Universal Bargraph Voltage Display

Monitor oxygen sensors, MAP sensors or airflow meters with this universal voltage display

by Julian Edgar

Click on pics to view larger images

At a glance...

  • LED bargraph displays voltage output of sensors
  • Adjustable for range and highest and lowest voltages
  • Can also monitor battery voltage
  • Will not load down sensors
  • Kit and prebuilt versions available

Here’s a cheap and straightforward electronic project that will allow you to display the output of lots of standard engine management sensors. If the sensor has a voltage output, you can use this display – whether that’s a MAP sensor, airflow meter, oxygen sensor or throttle position sensor.

Uses? Well, by monitoring the output of the standard MAP sensor in a turbo car, you get a coloured LED boost gauge. By monitoring the output of the airflow meter, you can see at any time exactly what proportion of full power you’re developing. Or by monitoring the output of the oxy sensor, you can see whether the car is running lean, rich or at stoichiometric.

The kit is called the Voltage Monitor and it’s available from the AutoSpeed shop in built and unbuilt versions. (See Voltage Monitor Kit - Pre-assembled and Tested and Voltage Monitor Kit). The kit can also be bought at Jaycar Electronics stores.

The positives of the project are many.

Firstly, tapping into the factory sensor won’t change its output, so the engine management system will work as normal.

Secondly, you can adjust the voltages at both the low and high ends of the LED bargraph, allowing (for example) just 3.2 – 4.8V to be shown. That means there are never any unused LEDs, so improving display resolution.

Thirdly, any in-car voltages can be measured, including battery voltage.

Fourthly, you can configure the LED colours to show what you want – for example, you might make low voltages appear on red LEDs – or alternatively, make these green LEDs. If you build the kit, the choice is yours.

Finally, the design is rugged and well protected, so it’s very unlikely to blow up.

Building the Kit

If you’ve ever built an electronic kit before, it is straightforward to construct. (If you don’t want to build it, you can buy it pre-built, but of course at a lot higher cost.)

Make sure that you solder the polarised components (like the LEDs!) the right way around and check all resistor values with a multimeter before soldering them into place. Don’t lose the selectable links that allow you to configure the project for different voltages.

Full build and use instructions are included with the kit, so this article just covers the main elements of set-up and construction.

As you may wish to change the LED colours or positions, before building the kit we advise that you fully read this article.

Configuring the Kit

The first step is to use a multimeter to measure the range of voltage you want to measure. So for example, if you want to monitor the output of the MAP sensor, use a multimeter to measure the highest and the lowest voltages that occur in driving. That means you’ll need to try both full throttle and full lift-off. An airflow meter will exhibit its greatest range at idle and at peak revs, full throttle.

Let’s say that the MAP sensor output varies from 1.3 to 4.3V.

The Voltage Monitor can be configured for 0-1V, 0-5V and 9-16V. These ranges are set by the removable links.

Range

Link 1

Link 2

Link 4

0-1V

Out

Out

In

0-5V

In

Out

In

9-16V

Out

In

Out

So where are these links? This photo shows them.

You might wonder what Link 3 is used for. Putting this link into place turns the LED display into a bar display; leaving it out makes the display show one LED at a time (ie dot display).

Calibration

In our MAP sensor example, where the lowest voltage to be displayed is 1.3V and the highest, 4.3V, the links would be set for a 0-5V display. Now with ten LEDs and a 5V range, you’d expect each LED to light up as the voltage rises by 0.5V. With our MAP sensor, in bargraph mode that would cause the bottom couple of LEDs to be always on. However, the tricky thing about this project is that the bottom and top voltages can be set individually. This results in the display always showing as much information as possible - because it auto-scales between these top and bottom voltages.

Pot VR1 sets the top voltage and Pot VR2 sets the bottom voltage. You could set these pots by trial and error and testing in the car, but it’s easier and much quicker to set up this circuit, that uses an additional 10 kilo-ohm external pot. Adjustment of this pot allows you to vary the voltage going into the Voltage Display, from 0 volts with the pot at one end, to battery voltage with the pot at the other end.

By monitoring the voltage with a multimeter, you can turn the pot until the highest voltage you will be monitoring is being fed to the meter. (In our example, that’s 4.3V.) Turn Pot VR1 (circled) until the top LED just lights at this voltage.

Then turn the external pot until you have 1.3V going to the display. Adjust Pot VR2 (circled) until the other end LED just lights.

You’ll then need to redo this process with the pots a few times as the adjustment of one slightly affects the other.

When you have finished your adjustment, the bottom LED should light at the lowest monitored voltage (in our example 1.3V), and the highest LED light at the highest monitored voltage (4.3V). You can see from this process that all the LEDs will then be used in normal operation.

LEDs

Now that you’ve seen how the LED range is configured, you can think about how to build the kit to best suit your needs.

If you are using it to monitor the output of a 0 -1V oxy sensor, the standard positions of the different coloured LEDs will be fine. That’s because low voltages are shown as red – and low voltages from a traditional narrow band oxy sensor occur at lean mixtures.

But if you are monitoring the output of the airflow meter, you probably don’t want to see red LEDs on all the time at idle. You could swap these red LEDs for green or yellow.

You can also change the provided rectangular LEDs for round 5mm or 3mm LEDs – any LEDS with a voltage drop around 2V are fine. Finally, if you don’t mind some fiddly wiring, the LEDs can be remote mounted from the board, which makes them easier to fit in anywhere convenient.

If the thought of trying to squeeze in 10 LEDs in a crowded dashboard is a concern, consider mounting just one of the LEDs remotely.

For example, if you are monitoring a narrow band oxy sensor, you could place on the dash a single red LED to indicate lean mixtures. Or you could place a single yellow LED on the dash, with the Voltage Display calibrated so that this LED flashes on and off in closed loop cycling.

Conclusion

Be able to continuously monitor sensor outputs is useful and can allow you to pinpoint tuning or engine management problems. In fact, the more you think about it, the more uses can be thought of for this project!

Battery Monitoring on the Cheap

If you frequent places where people throw away old cordless drills, you may well be able to pick up a LED bargraph display for nothing. In addition to its lower cost, it also has an advantage over the kit version – it’s much smaller. However, it is not nearly as versatile as the ‘proper’ project.

You’ll need a battery pack from a cordless drill that has...

... a test panel on one face. Pushing the button lights up a row of tiny LEDs, with the number of illuminated LEDs indicative of battery voltage.

Opening-up the battery pack is easy and removal of the LED circuit board a 2-second job – just snip the leads close to the battery pack and slide it out.

Here’s what the circuit board looks like. The LEDs are switched on by pressing the button on the right....

...but it’s easy to solder a bridging wire across the switch so that the display is on whenever power is applied.

Some testing with a variable voltage power supply showed that this display (from a nominally 15.6V pack) showed full battery voltage on 6.0V – clearly, in the battery pack installation, the module wasn’t wired across the whole pack. Also, despite having 10 LEDs, the module actually works in 5 pairs of LEDs.

The lowest two red LEDs vary in intensity with voltage, being off below about 3.3V and at full brightness by 4.6V. The next two red LEDs come on at 4.9V, the first two yellows light at 5.5V, the next two yellow at 5.8V and the final two green at 6.0V. Peak current draw is about 40 milliamps. The response rate is very fast.

It’s easy to increase the voltage at which full voltage is displayed. By using a pot (eg 10K – but the value may need to be adjusted to suit the application) it’s possible to adjust the voltage going to the module until the LEDs are illuminated at (say) 12.0V. If you do this, the other steps become 11.8V, 10.7V, 9.6V, 7.3V and about 6.3V.

Note: because this LED module is powered by its signal, you cannot use this module to measure the output of an oxygen sensor. It may also pull enough current to drag down MAP or airflow meter sensor signals. However, it’s ideal for measuring battery voltage.

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