This is the second try for LOW OHM METER
I added three ohms range 01 ohm. 010 ohm and 0— 100Ohm.
It was easy with dual amplifier MCP6442.
Then I heard, that others has use LiOn battery as power source.
It was not quite easy, still LCD uses 5V power supply and the battery works about 2-5 to 4.1V.
Then I used DC/DC converter, which I have used about three years.
It is very small pin SMD-device and very cheep.
It works also very well.
By DC/DC converter is not enough.
It needs also battery charger.
USB connector is the most easy to find.
But it needs also a charger, which limit charging voltage to 4.1V.
The charger is normally 5V, so it needs only current limiting resistor (5-4)/ 0.1A= 10R.
But how to limit the voltage?
There are lots of schematics in the Internet.
The Michochip MCP73831 is a small and cheap charger. but it is not available for half of year.
I need only the the switch off voltage.
do I need a reference device like TL431?
It need lot of stuff, so I decided that I use a normal P-Pet with potentiometer.
The Fet are not exactly precise how the connect , so I added a trimmer resistor how to connect the FET on.
Then I need the current generator for measuring the ohms.
I thought thät it will measure also positive voltages and I connect a N-FET to switch current generator off.
The voltage measurement needs many ranges.
How is it done with on double amplifier?
Fist is 10 * voltage range amplifier, which is done for ohms measuring.
It can measure open circuit voltage up 0.5V.
Then there is direct measuring range up to 5V.
Then I put only two resistor to decrease the voltage to 50V.
How is the voltage display?
I selected a little smaller LCD, a 2*8 characters LCD,
It has a LED back light, but it has not always be on.
CPU a the my normal 28 pin SMD device Microchip PIC16F886.
It has two buttons, which can select
- a low ohm measurement ( 0-100mohms)
- a medium ohms measurement ( 0-1 ohm)
- high (?) ohms measurement ( 0-10ohm)
- a low DC voltage measurement ( 0-0.5V)
- a medium DC voltage measurement (0-5V)
- a high voltage measurement ( 0-50V)
- select LCD light on/off.
Here is the schematic diagram
Here is PCB
Here is the black diagram of the meter.
Pekka OH3GDO
