One of the good things about cars of the last fifteen years is that there is a heap of sensors feeding information back to the ECU. Coolant temp, inlet air temp, manifold vacuum, airflow, throttle position - these sensors are probably all there under your car's bonnet, churning out signals. But what about making these sensors work for you, as well as for the ECU? Well, you can - and for under $20.
Sensor Outputs
The output of most EFI sensors is a varying voltage. As an example - as the amount of airflow passing into the engine increases, so does the signal voltage coming out of the airflow meter. What the electronic kit shown here does is allow the output of a sensor to be monitored, with a warning light being triggered when the sensor output reaches a certain voltage. (It won't work on the rare sensors that output a varying frequency.)
Available from Dick Smith Electronics, the $15.65 kit was originally designed by Silicon Chip electronics magazine. They didn't actually intend that it be used in performance applications, so reflecting its name of 'Low Fuel Indicator'. Yep, that's right - the kit was originally intended to illuminate a warning lamp when the fuel tank level got down too far. But, in the same way as switching on the warning light relied on the voltage output from the tank sender unit dropping below a pre-set point, so the signal output being monitored can instead be from the engine management sensors.
To make the kit really versatile, it has been designed so that the light can be triggered when the signal voltage drops to below a certain level, or alternatively rises above a certain level. On the printed circuit board, wire link #1 is placed into position if triggering is required when the signal rises to the level, and if the device needs to trigger when the signal falls to the critical level, then link #2 is installed instead.
And if you're thinking 'Yeah, it looks interesting but I couldn't put it together', don't worry - we cover step-by-step the construction of the kit. Add in the extensive instructions included with the kit itself, and - if you've got wire cutters, a multimeter and a soldering iron - you can make it!
Building the Kit
To assemble the electronic switch you'll need a multimeter, a pair of side cutters, and a soldering iron. Solder is supplied with the kit and there are full instructions. The most important part of the instructions is the diagram that shows where the different components go on the PCB.
Unpack the kit, and examine the components. The Printed Circuit Board (PCB) is the flat plastic board, the resistors are the small cylinders with coloured bands on them, the capacitors are the cylinders which have two wires coming out of one end, the diode is the small cylinder with a black band at one end, the Integrated Circuit (IC) is the package with lots of legs, the transistor is the package with three legs, and the trim-pot is the screwdriver-adjustable three-legged package.
Place the three 10 kilo-ohm resistors on the board. Find the right resistors by using the multimeter to measure the resistor values, and then bend the resistor wires so that they will fit through the right holes. Turn over the board, and carefully solder the wires to the PCB pads.
Next place the two 47 kilo-ohm resistors on the PCB, and solder them into place.
The remaining 3 resistors can then be soldered to the PCB. Make sure that the right resistors go in the right places. Note that it doesn't matter which way around the resistors are placed - they aren't polarised.
In the next step you'd expect to put the two capacitors on the PCB. However, we'll install only one of the capacitors - the one marked 100uF. Leave the other one (220uF) off the board. Note that the capacitor
is polarised - this means it has a 'plus' and a 'minus' wire. The 100uF capacitor (the value is written on it) is placed so that its minus (-) terminal - which is marked on the body of the device - is facing away from the holes for the IC. This then means that its plus wire is nearest the IC, as is shown on the component overlay diagram in the instructions.
The diode is next. This is another component which needs to be mounted the right way around, and it goes in so that its black band is nearest the IC.
The transistor goes on next, and must be mounted so that when it's laid flat on the board, the writing on it faces upwards. (Most times the other side of it has a bare metal part, which should be flat against the board.) Make sure that you bend the leads in the right place so that transistor lines up with the screw hole in the PCB.
Solder in the trim-pot next - it goes onto the board in only one direction.
The IC is next. This must be placed the right way around, with the notch at one end of the package facing away from the 12 volt, ground and signal input wires. When soldering the IC, try to use as little heat as possible, and - if you're heavy-handed - give the thing a chance to cool down between soldering pins. Make sure that solder bridges between different pins aren't inadvertently formed. Solder in the wire link in either Position 1 or 2 (depending on the application of the switch), and then solder on the PC stakes. Make the external wiring connections to the board, using red for plus 12 volts, black for earth, and white for the signal wire. Wire the warning light in as well. You've finished it!
Ooops - just remembered something. Before you connect the lamp, open it up like this and make sure that the bulb is fully screwed in. It's most frustrating when the lamp won't light, just because the bulb's vibrated out!
Module Modifications
One of the really good things about a simple kit like this one is that making changes to its functioning is pretty easy. You might need a couple more components, or even to try a few different values until you get the behaviour that you want - but because the bits and pieces are so cheap, that's not a big concern.
Varying Lamp Brightness
If you want the lamp to shows you more than just 'on' or 'off' conditions, you can easily make it change in brightness as the signal gets closer to the pre-set voltage.
To make the module work in this way, simply remove the 1 meg-ohm resistor from the board. Don't replace it with a wire link or anything - just take it right off. With the voltage trim-pot set to its middle position, the light will just start coming on at 3V and be fully on at 5V. With the adjustment pot at one end, the voltages will be 5.5V (just on) and 9V (fully on). At the other extreme of the adjusting pot's travel, 0.5V and 1.5V will be the approx 'just-on' and 'full-brightness' voltages.
You can easily use this if you want an indication of where an engine management sensor is in its output. For example, say that you use a multimeter to measure the output of the airflow meter and find that - because of your engine mods - it is maxing out early (ie reaching full voltage output while engine power is still climbing). In that case you can use the varying intensity warning light to show you when that happens - and at other times how close it is to happening. So if max output of the airflow meter is (say) 5V, you could set the module so that the lamp is fully lit at 5V. However, it will start to just glow at 3V, then becoming brighter and brighter until 5V is reached - giving you an easily seen and graphic guide as to what's going on with the airflow meter output signal.
Inbuilt Delay
It's easy to configure a delay before the light operates - in fact the circuit as designed already has this facility. Remember the 220uF capacitor that we left off the board? Well, if you put that back on where the instructions show (and as shown in the pic here), there will be a delay of about 14 seconds before the light comes on, after the signal input voltage has reached and constantly held greater than the trip level.
This is ideal for the application for which the kit was originally designed - monitoring the fuel tank level. With the delay, the fuel level has to be below the trip-point for the 14 or so seconds - so the light won't come on when the fuel level float momentarily drops as you're going around a corner. Reduce the value of the capacitor if you want a shorter delay - increase it for a longer one.
Buzzer
If you want to trigger a warning buzzer instead of (or together with) the light, simply connect the buzzer in parallel with the warning light, with the positive wire from the buzzer going to the light terminal nearest the chip.
Uses
Because the module draws almost no current on its input signal wire, it can be used on any device that has a varying (or switched) output voltage. Example uses include:
- Warning light or alarm for engine coolant overheating or engine coolant still cold (from gauge sensor)
- Warning light or alarm for engine oil pressure too low or too high (from gauge sensor)
- Warning light or alarm for high inlet air temps (from intake air temp sensor)
- ECU Check Engine light audible alarm (from Check Engine light)
- Low or high battery voltage indicator (direct from 12V supply)
- Overboost alarm in turbo cars (from manifold boost pressure sensor)
- And of course, as a low fuel warning light (from fuel gauge)!
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A Relay Output?
Yes, you can fit a heavy duty relay to the output so that you can switch on and off high current loads - that means that you control water injection pumps based on airflow, thermo fans based on temperature, and the like. But there are a few things you should know.... Firstly, fitting a big relay requires the addition or alteration of some components. That in turn changes some of the kit's parameters, like the difference between 'on and 'off' voltages and also the range of voltages than can trigger the device.
To add the relay, wire the its coil in place of the warning light, then adding a 1N5404 diode (cost is about 35 cents) as shown in this picture. Note the polarity of the diode!! If you look closely at the pic you'll also see another new component - a second 10 kilo-ohm trimpot - has been fitted. This has been wired onto the board in place of the 1 meg-ohm resistor, with only two of the three legs of the pot used. The reason that an adjustable device is needed here is because when a relay is wired to the output, it can be prone to chatter when the input signal is varying only slowly. By adjusting the new trimpot, you can dial out any relay chatter. As the main voltage setpoint is altered, so the other pot will also need some adjustment to stop this chattering.
But to make things just a bit more complex, as you adjust the pots, the difference between the 'on' and 'off' voltages will change as well. With the pictured relay being used and the second trimpot set to j-u-s-t stop relay chattering, the figures looked like this:
| Voltage trip pot at one end: |
On point: 9.8V |
Off point: 7.5V |
| Voltage trip pot in middle: |
On point: 6V |
Off point: 4.9V |
| Voltage trip pot at the other end: |
On point: 4.6V |
Off point: 1.2V |
So unfortunately the range of available trip points alters as well as the difference between the 'on' and 'off' voltages! We ran out of time to experiment, but we'd expect that altering the value of the main trimpot would also allow the trip point voltages to be reduced. Contact: Dick Smith Electronics: http://www.dse.com.au/cgi-bin/dse.storefront Cat No: K4210
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