Automotive Headlights Minder



What can you say on a bleak morning when you discover that you accidentally left your carís headlights turned on the night before, and your battery is now stone cold dead?


Because this minor tragedy has happened to me once too often, I decided to end this headache by designing an automatic head light circuitóthe Electronic Lightmaster. It waits patiently every time I switch off my carís ignition, looking to see whether or not Iíve also switched off the headlights. If I havenít, after a reasonable length of time (about 20 seconds), it simply switches them off for me . . . no ifs, ands, or buts.

Electronic Lightmaster is an apt name, because at the flip of a switch I can convert the circuit into a time-delay relay that purposely keeps my headlights turned on for about 2Ĺ minutes after I leave my car. This feature transforms my carís lights into mobile street lights that can illuminate dark driveways or paths during the short time it takes me to walk from my car to a doorway or some other entrance.


In the Works.


The circuit is built around a Darlington amplifier. This is a two-transistor design that has the very high input resistance needed to build a time-delay relay. Essentially, the amplifier looks at the voltage stored in capacitor C1 and applies almost the same voltage to the coil of relay K1.


To achieve the lighting effects we want, the circuit is connected to your carís electrical system in an unusual fashion. To begin with, one of K1ís pair of contacts is wired in series with the headlight power line. Your lights will only work if K1 is activated, keeping the contact closed.


With switch S1 in its NORMAL position, the amplifier circuit is powered by the head light power line. Thus, when you flip your carís light switch on, power is applied to the circuit. However, the relay will close only if capacitor C1 is charged to +12 VDC. Note that C1 is charged by a wire connected to your carís ignition switch. This means that if you flip your light switch on and also have turned on the ignition, then your headlights will light up.


If for any reason you should turn off the ignition, capacitor C1 is no longer connected to a source of voltage. It will begin to discharge through resistor R2. As a result, its stored voltage drops, bringing along with it the voltage applied to relay K1. After 20 seconds, the voltage at the relayís coil is too low to keep the relay activated and it drops out, thereby switching off the headlights.

When you move S2 to its DELAY position, the Electronic Lightmaster is modified slightly. Now R2 is removed from the circuit, so that the only resistance which can discharge C1 is the very large input resistance of the amplifier itself.


Practically speaking, this means that it now takes 2Ĺ minutes for the relay voltage to fall low enough to deactivate the contacts. Also, K1ís pair of contacts is placed across your carís headlight switch. This means that you can turn off the switch as usual when you leave the car without disrupting the Lightmaster circuit. Pushbutton switch S2 brings capacitor C1ís voltage up to its full value when it is pressed. Therefore, it functions as a RESET button. You need to use it only on those occasions when you have already used up a good part of the time delay before youíve left your car. For instance, say you take some time retrieving a lost item under your seat after youíve switched the ignition off. Pushing S2 for a full second restores the time delay period.


Switch S3 is the systemís power switch ó Iíve labeled it BYPASS. One pair of its contacts is wired in series with the relay coil, the other across the relay contacts that control the headlights. If you flip S3 to BYPASS, the circuit is effectively removed from the headlight wiring.


Building It.


As with any automotive project, itís difficult to design a single device that will fit into every car. So you may choose to use your ingenuity when you build the device and assemble it in a different kind of box. If you wish, you can even mount the guts underneath the hood and the three control switches on the instrument panel.


I wired the circuit into a 2Ĺ x 2Ĺ x 5-in, aluminum chassis box and hung it below the dash. My aim was to put the controls within easy reach. Parts placement isnít critical. But since anything installed in a car will be shaken up a bit, be sure that all connections are mechanically and electrically sound.


To make the most of the limited space, I mounted the relay above the bottom of the case by placing spacers on its mounting screws. All wiring is clone in point-to-point fashion, using terminal strips as soldering and mounting points. To guard against short circuits, you should place sleeving on all leads that might conceivably brush against other wires or metal objects. This is important because the case is grounded and part of the electrical circuit.

Double check the polarity of C1 when you install it. The circuit wonít work if its leads are reversed.


Transistor Q2 is a newly-developed, plastic-encapsulated power transistor. Cut off the unused collector lead (the lead next to the beveled edge), and make all collector connections to the metal tab. This tab acts like a small heat sink and must be positioned clear of other components.


Bring the four leads that connect to your carís wiring out to the four-terminal barrier strip. Use heavy duty wiring for these connections.




All required connections to your carís wiring are outlined in our diagram. The only tricky area is the head light power line (it may take a while to find the correct lead). A wiring diagram for your car will be helpful, so try and get a copy (your public library may have a copy of the shop manual). The lead you want is between the master light switch and the hi/low beam switch. This single wire carries the power to light your headlights.


Copyright by Bill Bytheway, K7TTY February 2012