Last week in
LED Dashboard Indicator, Part 1
we showed you how to mount a high intensity LED in a nylon housing
suitable for mounting on the dash or an A-pillar. Now it’s time to make the LED
work off 12V and as a bonus, dim itself at night. In addition, in this story
we’ll also show you how you can have adjustable brightness levels for both day
and night viewing.
Unlike a normal 12V pilot light, you can’t just
connect a LED across the car battery and expect it to last more than a second or
two. Instead, a resistor needs to be inserted in series to limit the current
flow to what is appropriate for the particular LED. Some people just use a rule
of thumb (like use a 560 ohm resistor when running a LED off a car battery) and
in the past that worked fine. But these days with lots of different LEDs
available, if you want the highest possible LED output, you need to tailor the
resistor value more closely to the LED specifications.
In addition to colour, intensity and package size,
LEDs have two other important specs. One is the forward voltage drop and the
other is the maximum current. With these two bits of information (which are
available from where you buy the LED), calculating the required resistor is
First up, what resistance value should be used?
The following equation gives you the answer.
Resistor = (Supply voltage – LED forward voltage
drop) x 1000/current
So, if you’re supplying a running car voltage of
13.8V, the LED has a forward voltage drop of 3.2V and a current of 100mA, the
Resistor = (13.8 – 3.2) x 1000/100
Resistor = 106 ohms
That 106 ohms will give a pretty bright LED. If
you want it dimmer, you can use a larger value resistor – say nearer 200 ohms.
You can decrease the LED current by picking a larger value resistor, but you
cannot increase the LED current beyond its specs without potentially blowing it
So that you can see exactly what’s happening, it’s
always wise to set your multimeter to measure milliamps and then insert
it in series with the battery, LED and resistor. That way, you can quickly and
easily confirm if what is supposed to be happening, actually is!
The second factor you need to calculate is the
amount of heat the resistor will dissipate. This power can be found by:
Power = (LED current/1000) x (LED current/1000) x
So is this case, that’s:
Power = 0.1 x 0.1 x 106
Power = 1.06 watts
That means that for this LED, you need a resistor
with at least 1 watt power dissipation.
Now there’s no such thing as a 106 ohm resistor –
the nearest value is 110 ohms. And you should always over-rate the resistor
power rating, so a 5 watt resistor would be a good thing. The final spec reads:
110 ohm, 5 watt resistor.
That’s a pretty big resistor – if you’re running a
LED that’s a lot less bright (say a current of 50mA and a forward voltage drop
of 2V), the resistor value in a car application calculates to 118 ohms (nearest
value: 120 ohms) with a required power rating of 0.3 watts (so a much smaller 1
watt resistor would be fine).
In use it’s easy to tell if the power rating of
the resistor is too small – it will get hot.
Unlike a conventional light bulb, a LED has a
polarity. That is, it will work only when connected the right way around. In
nearly all LEDs, the longer of the two leads connects to positive. You won’t do
any harm if you briefly connect it around the wrong way, so if it doesn’t work
one way, try it the other.
It’s important that a LED dash indicator auto-dims
at night because the LED intensity that you want in the daytime is waaay more
than you want at night. So how do we do it?
Let’s start off with the basic circuit – 12V
supply, LED, resistor. That’s the circuit we’ve been describing above, where the
current through the LED is limited by the resistor, which in this case we’ll say
is 110 ohms.
No we’ve added a potentiometer – a pot. (When it’s
wired like this, with only two of its connections being used, it’s sometimes
called a variable resistor.) By turning the pot, we’re able to vary the
brightness of the LED from a maximum (dictated by the fixed resistor) to a
minimum (dictated by the max value of the pot). If the pot is 1000 ohms, we’ll
be able to dim the LED right down – excellent for night viewing. The circuit
could be left just like this – but you’d have to turn down the LED brightness
each time you drove the car at night (and then remember to turn it back up in
Now we’ve added a switch in parallel with the pot.
If we close the switch, the pot is bypassed and so has no effect. If we open the
switch, the pot comes into operation. If the pot is adjusted so that the LED is
dim, operating the switch will cause the LED to change between day and night
And now here’s the final circuit. What we’ve done
is added a relay to operate the switch. The relay, which needs to be a Single
Pole, Double Throw design (see
Using Relays), is wired in a
normally closed configuration. That is, when there’s no power being applied to
its coil (blue in this diagram), the relay is closed – bypassing the pot. But
when power is applied to the relay, the relay switch opens and the pot comes
into operation. Feed power to the relay’s coil from the dash or parking lights
and you now have an auto-dimming LED indicator!
So to take a step backwards, what parts do you
need? Well, you need a resistor and a trimpot (that’s a little pot) will
probably 10,000 ohms max resistance. (It won’t need a high power rating because
there’s that big resistor already in series.) You’ll also need a SPDT 12V relay.
You don’t need a huge automotive relay that can handle heaps of current – a
miniature electronics one is fine. And maybe some punched board to build it all
on – or alternatively, you can solder it in space and wrap it with tape. From
any electronics store, maybe AUD$10 for the lot.
Turning on the LED
But how do you turn on the LED? That depends on
what you’re monitoring!
If you just want to see when boost pressure is
occurring, wire a boost pressure switch in series with the LED power feed. If
you want to see when an intercooler water spray is squirting, wire the system in
parallel with the pump. If you want to see when engine revs are at the redline,
use the Frequency Switch kit (see
Frequency Switch, Part 1
). If you want to see when
the air/fuel ratio is rich, use a Simple Voltage Switch kit (see
The Simple Voltage Switch) working off the
oxygen sensor. If you want to be alerted to an over-temp scenario, use the
adjustable Temperature Switch kit (see AutoSpeed Shop)
or a simple normally open temperature switch (see
Basically, you can turn on that LED by monitoring
nearly any parameter in the car, from the simplest to the most complex.
The two adjustments you can make in LED brightness
are for daytime (by changing the fixed value resistor) and for night time (by
altering the value of the pot). Assuming that you want the maximum possible LED
brightness in daytime, calculate the value of the fixed resistor to give maximum
LED current, as described above. If you want it dimmer, use a higher value
The night LED brightness can be set with the
system installed in the car – you just need to be able to access the pot. Note
that a 10,000 ohms pot (usually called a 10K pot) will allow you to dim the LED
until it’s barely visible, even in very dark conditions. It’s unlikely you’ll
need it this dim, but with a pot of this value you have the choice.
Neat simple, cheap and effective! It’s amazing how
informative just one or two LEDs can be as to your car’s operating
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