| The Smarter home-made Continuity tester |
Unlike other continuity testers, The Smarter Continuity Tester allows the testing voltage to be much lower than the voltage used for actuators and the notifiers that there is a short, and selectable too. It also avoids the possibility that a small discharge in circuit could cause a potential difference and a false reading.
The system is simply a dual stage voltage divider, and a comparator (op-amps work fine and that's what im using eg. lm358). The first divider lowers Vcc (eg 3v) down to the chosen testing voltage (eg 1v), which is then halved with the second divider (1v). the comparator (op-amp) receives the 1v into Non-Inverting-input (+) via the testing leads when connected by a short, otherwise pulled down by a 10k to ground. The Inverting-Input (-) is connected to the second division directly. The comparator is powered by Vcc.
This means when there is no short the comparator outputs 0V as the Non-Inv-inp is less than the Inv-Inp. as soon as the Non-Inv-Inp is greater than the Inv-Inp ie the leads are connected by a short the output is saturated to Vcc - A selectable Test Voltage that always outputs Vcc when shorted!
I would recommend a regulated Vcc so the values are constant, but the idea is that all values are selectable, and can therefore be changed by simply modifying resistor values.
The outputted Vcc can be connected to an led directly or multiple with a transistor. An RGB led could also be used, along with a not gate so a green could be shown if there is No short, and a red if there is or vica versa.
Included are schematics for the basic system as well as an add-on led board with a LED and a NOT LED.
Png schematc as well as board files in eagle are also provided.
MAIN BOARD - r1-r4 are selection resistors, r5 is 10k, designed around LM741.
LED BOARD - transistors - npn (bc547), r1 - 10K - led - 3mm