Coffee Machine Water Level Gauge

Specification

This design uses a standard 0->190 ohm fuel/water level gauge and lights some RGB LEDs in a sequence depending on the level.

The design was fitted to our coffee machine, with the level gauge in the water tank, and the LEDs mounted underneath. When it starts glowing red, it’s time to top up. We did this because the machine is on a timer which comes on every morning. It takes 15 minutes or so to warm up, so it’s really annoying if it has run out of water.

Power	12V 10W max
LEDs	1W RGB LEDs, 2 in series
LED current	0 – 200mA PWM, 100Hz
Colours	8 bit RGB, gamma corrected
Level look up table	8 steps (to match the 8 steps of the level gauge)

Circuit

Technical Description

LED driver

The LEDs are driven with a simple NPN transistor and 30 ohm series resistor from 12V. Assuming 3V LED Vf, this gives around 200mA LED current. The 30 ohm series resistor was made from 2 off 15 ohm 1W resistors in series, but you can use whatever you have available with suitable power rating. This isn’t very efficient but a lot simpler than using a switch-mode LED controller. If you have suitable LED controllers available, you can input the PWM signal into them instead.

Level Gauge

The level gauge is based on a sliding magnet in the liquid, and reed switches within the rod. The type I used has 8 steps of 0 to 190 ohm. 0 ohm when empty, 190 ohm when full. This is connected in a potential divider running from 12V to obtain a range of 0.5 to 2V, which is input to the microcontroller comparator.

There is a microswitch under our water tank, so we made sure it goes red well before this switch activates. This was done by checking the gauge reading at the point the switch activates.

You can order such gauges on Amazon from China in around 30 days..

Level Gauge ADC

The level gauge voltage is input to the positive side of the comparator of the microcontroller. The negative side is connected to a DAC. By adjusting the DAC until the comparator toggles, we form an ADC which can read the level gauge voltage.

The DAC is formed by low pass filtering a PWM signal from pin RB5. This runs at 100Hz, using the same software routines that generate the PWM signals for the LEDs. This PWM DAC has an 8 bit control range.

The software has a loop which increments the PWM DAC when the comparator is high, and decrements when low. This loop runs every approx. 100ms, so it takes around 26 seconds for the level gauge ADC to ramp from one end to the other. This is fast enough for the level gauge, which only changes slowly.

LED PWM

The LED driver is driven by the RB1, 3 and 4 pins at 100Hz. An interrupt routine runs every 10ms to generate this PWM. The internal microcontroller PWM generators are not used as they cannot generate the 4 independent PWM signals required by this application. Note an initial design at 50Hz was too flickery.

The level gauge ADC reading is looked up in a table of RGB values, and applied to the LED PWM settings. The window for each step is chosen to match the steps of the level gauge to avoid the design flickering between two colours.

Serial Port

The circuit shows a serial port input. This is not used in this application, but is there for debug purposes or other applications. The software would need modifying to use this, as currently the level gauge overrides the serial port values.

Software

Software was written for PIC 16F627 using MPLAB X 5.25. You can use other PICs/microcontrollers and build tools. I used 16F627 as I had some spare. Other microcontrollers might make the implementation easier if they include enough PWM generators and an ADC.

The software can be found here as a zip file.

It is based on previous software, with code added to implement the 8 colour steps as per below.

The PWM frequency was increased to 100Hz as I found 50Hz far too flickery.

With the serial port code removed, the memory usage is as follows.

Memory Summary:

Program space used 308h ( 776) of 400h words ( 75.8%)

Data space used 23h ( 35) of E0h bytes ( 15.6%)

EEPROM space used 0h ( 0) of 80h bytes ( 0.0%)

Data stack space used 0h ( 0) of 50h bytes ( 0.0%)

Configuration bits used 1h ( 1) of 1h word (100.0%)

ID Location space used 0h ( 0) of 4h bytes ( 0.0%)

The microcontroller is mounted in a socket and removed to program it. I used a Vellaman K8048 programmer with PicProg2009 running on my PC. There are more modern programmers available!

Level Gauge Steps

Level gauge configured as potential divider.

Resistance is 0-190 ohms with 8 steps as measured below.

Top resistor 1k2 to 12V. Bottom resistor 51 ohm to give offset of 0.5V to ensure comparator works.

Vpwm is 2.5V as I use a potential divider of factor 2 (two 10k resistors) to better match the range of the voltages from the gauge.

Rlevel	Rtop	Rbot	I	V	PWM	PWM decimal	PWM low	PWM high	LED Colour
0	1200	51	9.6E-3	0.489209	19.57%	50	0	73	RED
51	1200	51	9.2E-3	0.940092	37.60%	96	73	104	RED
71	1200	51	9.1E-3	1.107413	44.30%	113	104	121	BLUE
90	1200	51	8.9E-3	1.261745	50.47%	129	121	137	GREEN
110	1200	51	8.8E-3	1.419544	56.78%	145	137	153	GREEN
130	1200	51	8.7E-3	1.572773	62.91%	160	153	172	GREEN
160	1200	51	8.5E-3	1.794472	71.78%	183	172	194	WHITE
190	1200	51	8.3E-3	2.00694	80.28%	205	194	255	WHITE

The PWM high and low values show the window to avoid sitting on a step.

The following table implements the above steps in the program.

//Table of colours for each level step. There are 8 steps.

const unsigned char pwmdac_max[]={73,104,121,137,153,172,194,255};

const unsigned char led1_steps[]={200,200,0 ,0 ,0 ,0 ,180,180}; //red

const unsigned char led2_steps[]={0 ,0 ,0 ,200,200,200,180,180}; //green

const unsigned char led3_steps[]={0 ,0 ,200,0 ,0 ,0 ,180,180}; //blue

You can change these tables to match different level gauges with different steps, or to change what colours are used.

Construction

I built this on matrix board and bolted it under the drip tray of our coffee machine.

The LEDs are mounted on standard metalback PCBs which you can buy online. These were mounted on some matrix board. Here’s a couple of photos of the part finished matrix board:-