CONTINUITY TESTER
INSTRUCTION SHEET
Check Your Kit
Schematic Diagram
PCB Layout
This project will determine:
1. Power available at battery and polarity - Connect both clips to battery in the correct polarity : Both LEDs will light.
2. That a test point is earthed - Touch the test point with the probe : One LED will go out.
3. That a test point is alive (has power available)- Touch the test point with the probe : The other LED will go out. With a little ingenuity it can be adapted to show continuity also.
Technology
1. POWER AVAILABLE and POLARITY :
When the clips are connected to the battery in the correct polarity
current flows through the resistors and the LEDs. The
polarity of the LEDs is such that the
flow of electrons (current) between the battery poles is not
blocked. The 560R resistors meter the current to the
LEDs, both of which light up. Total resistance in the
Circuit is found by R1 + R2, viz. 560 + 560 = 1120 OHMS. You could verify
this with an OHM-meter. Taking the conventional current
flow, and working from the Positive end,
the resistance measured between the +ve clip and the probe will be 560
Ohms. Between the +ve and -ve clips the resistance
is 1120 Ohms.
2. The EMF must overcome 1120 Ohms to force current right around the circuit. If the probe is touched to earth then the EMF must overcome only 560 Ohms to reach the 0Volts battery pole (through the earth point). The current will flow to earth at the test point, but through the red LED and its resistor. This LED lights up. The other LED has its power robbed by the earth point so it goes out. Remember that current will ALWAYS take the path of least resistance.
3. If the probe is touched to a point carrying say 12Volts then this supply has to overcome only 560 Ohms at the green LED so it lights up sourced from the supply at the test point. The battery supply through the +ve clip has to overcome the full 1120 Ohms. The lesser resistance at the green LED offers the path with less resistance so the flow from the +ve clip ceases and the red LED goes off.
THE MESSAGE THE CIRCUIT TESTER IS TELLING YOU MUST BE INTERPRETED CORRECTLY.
TOTAL RESISTANCE IN THE CIRCUIT : (Resisters in series) = R1 + R2 = 560 + 560 = 1120 Ohms
CURRENT FLOW IN THE CIRCUIT : OHM'S LAW : I = E divide by R
I is Current in Amps
E is EMF in Volts
R is resistance in Ohms
I = 12 divide by 1120 = 0.011 Amps = 11 uA
THE RATED CURRENT DRAW FOR A LED IS 20uA. This is achieved (approximately) by using 560 Ohms with 12 Volts. A lower resistance will increase the current and could blow the LED. The metering resistance must always be calculated using OHM'S LAW.