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The Multicolour Dash Indicator

A cheap and simple way of showing the status of underbonnet electrical functions.

by Julian Edgar

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This article was first published in 2003.

The other day a guy I know fitted a low water level switch to his intercooler spray tank. He used a float switch which is cheap and readily available. (Best of all, it can be inserted through the wall of the tank and sealed into place without having to get a spanner inside!) He's got it wired to a dash buzzer, so as the water level gets low, the buzzer sounds. But sometimes he gets to feeling a bit restless: the buzzer might be off, but maybe that's not because the water level is high but because the buzzer is broken! (If you want to buy one of these switches, go to and search under cat no 317-932.)

Another bloke I know has got a temp switch buried in the fins of the intercooler, so that he can tell by a dash warning light when the temp's getting up. When it's high, he operates a dashboard switch that turns on the intercooler water spray. Then there's another light that tells him the spray is running.

In addition to both of these systems involving intercooler water sprays, each has something else in common. For a very low cost it's easy to add a single dashboard LED that can change from green to red to much better shows the status of these systems.

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Take the first case. The LED can be configured so that it's green when the water level is high and red when it's low. No problems with wondering if the LED is broken or a power feed has come adrift.

And the second? The LED can be configured to be off (ie white) when the intercooler temp is low, red when the temp is high (and the spray is switched off), and green when the temp is high (but the spray has been switched on). You can even leave the switch in an 'auto' position where the lighting of the green LED will show when the spray is running!

The use of a single LED also allows you to much more easily integrate the indicator into a busy dashboard or instrument panel.

So all of this must be really complex to do, eh? Nope.

Switch Position Indicator

Let's start off with a toggle switch, where you want to show via a green or red indication what position the switch is in. (Good for night viewing, as well.) You'll need a double pole, double throw switch, often abbreviated to DPDT. All that this means is that two circuits can be switched simultaneously in two directions. These switches are common and cheap; you can recognise them by their six connections. (For more on 'DPDT' and 'SPST' and similar names, see "DIY Adjustable Temp Switches".)

One of the circuits that the switch controls will cause the LED to be either red or green, while the other circuit is for whatever you're turning on or off. (Obviously it doesn't have to be an intercooler water spray - it can be any circuit at all.)

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Here the switch is shown with the LED and a resistor that you'll need (560 ohms) which controls the current flow through the LED. The 12V input is switched to one or other of the LED leads, while the earth is common for both colours. The upper switch connections aren't used yet. Simple, huh?

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Now we've added a pump, controlled by the same switch. The LED can be red with the pump switched off and green with it switched on. (Reverse the switch-to-LED input connections to reverse the colours.)

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Now try this one for size. Look the same? It is, but for a normally-open intercooler temp switch that has been added on the 12V feed. Let's say that the temp switch closes at 50 degrees C.

The LED now has three conditions, each informing the driver of what is happening:

  • LED is off (ie white): Temperature of intercooler is less than 50 degrees C
  • LED is red: Temp of the intercooler is above 50 degrees C and water spray is switched off
  • LED is green: Temp of the intercooler is above 50 degrees C and water spray is switched on

You can easily replace the intercooler temp switch with a boost pressure switch, which in addition to telling you by a change of colour when you're actually on boost, will also remind you of the spray switch setting each time the LED comes on. (Or you could place the boost pressure switch in series with the temp switch, so that both boost and temperature have to be high before the spray will trigger.)

A suitable temp switch for the above application is available from electronics suppliers - eg Jaycar Electronics cat no ST3831 at only AUD$4.45. One of the cheapest and most effective simple click-action boost pressure switches are the ones used in spa baths - again these are cheap (around AUD$12.)

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DPDT switches are available in all sorts of configurations. Here's a mixture: toggle, rocker, pushbutton and slide. Most of these switches were salvaged from old consumer goods which were acquired for nothing (ie they were being thrown out) or for a dollar or two.

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Note how the DPDT switches all have six connections - invariably the switch layout means that each of the two centre pins switches to one or the other of its nearest pair of pins when the switch is thrown.

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Together with a bi-colour LED, this DPDT pushbutton would be ideal for switching in a higher level of boost. 'Power' indeed! The LED could show green for normal and red for the higher boost level (which could be easily achieved by using a solenoid to switch in a greater wastegate bleed.)

Warning Indicator

The above system is primarily for working with a manual switch. But what about that low water level indicator in the intercooler water spray tank? (Most switches of this type aren't the DPDT type used above.) The answer is to use a relay - a single pole, double throw (SPDT) or double pole, double throw (DPDT) that in turn is controlled by the remote switch.

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The above diagrams use a pictorial representation of what the back of a typical DPDT switch looks like - these switches are all pretty standard. However, the pin-outs of relays do vary from model to model, so it's wise to check out the relay that you're working on, using a multimeter to assess that the right connections are being made when the relay pulls in. For more on how these relays work, see our story on "DIY Adjustable Temp Switches".

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You can see that the circuit is very like the others, except a SPDT relay replaces the DPDT switch. SPST 12V relays are also common and cheap - you don't need to use a high current switching relay. The two upper connections are for the coil that operates the relay, while the three lower connections are the SPDT part of the relay.

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Here is the same circuit, but drawn so that you can more clearly see inside the relay.

So what happens with this circuit? When the remote switch (which could be a boost switch, a temp switch or a level switch) is open, the LED is one colour. When it is closed, the LED is the other colour. Taking this approach also allows you to use a temp switch that is 'normally closed' and which opens when the temp is exceeded. (This type of temp switch is more common than 'normally open' designs.) The use of a relay also allows you to use a clothes iron or frypan adjustable thermostat, as covered recently ("DIY Adjustable Temp Switches".)


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The 3-wire bi-colour LEDs are available for chickenfeed from electronics parts distributors, eg the Jaycar Electronics ZD1734 costs well under AUD$1 each. DPDT switches and SPDT (and DPDT) relays are also common and cheap, as are spa bath boost pressure switches and ex-frypan adjustable thermostats.

It ain't world-shattering stuff, but it's cheap, easy to do and effective. And they're the sort of DIY mods that are most likely to be actually carried out...

The DIY Voltage Indicator?
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We've already covered in AutoSpeed a single instrument LED that changes colour - the "DIY Voltage Monitor". However, that PCB-based kit design was specifically for monitoring voltage levels - especially battery voltage - while this article covers the use of bi-colour LEDs that can be used to monitor the on/off status of almost any 12V circuit in the car.

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