The Pressure Switch that's newly available form the AutoSpeed shop is an incredibly sensitive piece of gear - at its factory adjustment, it triggers at just 0.15 psi! That makes it super quick as a boost pressure switch, but it also allows it to be used to detect aerodynamic airflows and in other unusual applications. In fact, we used it in an article last issue "Siting Cold Air Intakes" to find the best high-pressure place at the front of a car to site the mouth of a cold air intake.
But it does have some deficiencies. Firstly, because of its sensitivity, it is quite limited in the amount of current it can switch. So you can't connect it directly to, say, a water/air intercooler pump. If you did, the switch would soon burn out. Another reason that it wouldn't last very well in this type of application is because when the pressure rises only very slowly, the switch chatters at the turn-on point. In the use we made of it for finding areas of high aerodynamic pressures at the front of the car, this characteristic was actually useful - you could tell when the switch was j-u-s-t about to turn on because it started chattering. But of course, if you wanted to control more than just a buzzer in the cabin, the chattering's no good at all.
So, before you can turn on and off things, you really need an anti-chatter mechanism (anti-bounce if you like) and also a relay interface that allows the switching of higher currents.
In this article we'll show you how to achieve both outcomes - and it will take less than 15 minutes and $8.
Electronics
Now even if you're no whiz with the soldering iron, don't panic! We'll take you through step by step how to put together this simple piece of gear, even if you've never built an electronic kit before. (And if you're a stunner with electronics, you'll prob be able to see some ways of achieving different outcomes again!)
We've used as the basis of the Pressure Switch Relay Module a Jaycar Electronics kit, the 12V DC Relay Card. It's designed to interface a low-current output (eg a LED) with a relay that can be used to switch real loads. But we've also tweaked it to include an anti-bounce function.
So, what will you need?
- The 12V DC Relay Card kit
In addition, you'll need two more components:
- 470 ohm ¼ watt resistor
- 470uF 25V electrolytic capacitor
Through the AutoSpeed Shop, all the components to make the Pressure Switch Relay Module can be bought for $7.95, a notable saving over buying them separately.
In addition, you'll need a fine-tip soldering iron, solder and some small diagonal cutters (sometimes called side-cutters).
Building It
If you aren't familiar with electronics, the first step is to identify the components. Open the kit and lay out the bits and pieces. (At this stage, keep the two extra components to one side.) What you'll see are two resistors (one marked with colour codes, the other not), one 3-legged transistor, a Printed Circuit Board (PCB) and a relay.
The first step is to mount the coloured resistor, which goes onto the PCB where the screen-printing tells you to put R2. This component can go in either way around, so simply bend the leads at right angles and then insert the wires through the holes, so that the resistor sits flat on the board.
After that, turn it over and carefully solder the wires to the pads. Don't use splashes of solder that can bridge tracks and make connections where there shouldn't be any - keep the soldering tidy. Once you've soldered the component in place, cut off the surplus lengths of wire. Mount the other resistor where it tells you to put R1, (it can also go in either way around), and again solder it into place.
The transistor is next. Mount it with the orientation shown in this pic, ie with the flat front on the body (it also has the writing on it) facing towards where the relay will go.
Next mount the relay, and solder its five legs into place.
OK, now comes the bit where you add the extra components.
Holding the board so that the relay is on the right, place one leg of the extra 470 ohm resistor in the top, left PCB hole - the one marked with a (-) symbol. Push the component right down and solder its leg into place. Cut off the surplus wire under the board and then shorten the other lead of the resistor right down so that there's just enough wire left to solder to.
Select the 470uF electrolytic capacitor (the other extra component) and carefully inspect it. One of its two wire leads will be marked with a series of negative (-) symbols down the side of the body. Solder this wire to the free wire of the extra resistor. Insert the other wire from the capacitor through the hole marked 'IN' and solder it under the PCB.
Now you've pretty well finished! Check again that the transistor is around the right way (flat front facing the relay) and that the coloured resistor on the PCB is closest to the relay. Also check that the new capacitor has its negative side connected to the (-) marked hole.
Solder the power, earth and 'IN' wires to the PCB, as indicated in this photo. These wires normally go through holes in the PCB, but because we've filled up some of those holes with extra components, just solder them directly to the pads underneath, as shown in this photo.
Incorporating the Pressure Switch
Pull the back mounting plate off the switch to reveal the terminals. Solder the wire from the 'IN' terminal of the Pressure Switch Relay Module to one terminal of the switch. Connect the other side of the switch to the +12V power lead going to the Relay Board. Check the diagram above if you have problems.
Testing
Connect 12V and earth to the board. (Don't get them around the wrong way!!). Apply a temporary light pressure to the pressure-sensing nipple of the switch. The relay on the board should close and then re-open about 2-3 seconds after you stop applying pressure. Very, very gently blow into the switch sensing port, slowly increasing pressure until the switch just turns on. There should be no chatter - the switch should either be on or off.
As you've probably figured out, the anti-bounce works by holding the relay on for at least 3 seconds each time the switch closes. So if the pressure is j-u-s-t at the switch on point and the switch is chattering, the fastest that the relay can chatter is once per 3 seconds. (If you want to increase the length of this delay, increase the value of the capacitor.)
Mounting
The Pressure Switch Relay Module can be mounted in a UB5 plastic jiffy box (eg Jaycar Electronics - $2.75 and cat no HB-6015), with the pressure switch located at the other end. When placed on its side, the relay board is firmly held in place by the lid, while the pressure switch can have its port projecting outside of the box through a drilled hole, being then held in place by double-sided tape. The wires can be fed out through holes drilled in the walls of the box. Note that the box should be mounted within the cabin - it isn't waterproof.
Using it
Configured in this way, the Pressure Switch Relay Module can be used to switch on something when there is pressure present, or switch off something when there is pressure present.
To switch something on, use the Common and Normally Open connections of the relay. ("Normally Open" means that the relay is open until power is applied, upon which it closes). This could be used to turn on a boost pressure light, for example.
To switch something off, use the Common and Normally Closed relay connections. For example, this function could be used to turn off an intercooler fan when there is enough airflow being generated by the forward movement of the car (more on this technique in an upcoming article).
The relay on the board is rated at 6 amps (ie about 80 watts). If you want to switch on very big loads (eg a large thermo fan, really big water/air intercooler pump or the like), use the Pressure Switch Relay Module to operate another, heavy-duty automotive relay as shown here.
If you want to switch off very big loads, use the circuit shown here.
Conclusion
For around $20 you can have a switch that is sensitive enough to detect the passage of passing airflow, yet still be able to switch real loads. In upcoming issues we'll show you some actual performance applications.
The pressure switch described in this article has sold out.
