Fluid Level Sensor
Author
D18 forum member misfitjackie (Jack) is NOT a qualified electronics technician and such can give no concrete advice in this area. This document is only a guide and should be used as such. Where there are concerns regarding electronic devices, please refer to your own electronics expert. Any other queries, feel free to email me.
Design Requirements
A general purpose low/high fluid level sensor was required during construction of D18 and brute force electrolyser units, as well as for header tank monitoring. Basic requirement was to be able to monitor the level of electrolyte/fluid and if the level dropped below a certain point (for a certain amount of time to allow settling), switch a pump or solenoid on to allow external supply to refill the level to the maximum point, then switch it off when high sensor reached.
Simple idea, but certain situations made sensor setup more critical to prevent from over/under filling. Design had to allow for fluid sloshing about while the vehicle was in motion or being more against one side/end of the unit depending on the vehicle attitude in relation to normal fluid level. These latter situations happen when the vehicle is ascending or descending a long incline (tilted up or down), or is affected by a large positive or negative camber on the road (tilted sideways). Multiple high and multiple low sensors at opposite ends allow an averaging effect to accomplish this.
The 1-2 second time delay is critical to stop switching on/off constantly as the fluid splashes around. In effect the low level should ONLY be registered if ALL sensors are out of water for the whole delay time. This would normally only happen on a long level stretch of highway, idling at the lights, when starting up on level driveway, or the level has actually fallen low enough to register while some minimal movement still evident (low-low).
Circuit Design
The circuit was designed by D18 forum member Me262 starting with other circuits available on the Internet. Others designs relied on a relay doing the actual switching of the pump. Me262’s approach was to replace the relay with a power transistor – no moving parts, less to go wrong. Initial designs did not allow for high/low monitoring but instead a simple one sensor switch. Several designs were worked through until the present design evolved which, although simple, meets all project requirements.
Jack's way - two pair sensors - current Me262 circuit schematic
Printed Circuit Board
Current misfitjackie pcb design is a two-up format (with three led boards if required).
Individual requirements on where the circuit is to be mounted will dictate the actual size and shape of the final pcb. If the small led mount boards are not required, cut them from the pcb image before printing out. The small led boards can either be mounted in the lid of an enclosure, or remotely in the cabin of the vehicle.
The same procedure is needed to print the component side but you don’t need the step that dissolves the copper tracks. The component side of the board is in reverse ready to print the layout on the top of the board if required, it will be the right way when complete. If you trim both printouts to use the same corner for board registration, you can get pretty accurate component placement.
ExpressPCB online manufacturer
Pcb's can be ordered online. ExpressPCB source file can be supplied for online manufacturer ExpressPCB, who would manufacture boards professionally. If source file is required please email [email protected].
DIY - Creating the pcb at home (cheapest way)
It’s easy, all you need is a laser printer, an image of the board, a small amount of ammonium persulphate or ferric chloride (from local electronics stores) and a blank copper plated pcb board (plus a small tray, rubber gloves, spare time and perseverance).
View these URL’s to see a couple of methods on how to create your own pcb’s at home – it’s so easy…
The author tested a few types of paper that were available. Found personally that Epson matt photo paper and HP gloss photo paper both provided reasonable results. When a mistake was made or one type of paper didn’t work properly it wasn’t a problem – thinners removed the toner and gave a clean board to start again with.
The hardest or tedious part of producing your own pcb board is to drill out all the tiny little holes required (author’s opinion)
Here are a few photos of the authors first attempt at home made pcb boards:
Photo1 Photo2 Photo3 Photo4 Photo5 Photo6 Photo7 Photo8 Photo9 Photo10
If you cannot get a clean image from the circuit images above, or you require the ExpressPCB source file, email me..
Building the sensor unit
Electronics knowledge required
This is a beginner’s project. The only real care required is when soldering the transistors. If you have not any experience building electronic kits, it is suggested to have a look through Patrick Kelly’s electronics tutorial. This is an excellent document for learning the basics.
Each target system will determine if you require a small heat sink on the TIP 3055 or not (author mounted straight to aluminium case instead – bit cramped, will choose a larger box for next unit)
Resistors are recommended to be ½ watt if proposed windscreen washer pump draws a few amps (max 4 amps). If only a small electronic solenoid to be used that draws less than an amp, ¼ watt resistors should be ample.
Here are a few photos of the author's initial test unit being built:
Photo1 Photo2 Photo3 Photo4 Photo5 Photo6 Photo7 Photo8 Photo9
Finshed unit ready for vehicle mounting:
Testing the sensor unit
It’a good idea to test everything is working OK before mounting your pcb into the case/box. That way if you’ve missed a solder point, wire or whatever it is easier to fix.
A simple circuit test can be done with a small 12V 5 watt auto taillight globe in place of a pump/solenoid, and a small container of tap water.
Mount the 5w auto light across the pump wires
Place the 12v sensor return wire in the small tub of water and clip to side to keep out of way. Place both low and high sensor wires in the water, connect 12v & gnd to 12v battery. The pump light may flash quickly then power applied but that is normal.
Withdraw the high sensor wire from the water – no change.
Withdraw the low sensor wire from the water – pump light should come on.
Place low sensor wire back in the water – pump light should still stay on.
Place high sensor wire back in water – pump light should go out.
If all went alright, then at this point it is OK to mount the unit in your pcb container/box.
Sensor requirements
It is presumed the electrolyser/header_tank will be mounted in a level plane in the vehicle. If this is the case, the sensor bolts can be mounted once the range (if any) is determined. To cover all possibilities of vehicle attitude (no, not good or bad), it is recommended that four sensors/bolts be used in all.
Header Storage Tank
Only two sensor/bolts plus a return +12v are needed in the storage tank situation.
Electrolyser
Two pairs of high amd low sensors are required.
Suggested electrolyser arrangements
NOTE: Where electrolysers have insulated cases, a +12v return sensor is required near the bottom of the tank (or at least below the low sensor).
Vehicle wiring
The sensor unit must wired to the vehicle using a relay to isolate it so that it is only switched on, when the ignition is on.
If using this unit with a D18 type cylindrical stainless steel electrolyser, the 12v return sensor is not required. Positive 12v return path available within the cell itself. The sensor unit can be wired up to the D18 +12V power as that already would be wired with a relay isolator.