12V (Vcc) supply via the 470kΩ resistor. The capacitor voltage is monitored by pin 6 and when it reaches 2/3Vcc at the end of the timing period, pin 3 goes low and the 220µF capacitor dis­charges via pin 7. Q2 now turns off and so Q1 also turns off and the lamps go out. For a monostable circuit such as this, the period (T) for which pin 3 is high is given by the equation T = 1.1RC. In this case, R = 470kΩ and C = 220µF and so the period works out to 114 seconds (ie, slightly less than two minutes). In practice, the time delay is slightly longer than 114 seconds due to leakage and the fact that most electrolytic capacitors have a capacitance which is greater than the marked value. To obtain correct operation, IC1 must be triggered each time a door is opened and one of the door switches closes. This has been achieved by AC-coupling a low-going trigger signal to pin 2 of IC1 via a .01µF capacitor. Let’s see how this works. Initially, when all the door switches are open, Q1’s collec­tor is pulled to the +12V supply via the 1kΩ and 10kΩ resistors. IC1’s pin 2 input is also held high (ie, to +12V) via its associated 100kΩ pull-up resistor. When a door switch closes, Q1’s collector is initially pulled to ground via the lamp filament. This low is coupled to pin 2 of IC1 via the 1kΩ resistor and the .01µF capacitor and so IC1 triggers and begins its timing cycle. When this happens, pin 3 goes high and Q2 and Q1 turn on to provide power to the lamps (ie, Q1’s collector quickly reverts to +12V). Note that during the timing period, one side of the .01µF capacitor is held low via diode D2 and transistor Semiconductors 1 BD650 PNP Darlington transistor (Q1) 1 BC337 NPN transistor (Q2) 1 ICM7555 or LMC555CN CMOS timer (IC1) 1 16V 1W zener diode (ZD1) 2 1N4148 diodes (D1,D2) Capacitors 1 220µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 1 0.1µF MKT polyester 2 .01µF MKT polyester Resistors (0.25W, 1%) 1 470kΩ 1 1kΩ 1 100kΩ 1 470Ω 1W 5% 1 10kΩ 1 10Ω 0.25W 1 4.7kΩ Miscellaneous Automotive cable, insulated bullet connectors, automotive eyelet connector, cable ties. Q2. The voltage at pin 2 of IC1 goes high shortly after triggering, when the .01µF capacitor is charged via the 100kΩ resistor. If the door switch is now opened (ie, the door is closed) before IC1 times out, the lamps immediately go out. If, however, the door switch is left closed, the lamps will go out at the end of the 2-minute timing period, as described previously. When this happens, Q1’s collector will be pulled low via the lamp filament and the closed door switch but IC2 cannot be retriggered because D2 and Q2 have kept one side of the .01µF capacitor low during the timing period. When the door switch is subsequently opened, Q1’s collector will be pulled to +12V via the 1kΩ and 10kΩ resistors. Diode D1 clamps the pin 2 input of IC1 to the supply rail to protect the IC from damage when October 1993  31 TO DOOR LAMPS TO +12V 10  470  1W TO CHASSIS (GROUND) Q1 ZD1 Q2 470k 1k 10k 100k IC1 7555 D2 .01 4.7k D1 .01 220uF 0.1 1 100uF Fig.2: install the parts in the PC board exactly as shown in this layout diagram & don’t forget the wire link. The external wiring should be run using automotive cable. this occurs. The circuit is now armed and will be retriggered the next time a door switch closes. The supply for IC1 is decoupled from the main +12V rail using a 10Ω series resistor, a 100µF capacitor and a 16V zener diode (ZD1). ZD1 protects IC1 from the voltage spikes that occur in automotive supplies. Note that IC1 is powered all the time. It only draws a nominal 400µA and so has negligible affect on the battery. Construction The PC board for this project is coded 05209931 and measures 46 x 61mm. Fig.3 shows the component locations on the board. Begin the board assembly by installing PC stakes at the three external wiring points. The remaining parts can be in­stalled in virtually any order but take care to ensure that IC1, transistor Q2 and the diodes are all correctly oriented. The 470Ω 1W resistor should be mounted slightly proud of the PC board to aid heat dissipation. The two electrolytic capacitors must also be correctly oriented. Note that the 220µF capacitor is mounted on its side so that it doesn’t later foul the case lid (see photo). Position it so that it lies on top of the 4.7kΩ resistor and diode D1, and bend its leads at right angles so that they pass through the appropriate holes in the PC board. Transistor Q1 is the last component to be installed. It is fitted with a small finned heatsink to keep it cool and is bolted to the board using a screw and nut. Bend the leads of the tran­sistor at right angles so that they fit the holes in the board before finally tightening the nut. Once the PC board has been completed, you can drill a hole in the end of the case for the rubber grommet. The PC board can then be clipped into position and the three external leads fitted (use automotive cable). Installation Before commencing the installation, check the car’s wiring diagram to determine the best place to connect the cir­cuit. In some cars, you may be able to make the connection at the fusebox, provided that the fuse only supplies the interior lamps. The circuit should be installed directly after the fuse as shown in Fig.1. Do not bypass the fuse otherwise you could get a fire if a fault develops in the car’s wiring. In most cars, however, other equipment will be powered via the same fuse (eg, the clock, radio, boot light and instrument lights). If this is the case, you will have to tap into the wiring further down the line, after the supply points for this equipment. Disconnect the battery before installing the wiring, to prevent any Fig.3: this is the full-size etching pattern for the PC board. inadvertent shorts. The procedure is to cut the positive supply lead to the interior lamps and fit bullet connec­ tors to the cut ends. The appropriate leads from the timer are then plugged into these connectors (be careful not to get the leads transposed), while the ground lead is fitted with an eyelet connector and bolted to a suitable chassis point. The timer itself can be mounted beneath the dashboard and secured using cable ties. Check that the interior lights operate normally when a door is opened and that the lights go out after about two minutes if the door is left open, or immediately if the door is closed again. Finally, check that the interior lights can be made to come on again at the end of the timing period by closing and re-opening the door. All other items in the car should function as normal, regardless of the status of the timer circuit. If the circuit fails to operate correctly, check that all parts are in their correct locations on the PC board and that they are correctly oriented. SC RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 1 1 1 1 1 32  Silicon Chip Value 470kΩ 100kΩ 10kΩ 4.7kΩ 1kΩ 470Ω 10Ω 4-Band Code (1%) yellow violet yellow brown brown black yellow brown brown black orange brown yellow violet red brown brown black red brown yellow violet brown brown brown black black brown 5-Band Code (1%) yellow violet black orange brown brown black black orange brown brown black black red brown yellow violet black brown brown brown black black brown brown yellow violet black black brown brown black black gold brown