In a past few weeks, I started to realize, that it is much of an effort I have to make, in order to check circuit's continuity... Cut-off wires, broken cables are such a big problem, when every time there is a need to pull out multi-meter from the box, turn it on, switch to the "diode" mode... So, I decided to to build one by myself, in a very simple way, that would take me 2-3 hours to make it.

So, Let's build it!

Step 1: Parts and Instruments

I.Full list of a components, some of them are optional, due to needless functionality (Like a on/off indicator LED). But it looks good, so it is recommended to add it.

A. Integrated Circuits:

1 x LM358 Operational Amplifier
1 x LM555 Timer Circuit

B. Resistors:

1 x 10KOhm Trimmer (Small package)
2 x 10KOhm
1 x 22KOhm
2 x 1KOhm
1 x 220Ohm

C. Capacitors:

1 x 0.1uF Ceramic
1 x 100uF Tantalum

D. Other Components:

1 x HSMS-2B2E Schottky Diode (Can be used any diode with small voltage drop)
1 x 2N2222A - NPN small signal transistor
1 x LED blue color - (Small package)
1 x Buzzer

E. Mechanical and Interface:

2 x 1.5V coin-cell batteries
1 x 2 Contacts terminal-block
1 x SPST Push-putton
1 x SPST Toggle switch
2 x Contact wires
2 x Endpoint knobs

II. Instruments:

Soldering Iron
Sharpening file
Hot-glue gun
standard gauge wires
Soldering tin
Electric screwdriver

Step 2: Schematics and Operation

To make it simple to understand circuit's operation, schematics are divided into three parts. Each part explanation corresponds to a separate operation block.

A. Comparison stage and idea explanation:

In order to check wire's continuity, there is need to enclose electric circuit, so the stable current will flow through the wire. If the wire is broken, no continuity will be present, thus current will equal zero (cut-off case). The idea of circuit that is shown in the schematics, is based on voltage comparison method between reference point voltage and the voltage drop on a wire under test (Our conductor).

Two device input cables connected to the terminal block, since it's much easier to replace the cables. Connected points are labeled "A" and "B" in the schematics, where "A" is compared net and "B" connected to the circuit's ground net. As seen in the schematics, when there is disruption between "A" and "B", voltage drop will occur on the "A"-split components, therefore Voltage on "A" becomes greater than on "B" thus comparator will produce 0V at the output. When the tested wire is shorted, "A" voltage becomes 0V and comparator will produce 3V (VCC) at the output.

Electric operation:

Since the tested conductor may be any type: PCB trace, power lines, regular wires, etc. There is need to limit the maximum voltage drop on the conductor, in the case we don't want to grill the components that current flows through them in a circuit (If 12V battery is used as a power supply, 12V drop on FPGA part IS very harmful). Schottky diode D1 pulled-up by 10K resistor, maintains constant voltage ~0.5V, the maximum voltage that can be present on a conductor. When conductor is shortened V[A] = 0V, when abrupted, V[A] = V[D1] = 0.5V. R2 splits voltage drop parts. 10K Trimmer is placed on the comparator's positive pin - V[+], in order to define minimum resistance limit that will force comparator unit to drive '1' at it's output. LM358 op-amp is used as comparator in this circuit. Between "A" and "B" SPST push-button SW2 is placed, in order to check device operation (if it is working at all).

B: Output signal generator:

Circuit has two states that can be determined: either "short-circuit" or "cut-off". So, comparator's output is used as enable signal to the 1KHz square wave generator. LM555 IC (available in small 8-pin package), is used to provide such wave, where comparator's output connected to the RESET pin of LM555(i.e. chip enable). Resistors and capacitor values adjusted to 1KHz square wave output, according to the recommended manufacturer values (See datasheet). LM555 output is connected to the NPN transistor used as a switch, making buzzer provide audio signal at the appropriate frequency, every time when "short circuit" is present on the "A"-"B" points.

C. Power supply:

In order to make device as small as possible, two 1.5V coin-cell batteries attached in series are used. Between the battery and the VCC net on the circuit (See schematics), there is SPST on/off toggle switch. Tantalum 100uF capacitor is used as regulating part.

Step 3: Soldering and Assembly

Assembly step is divided into 2 essential parts, first describes soldering the main board with all the internal components, and second expands about interface enclosure with all the external components have to be present - LED on/off indicator, on/off toggle switch, buzzer, 2 fixed probe wires and device check push button.

Part 1: Soldering:

As seen in the first picture in the list, aim is to make board as small as possible. So, all the ICs, resistors, capacitors, trimmer and terminal block are soldered in a very close up distances, according to the enclosure size (Depends on total size of the enclosure you would choose). Make sure, terminal block direction is pointed OUT of the board, to make it possible to pull fixed probe wires from the device.

Part 2: Interface and Enclosure:

Interface components should be placed in appropriate areas on the boundary of enclosure, so it will be possible to connect between them and main internal board. In order to make power supply controlled by a toggle switch, the connecting wires between the toggle switch and circuit/coin-cell batteries are placed outside the main board. In order to place rectangular objects, like a toggle switch and terminal block inputs, where it is located, it was drilled with relatively big diameter bit, when rectangular shape was cut with a sharpening file. For buzzer, push button and LED, since they come with round shapes, drilling process was much more simple, just with a different diameter drill bits. When all the external components are placed, there is need to connect them with a thick, multi-torsional wires, in order to make device connections more robust. See pictures 2.2 and 2.3, how the finished device looks after the assembly process. For the coin-cell 1.5V batteries, I've bought small plastic case from eBay, it is placed just underneath the main board, and connected to the toggle switch according to the schematics description step.

Step 4: Testing

Now, when the device is ready to be used, final step is calibration of state, which could be determined as "Short Circuit". As it was previously described in the schematics step, trimmer's purpose to define resistance threshold value, that below it, short circuit state will be derived. The algorithm of calibration is simple when resistance threshold can be derived from a set of relations:

V[+] = Rx*VCC / (Rx + Ry),
Measuring V[Diode]
V[-] = V[Diode] (Current flow into op-amp is neglected).
Rx*VCC > Rx*V[D] + Ry*V[D] ;

Rx > ( Ry*V[D]) / (VCC - V[D])).

This is how minimum resistance of the tested device is defined. I calibrated it to reach 1OHm and below, so the device would indicate conductor as "Short Circuit".

Hope you'll find this instructable helpful.

Thanks for reading!