Price - Stock No 550


This kit measures battery voltage.  The red LED (closest to the IC) indicates a low status - 11.5Volts or less.  The yellow LED indicates 50% charge - 12.0V.  The green LED indicates 12.6V or full charge.

This kit also measures the charging status.  Turn the charging system on - or start the vehicle.  The red LED (at the extreme end from IC) indicates overcharge - 15.5V.  The green LED indicates maximum charge voltage - 14.5V.  The yellow LED indicates normal charging voltage - 13.2V.


PCB Layout


  1. Inspect the tracks for fine breaks and test the continuity of each track with  an  electronic  circuit  tester  or  ohm-meter.
  2. Identify the resistors using a multimeter or the Resistor Colour Chart
  3. Bend  the  legs of the resistors to match their respective holes and push them through from the plain side of the P.C.Board.  Solder in place - refer Soldering Technique   
  4. The two 9.1V Zener diodes can be bent to shape and  mounted  with their band to the K end of the space. - see the placement sheet.
  5. The 16 pin  IC  socket can be pushed carefully into place. Solder in place.
  6. The trimpot can now be inserted and soldered.
  7. Two pins can be pushed into the holes where the leads connect.  These can be soldered off. 
  8. The six LED's can be inserted observing polarity and colour position, then solder in place.
  9. Using wire strippers, bare and tin the ends of the red and black wire - see refer Tinning wire.  
  10. Solder the clips to each wire to finish the leads.
  11. Solder the red lead to positive and black lead to negative.
  12. Establish the correct orientation of the ULN2003 with the locating dot on the IC and on the diagrams.  Insert the ULN2003 into the socket with care.
  13. The 12 V Battery Tester is now complete and ready for calibration.


A multimeter will be required to complete calibration.

  1. With the multimeter set to measure the DC Volts range up to 20V.  Using a charged 12V battery, connect the multimeter to the terminals, observing polarity. 
  2. Record the voltage.  If the voltage is less than 12V then choose another battery.
  3. Refer to the schematic diagram and choose the voltage and LED which best represents the battery voltage (as read on the multimeter).
  4. Connect the 12V Battery Checker and adjust the 5K trimpot until the chosen LED just begins to illuminate.
  5. To optimise the calibration, repeat on a couple of batteries.

Trouble shooting

Troubleshooting  if necessary will involve careful checking of locations and polarity of components, the diodes and that the locating DOT on the ULN2003 is as the drawing shows. Re-solder all joints and check to make  sure  you  have  not  bridged across  between  any  two adjacent component legs.  The end  of  a  broken  hacksaw  blade  sharpened  on  an  emery wheel is a good tool for cleaning between soldered joints.

Technical notes

First we must understand what the ULN2003 is. Datasheet for the uln2003 .

In essence, seven darlington transistors.

A darlington transistor is made when two transistors are used in the fashion shown in the diagram below.  It is a useful combination as it allows a large current flow from the output to ground but is triggered by a small current flow from the Input to ground ie. a voltage is applied to the input.

The simplified diagram for this type of darlington configuration also appears below.  This symbol is widely used in digital electronics and is also known as an inverter (or inverted buffer).  It is so called because it inverts the input signal, When input is HIGH, Output is LOW and vice versa.

So now we have a simplified diagram for the ULN2003.

The 11.5V LED is illuminated from a low voltage (around 3V) and shows the unit is working even if the battery is almost completely flat. 

From the schematic diagram, we can observe that the outputs of five of the inverters in the ULN2003 are used to activate their respective LED by taking the K leg of the LED to ground.   Each input of the inverters are fed from the junction of the 9V1 Zener Diode and the 5K trimpot, this forms a voltage divider with adjustment to calibrate the 12V Battery Tester.  This junction point is fed to the inverters through resistors which are specifically chosen to activate each buffer at the required voltage.  These resistors form a second voltage divider when combined with the internal resistances of each inverter in the ULN2003 (shown above).

Some of the resistors have been used in series or parallel to achieve the required resistance.  If time is taken to calculate the total resistance of these series / parallel networks, you will find that the resistances vary from 12K for the 15.5V LED to 50K for the 12.0V LED.  See Resistors .

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