by R.E., Survival Blog:
(Continued from Part 1. This concludes the article.)
Back to First Principles
What would be really useful would be a series of lights which would reflect how full the tank was, and if in addition to that, an audible alarm for high water-level. Perhaps I could build something like this using basic off-the-shelf electronic components and my rather rudimentary knowledge?
Like most reading this, I am no engineer, and no electrician. My only personal asset seems to be that, I like to tinker with stuff. So, I dug out the multimeter and an old breadboard and began to experiment. To save time for others who are trying to solve a similar problem, a description of what I finally concocted follows.
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I ended up using five NPN transistors (BC547), as shown in the schematic diagram below as having the arrow pointing away from the base towards the emitter.
Transistors have three pins: collector, base, and emitter. A transistor acts as a solid state switch turning on with a positive voltage applied to the base. To figure out which pins are which, refer to the spec sheet that came with the transistors. The BC547’s I used came in what is known as a TO-92 package which, with the flat face up, leads towards you, left to right: collector (1), base (2), and emitter (3).
By creating my probe using a fiberglass rod, I placed six bands of copper, at the bottom and at 9 inch intervals. The bottom band was wired to the plus (+) power supply. At heights of 9, 18, 27, 36 and 45 inches each was wired to the base of an NPN transistor through a 330 ohm resistor.
The fiberglass rod itself was salvaged from left-overs from a six-foot-high chainlink fence project. It turned out this was the perfect length for covering the depth of this cistern. Bands of copper were created using strapping of the sort used by plumbers. Stainless steel machine screws were used to attach each copper band to a soldered terminal connector on each level wire – six wires in total.
Each emitter of each of four NPN transistors connected directly to a green light emitting diode (LED). The LEDs are oriented shown with the bar on the symbol connected to the negative (-) power supply. On my LEDs the longer leads were the negative.
Each NPN collector connects to the positive power supply via a 220 ohm resistor.
For the overflow indicator, I might not always want something that makes noise, so placed a switch on the overflow buzzer, connected in parallel with an unswitched red LED. The red LED will always come on when the cistern is close to being full – the buzzer can be switched off.
One additional LED, yellow, indicates that the power supply is on. So, even when the cistern is empty a working device will always show yellow.
I started out using a 9 volt DC regulator (LM7809) to power the circuit. In my small tank circuit testing, 9 VDC was sufficient. But, I subsequently determined via trial and error that a 12 volt DC regulator (LM7812) was more appropriate for the cistern application. The DC voltage regulator allows this device to be powered by my solar panel battery system which runs 23vdc through 30vdc.
The tab of the voltage regulator LM7812 on the table extending out the top, the pins towards me, left to right, are input (1), negative/ground (2), and output (3). I supplied DC from my solar power system battery, nominal 24 vdc (+) on the input pin 1, 12 vdc (+) available on the output pin 3 connected to the water level circuit, with the negative conductor from the 24 volt battery (-) tied to pin 2.
The next question I needed to settle was how much of a heat sink would I require on the LM7812? With the tank full, I measured a maximum power consumption of 6.5 watts, translating into roughly half an amp of current draw at 12 volts. The spec sheet for the LM7812 rates the max current at about 2 amps if adequately sunk. Therefore, I figured that not much of a heat sink would be required.
