Update(2010-10-17) I’ve made some changes to the PCB design. There is now a RC filter on the sense input and a polarity protection diode on the power supply. I’ve also corrected some parts values. The changes are available from the GIT repository.
My parents are very keen on 4×4 and getting away into the bush. They have two lead acid batteries in the Land Rover, the main one and a second battery for running the fridge, lighting etc. What they wanted was, a small device that could detect when the vehicles engine was running and after a set time-out start charging the second battery. The design presented here detects the charging voltage of the alternator and will switch a low current relay after a set time. You can use the relay to control a high current contactor or other solid state switch.
Theory of operation
Looking at the schematics you can see there is not much to this design. It only consists of 4 ICs and a couple of caps and resistors, but I’ll try explain how the system works.
When your car’s engine is off the battery voltage is around 12v (if you have a 12V lead acid battery), but once the engine is running the alternator starts charging the battery and the voltage rises to 13.5V – 14V. This rise in voltage is detected by the comparator IC1. Once the Sense voltage rises above the set point the comparator resistor R6 pulls the CONTROL line high and triggers the monostable 555 timer IC3A. The output of IC3A is active low, so the TIMER line is held high while the 555 timing. The TIMER line powers the timer circuit of the Astable configured IC3B, this blinks LED2 to show the user that the timer is actively counting down, it blinks at about 2HZ.
The comparator set point can be adjusted with R3, the IC3A time-out via pot R8, and the rate LED2 blinks with R10,R11 or C8.
The 74HC00 Quad NAND gate controls the logic behind LED2 and activating the relay K1. The following truth table describes the states of the system.
Remember that LED2 is active low and the Relay is active high.
The PCB was laid out as a single sided board. The wide track widths makes this board very easy to etch at home.
I used Eagle3D to generate the 3D renderings of the board. All the parts are there except the relay, I couldn’t find a suitable model for it. I hand edit the POVRay file to make sure all the components are in the correct orientation and position. You can download the POVRay file from the git repository (see bottom of this page).
Etching and building the prototype
I used a piece of CIF presensitised PCB. The board has a UV sensitive coating on it, you expose the board to a positive image of the design you want to etch. Where ever the coating is exposed it washes off during development. Once the board has been developed you can etch it.
As you can see in the image below, there was a small mistake with the layout of the PCB. The power supply pins of the comparator (IC3) where reversed on the schematic and I only noticed it after etching. I guess that’s why you build prototypes!. The fault has been corrected in the schematics and the PCB layouts.
You can download the schematics, board layouts and POVRay files from the git repository.
git clone git://stewartallen.org/dual_battery.git
or browse the repository here.
Dual Battery Switcher by Stewart Allen is licensed under a Creative Commons Attribution-Non-Commercial-Share Alike 2.5 South Africa License.
Based on a work at git.stewartallen.org.