Headlight reminder for cars

I have just had an embarrassing situation in that the internal lights on my car were left on for three days, flattening the battery. This has cost me a new battery, a lot of lost time, plus the wrath of the other family member. So after a cursory examination of the "Headlight Reminder For Cars" kit (SILICON CHIP, August 2001) at my local Jaycar store, I purchased one.

However, I now believe it will not do what is required of it. I have three separate circuits in the car which need monitoring: headlights, parking lights and interior lights, as these are on separate switches, as well as the door switches.

The first two I can monitor by taking a feed off the instrument lights. I do not know about the third circuit but I wondered what would happen if I put a diode between the door switch circuit and the light circuit. Please advise (P. D., via email).

A diode from the door switch and another from the interior light switch could be used to detect when any light is on. The direction (polarity) of the diodes depends on the car wiring.

If one side of the lamps is directly connected to the chassis, then the diodes connect with their anodes to the lamps where they are powered by 12V. The cathodes connect together and form the connection for the door switch input on the Headlight Reminder. In this case LK3 is out, LK4 is in and LK5 is out.

Conversely, if one side of the lamps is directly connected to 12V, the diodes are connected with their cathodes to the negative side of the lamps. The anodes form the connection to the Headlight Reminder and LK3 is in, LK4 is out and LK5 is out.

Extending the SC480’s low-frequency response

I am using a pair of SC480 amplifier modules from the January & February 2003 issues of SILICON CHIP. I know that the SC480 module has excellent low-frequency response but I am hoping to improve the response at near DC to facilitate another mode of testing. I’ve built a vibration-testing platform and I am using the SC480s to drive the actuators.

Studying the circuit diagram, I can’t find the components that are limiting the low frequency gain. Any help on this matter would be very much appreciated. (J. M., via email).

Improving the bass response below the existing -1dB point at 14Hz is simple. Just increase the input 1μF BP capacitor to 2.2μF or 4.7μF. Similarly, increase the 47μF BP feedback capacitor to 100μF or 220μF.