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Universal circuit fits all vehicles: Courtesy light delay for cars Give your car that luxury feel by extending the time that your cabin lights remain on once the car doors have closed. For that extra touch of class, the lights fade to darkness at the end of the time period. By JOHN CLARKE A COURTESY LIGHT DELAY is a great feature for your car. It enables you to see to insert the ignition key and find your seatbelt when it is dark outside, without having to leave the door open. However, many cars lack this feature, particularly older models. When the car door is opened, the cabin lights do light up but as soon as the door is closed, the lights go out. This happens just when you are about to get settled into the seat. Of course you can fumble around and find the interior switch but wouldn’t it be nice if the lights stayed on automatically for a short time instead? And wouldn’t it be classy if the lights faded out at the end of the timing period instead of a sudden switch off? Another feature that would be useful is to have the courtesy light(s) automatically switch off whenever the parking lights are switched on. This would allow you to drive off if ready to go, before the courtesy lights had timed out. The final feature of this new design is its ease of installation. Past courtesy siliconchip.com.au light delay circuits have presented real problems for installation because of the various wiring combinations for courtesy lights in modern cars. In presenting this new design, we particularly wanted solve the connection problems presented by the popular “Electronics Australia” design from the April 1997 issue. This design needed to be built in one of four versions, meaning that it was a game of chance if the car’s wiring configuration was not known. By contrast, in our new design, the same circuit will work in all cases. Courtesy light circuits The automotive industry is renowned for its lack of standardisation when it comes to car wiring and this is certainly revealed when it comes to lighting circuits. Fig.1(a) and Fig.1(b) show how the courtesy lights can be wired. Some cars will have the lights connected to the +12V supply rail and the door switches connecting to the car chassis, while other cars will have the opposite connection, with the courtesy lights connecting to chassis and the door switches connecting to the +12V rail. Note that we have shown only two lights and two switches. Some cars will have more switches (one in each door plus a manual courtesy switch inside) and more lights. The switches are all wired in parallel and extra lights are also wired together in parallel. All of the courtesy lights switch on whenever one of the door switches is closed. This occurs when a door is opened. When all doors are closed, all the switches will be open and the courtesy lights will be off. Similarly, the two possible tail light connections are shown in Fig.1(c) and Fig.1(d). The tail lights are on when the lights switch is closed. This switch would also power the parking lights at the front of the car but this is not shown in this circuit. Main Features • • • • • • • Adjustable delay period from 7-40s Lights fade out at end of time period Courtesy lights switch off if parking lights switched on No standby current drain from battery when lights are off Universal circuit works with any 12V car system Low parts count Easy to install June 2004  71 Fig.1(a) Fig.1(b) Fig.1(c) Fig.1(d) Fig.1: the two possible wiring configurations for the courtesy lights are shown at Fig.1(a) and Fig.1(b), while Fig.1(c) and Fig.1(d) show the alternative tail light wiring configurations. For our Courtesy Light Delay circuit to work, we simply need to connect it across one of the door switches. We also need to connect it to the tail light wiring, so that the courtesy lights are immediately switched off if the tail lights are switched on during the timing period. In practice, this means that the Courtesy Light Delay requires just four connections to the car’s wiring. Two wiring leads connect across the door switch, while the other two connect directly across one of the tail light filaments. How it works Fig.2 shows the full circuit details of the Courtesy Light Delay. It comprises a Mosfet (Q1), an optocoupler (OPTO1), a diode (D1), a diode bridge (BR1) and a few capacitors and resistors. Q1 acts as a switch. It’s effectively wired in parallel with the door switches and switches power to the courtesy lights during the timing period, when all door switches are open. Fig.2: the circuit uses Mosfet Q1 to switch power to the courtesy lights when the car’s door is closed (ie, the door switch opens). Trimpot VR1 sets the time delay, while bridge rectifier BR1 monitors the tail lights and switches off Q1 via optoisolator OPTO1 if the tail lights are switched on. 72  Silicon Chip Note that the door switches are marked with plus and minus signs in Fig.1(a) and Fig.1(b). The positive rail of the delay circuit connects to the plus side of the door switch, while the negative rail connects to the minus side. In operation, the circuit derives its power from the vehicle’s 12V battery via the courtesy lamp filaments. As a result, the lamps act as low-value resistors in series with the supply. However, because the circuit draws so little current when it is operating, there’s very little voltage drop across the lamp filaments and so the circuit operates from almost the full battery voltage. Note that the current flows via the courtesy lamp filaments– it doesn’t matter whether the lamp filaments connect directly to the +12V supply as shown in Fig.1(a) or to ground as in Fig.1(b). The circuit operation is as follows. When a car door is opened, one of the door switches closes and the courtesy lights switch on as normal. During this time, the switch shorts out Mosfet Q1 and so there will be no voltage across the courtesy light delay circuit; ie, between its plus and minus terminals. As a result, capacitor C1 will be discharged via R1, while C3 will be discharged via resistors R3 and R4. Subsequently, when the door switch opens again (ie, the door is closed), the courtesy lights will go out and there will be close to 12V across the drain and source of Q1. This voltage also immediately appears across a series connected network consisting of capacitor C1, diode D1 and capacitor C2. Initially, C1 has a much lower impedance than C2, since it has 10 times greater capacitance – ie, 470µF vs 47µF. As a result, C2 is rapidly charged via C1 and so has almost the full supply voltage across it soon after power is applied to the circuit. In practice, if we ignore the voltage drop across diode D1, capacitor C1 will initially have about 1.1V across it and C2 will have 10.9V across it. What happens now is that C1 charges to the 12V supply via resistor R1. During charging, the voltage on the negative side of C1 gradually drops to the negative supply rail. At the same time, diode D1 prevents C2 from discharging since it is reverse biased. As a result, C2 remains with about 10.9V across it. At this point we need to understand siliconchip.com.au how Mosfet Q1 works. These devices have three terminals, called “gate”, “drain” and a “source”. When the gate voltage is at the same voltage as the source, the Mosfet is off and no current flows. However, when the gate voltage rises to its threshold of around 3-4V, the resistance between the drain and source suddenly goes low and so current can flow between these two terminals. In practice, the drain-source resistance depends on the gate voltage and is at its lowest (about 0.1Ω) when the gate voltage is more than 10V above the source. Now take a look at the circuitry involving capacitor C3, resistors R3 & R4 and the optocoupler (OPTO1). When power is first applied (ie, when the door is closed), C3 initially behaves like a short circuit (since it is discharged). As a result, current flows via R3 and switches on the transistor inside the optocoupler, thus clamping Q1’s gate at its the source voltage. At this point, C2 has about 10.9V across it (as already stated) but is prevented from quickly discharging since it is isolated from the optocoupler by resistor R2 (100kΩ). Capacitor C3 now quickly charges via resistors R3 & R4 and removes the base drive to the optocoupler’s transistor about 1ms after power is applied. However, this time period is so short that it does not allow C2 to discharge to any extent. Now that the optocoupler’s transistor is off, Q1’s gate voltage will be equal to the voltage that’s across C2. As a result, Q1 switches on to drive the courtesy lights. From this, it might appear that the courtesy lights will briefly switch off when the door is closed, before the circuit switches them back on again. In theory, this is true but the “offtime” is so short that it is virtually unnoticeable. So why do we use the optocoupler to briefly hold Q2’s gate low (ie, for Fig.3: install the parts on the PC board as shown here, taking care to ensure that the polarised parts are all oriented correctly. that 1ms period)? The answer is that without this feature, Q1 would switch on as soon as C2’s voltage reached the Mosfet’s conduction threshold of 3-4V. This would effectively “kill” the supply to the circuit and prevent C2 from charging any further. C2 would then quickly discharge via VR1 and the 220kΩ resistor to below Q1’s gate threshold and so the courtesy lights would go out again almost immediately. By contrast, by using the optocoupler to hold Q2’s gate low for 1ms, C2 charges to above 10.9V before Mosfet Q1 switches on. And that means that C2 must then discharge from 10.9V down to below 4V before Q1 switches off (and switches off the courtesy lights). The time it takes to do this gives us the delayed on period for the lights. VR1 allows this delay period to be adjusted by varying the discharge resistance for C2. At the end of the timing period, the lamp fades out as Q1’s resistance rapidly increases as its gate voltage falls below about 5V. This means that the voltage across Q1 gradually rises from about 0V when it is fully on to 12V when it is off. As a result, capacitors C1 & C3 slowly charge to the 12V supply, via R1 and R3 & R4 respectively. This slow rate of charge prevents C1 from recharging C2 and stops C3 from switching the optocoupler’s transistor on again. Tail light circuit As mentioned earlier, the circuit turns the courtesy lights off immediately if the parking lights (or the headlights) are turned on. This is achieved using bridge rectifier BR1 and the optocoupler. In practice, we don’t monitor the parking lights or the headlights directly. Instead, the circuit monitors the tail lights, since these are always on with both the parking lights and the headlights. As shown, the bridge rectifier is connected directly across the tail lights (ie, in parallel with one of the lamps). When the tail lights are on, there is 12V across them and this is applied to BR1, which then drives the LED inside the optocoupler via a 680Ω current-limiting resistor. This in turn switches on the transistor inside the optocoupler and so Q1 switches off and the courtesy lights go out. So the optocoupler performs a dual function: (1) it forms part of the initial 1ms delay circuit and (2) it plays a vital role in switching off the courtesy Table 1: Resistor Colour Codes o o o o o o o siliconchip.com.au No.   1   1   1   1   1   1 Value 220kΩ 100kΩ 22kΩ 10kΩ 680Ω 470Ω 4-Band Code (1%) red red yellow brown brown black yellow brown red red orange brown brown black orange brown blue grey brown brown yellow violet brown brown 5-Band Code (1%) red red black orange brown brown black black orange brown red red black red brown brown black black red brown blue grey black black brown yellow violet black black brown June 2004  73 they fit into the allocated holes. This device is fitted with a small U-shaped heatsink and the assembly is secured to the PC board with a screw and nut. The PC board is mounted inside the case by simply clipping it into the mounting clips. Before doing this, you will have to mark out and drill two holes in one end of the case, to allow for wire entry to the screw terminals. These holes are located 11mm down from the lip and 18mm in from the outside edge of the case and are made using a 6mm drill. Note: for 24V operation, change both C1 and C2 to 470µF 25V and change the 680Ω resistor to 1.2kΩ. Installation The completed PC board clips into the side pillars of a standard plastic case. Note the small heatsink fitted to Mosfet Q1, to keep it cool. lights when the tail lights are switched on. Note that the connections to the tails-lights can be made without any regard as to the polarity. That’s due to BR1, which ensures that the positive voltage rail is fed to the anode of the Parts List 1 PC board, code 05106041, 78 x 46mm 1 front panel label 1 plastic box, 82 x 54 x 31mm 1 mini heatsink, 19 x 19 x 10mm 2 2-way PC board mount screw terminals, 5.08mm spacing 1 M3 x 10mm screw & nut Semiconductors 1 MTP3055E 14A 60V Mosfet (Q1) 1 4N28 optocoupler (IC1) 1 W04 1.2A bridge rectifier (BR1) 1 1N914, 1N4148 diode (D1) Capacitors 1 470µF 16V PC electrolytic (C1) 1 47µF 16V PC electrolytic (C2) 1 100nF MKT polyester (C3) Resistors (0.25W 1%) 1 220kΩ 1 10kΩ 1 100kΩ 1 680Ω 1 22kΩ 1 470Ω Miscellaneous Automotive wire, connectors, mounting brackets, etc. 74  Silicon Chip optocoupler’s internal LED. The wiring arrangement of the tail light circuit is also unimportant since the circuit simply monitors the voltage across the lamps. Construction All the parts for the Courtesy Light Delay are mounted on a PC board coded 05106041 (78 x 46mm). This then clips into a standard plastic case measuring just 82 x 54 x 31mm. Fig.3 shows the assembly details. Begin by checking the PC board for any shorts between tracks or breaks in the copper. That done, remove the corners of the PC board if this hasn’t already been done, so that the board clears the four pillars inside the case. Now for the parts assembly. First, install the resistors in the positions shown, followed by diode D1 and the optocoupler (OPTO1). Table 1 shows the resistor colour codes but it’s also a good idea to check each one using a digital multimeter before installing it on the board. Take care when installing D1 and OPTO1 – they must be oriented as shown (see also Fig.1 for the device pinouts). Next, install trimpot VR1 (this may be coded 105), then install the three capacitors, bridge rectifier BR1 and the two 2-way terminals. Again, check to make sure that BR1 and the two electrolytic capacitors (C1 & C2) are oriented correctly. Finally, install Mosfet Q1 by bending its leads at right angles so that The Courtesy Light Delay can be mounted in any convenient location under the dashboard. It’s up to you how you secure it, since the circumstances will vary from vehicle to vehicle. To connect the unit, you will need to access one of the car door switches and the tail light connections. Note that some door switches will have two wires, while others will only have a single wire connection. In the latter case, one contact is connected directly to chassis at the switch mounting position. Note also that it’s important to get the door switch connections to the unit the right way around – ie, the positive door switch connection must go to the positive rail of the Courtesy Light delay. You can quickly determine which is the positive door switch connection by using your multimeter to measure the voltage across the door switch when it is pushed open. If there’s only a single wire running to the switch, this will be the positive (the chassis connection is negative). It’s a good idea to disconnect the vehicle’s battery before running the wiring, to prevent any inadvertent short circuits. Note that all wiring should be run using proper automotive cable and connectors. The “Tail lights” terminals on the Courtesy Light Delay are simply connected across one of the tail lights. You can access this wiring either directly at the tail lights or at the lights switch or the fusebox. Alternatively, you can connect these terminals across one of the parking lights at the front of the car. It doesn’t matter which way around you connect siliconchip.com.au Fig.4: here are full-size artworks for the PC board etching pattern and for the front panel. them, since the bridge rectifier automatically caters for both polarities (as explained previously). Once the wiring is complete, reconnect the battery and check that the courtesy lights remain on after the door is closed. Now turn the parking lights on – the courtesy lights should immediately go out again. You can now trigger the courtesy lights again and set the “lights-on” delay period using VR1. Turning VR1 clockwise will increase the delay period. Troubleshooting If the courtesy lights are always on, it may be because the door switch terminals have been connected with reverse polarity. If that happens, the courtesy lights turn on via the intrinsic reverse diode inside Q1. Simply swapping the leads to the door switch will fix this problem. If the lights do not remain on after the door is closed (and the connections are correct), check that there is no voltage applied to the “Tail light” terminals on the PC board. If there’s no voltage here, the problem will be on the PC board itself. The first step is to carefully check the copper side of the board for missed solder joints and solder bridges between adjacent tracks. That done, check that all components are oriented correctly and that they are in their correct positions. Finally, check that there is 12V between the drain and source terminals of Q1 when the door switches are open (ie, with the doors closed). If there is no voltage here, check your wiring back to the door SC switch. siliconchip.com.au June 2004  75