One of the problems in incorporating extra gauges and monitors into the dashboard of recent cars is making the addition look factory. The days of cutting a great big hole in the dash and jamming in a gauge or meter are long past - if you want to retain resale value and keep your car looking good, anyway. Sure, you can add pods on the top of the steering column or on the A-pillar, but again it's a very obvious alteration.... something that's not always wanted.
So one of the criteria that we set for this instrument is that it had to consist of something very easy to incorporate into the dash - and a single LED is just that!
That's right, the readout of the instrument consists of just one LED - it's green when the voltage being monitored is greater than the preset level, and changes to red when it drops below that level.
So what use is that sort of instrument?
One of the best uses is as a battery monitor. You like playing your sound system loudly at shows, with your car's engine switched off? But you're always worried that you'll have the embarrassment of a battery too flat to start the car? Well, by using this monitor it's as easy as watching the colour of the LED. If it turns red you know that the battery is getting down. If it stays green, all is fine.
And of course the system can be used to monitor battery voltage in any situation where the battery is being drained - from cleaning your car with all the interior lights on and sound system playing, to using the car battery to power external lights or other electrical items when you're camping at a motorsport event.
The monitor has another car sound system function as well. If you wire it at the amplifier end of the system, it will monitor the actual power supply voltage reaching the amp - in real time. So if the cables that you're using aren't heavy enough to carry the current without a voltage drop, you'll see the LED flashing from green to red on the big bass thumps as the volts available at the amp drop momentarily.
Finally, the monitor is ideal for measuring battery strength in any battery-powered portable piece of equipment. So for example you can easily incorporate it into the DIY Knock Sensor MkII design ["DIY Detonation Detector - Mk II"] that we recently covered. In this type of application, the green LED also functions as a 'power-on' indicator.
So it's easy to build the indicator into the dash (or amp surround, or even mounted right next to the battery under the bonnet) - but is it cheap and easy to construct? The answers to those questions are 'yes', and 'mostly yes'.
The Kit
The kit is available from a number of suppliers - Dick Smith Electronics (cat no K3118 at AUS$11.48), or Jaycar Electronics (KA1778 at a substantially cheaper $AUS8.95). You can also purchase the kit through AutoSpeed if you wish - Voltage Indicator Kit.
The kit comprises a 25mm square printed circuit board (PCB) and just a handful of components. However, while it is a simple kit to put together, the very small size of the PCB does make it a bit trickier. It's for that reason that we'll cover the construction step by step.
The first step is to solder into place the 5 resistors. Use a multimeter to sort out which is which, and then use both the overlay diagram in the instructions and this diagram to put them each into their right places. The diagram in the instructions is a bit small and it's easy to make a mistake in this area, which is why we've included this extra diagram.
The installed resistors should look like this.
Once you have done that, place the trimpot on the board and solder it into place.
The capacitor is next. It is a polarised component, which means that it must be soldered into place with the right orientation. (The resistors, on the other hand, can go either way around.)
The capacitor's polarity is indicated by a 'minus' symbol near one lead. This lead goes closest to the edge of the PCB, ie the positive lead is nearer the IC.
The next step is to solder the LED onto the board. Note that again the instructions are slightly deceptive - better to follow this diagram. As can be seen here, the LED has three leads, each of different lengths. Orientate the LED and PCB so that the shortest lead goes into the top hole, the longest into the middle and the other lead into the bottom hole.
Note that if you get this around the wrong way all that will happen is that the LED will start off red and then turn green as the voltage falls, rather than the other way around.
Another way to mount the LED is to push its leads through from the other side of the board, soldering them on the same side as the newly positioned LED. If you arrange the length of the leads right, you'll be able to stick the flat side of the board to the back of a mounting panel with double-sided tape, with the LED just projecting through a single hole in the same panel. Of course, you can also extend the LED wires and mount the LED and PCB completely separately.
Finally in the component assembly there's the three-legged voltage reference and the 8-legged IC. The voltage ref is assembled with the orientation shown here - the flat facing away from the edge of the board, while the 8-legged IC goes with the notch (sometimes shown on the IC as a dot) facing in the direction shown here.
Then you simply need to solder into place the two wiring pins and the finished PCB should look like this.
Using It
The best way to set up the monitor is on the actual car. Grab your multimeter and connect the module to power and earth, making sure that you don't get these connections reversed! Depending on where the trimpot is set, the LED will then glow red or green. Turn the pot until the LED is green, and then back the other way until it just turns red. Then start the car. With the increase in battery voltage from the running alternator, the LED should immediately go green.
Now that you've shown it works, use your multimeter to measure the voltage feeding the module. On my car, the running-engine battery voltage is no less than 14.4 volts, while when the engine is switched off, the battery voltage immediately drops to about 12.4V. If I have the lights and ignition on (but the engine still off) the battery voltage plummets, dropping to the high elevens in a minute or so. However, if I leave the lights off and just play the sound system loudly, the battery voltage drops only very slowly.
So looking at all these voltages, I made the decision that I wanted the LED to change from green to red at about 12.0V.
By turning the adjustment trimpot very carefully, it's possible to set the module so that the LED changes to red very close to 12V - just what's wanted in this case. And of course, if you want the LED to change colour at, say, 11.5V, just adjust the pot a little further.
Cheap, easy and effective!
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