As electronics enthusiasts, we can never have too many diagnostic tools at our disposal. This particular project is compact enough to fit into a toolbox or even a shirt pocket, as it’s only the size of a business card. Remarkably, it contains three separate diagnostic tools all integrated into a single small PCB.
Each of these tools serves a unique diagnostic purpose, and for simplicity, the status is indicated using LEDs where applicable:
- Continuity/Diode Test (Dual LED): This function allows you to test for continuity in circuits and check diodes.
- 1kHz Square Waveform Generator (No LED): Useful for generating a steady 1kHz square wave signal for testing purposes.
- Contactless AC Voltage Indicator (Single LED): Detects the presence of AC voltage without needing to make direct contact, indicated by a single LED.
The entire unit is powered by a 3V coin cell, ensuring portability, with an option to connect an external power source ranging from 1.5V to 5V. Given the size and the specific applications this project is designed for, there are no adjustable components included to compensate for component tolerances, as ultimate accuracy is not the goal. Instead, it’s a practical, portable aid providing go/no-go indications and simple capabilities, not intended to replace more advanced quantitative tools like digital multimeters (DMMs) or oscilloscopes.
Specifications:
- Size: 50mm (D) x 90mm (L) x 6mm (T)
- Weight: 21g
Supplies
TLC551CD SOIC8
TSZ122 (Dual OPAMP) SOIC8
BAS286 (Schottky diode) SOD80
FDN5618P (PFET) SOT233 – Qty 2
BC849B (NPN) SOT233 – Qty 3
LT3092 Current Source SOT223
1MR resistor 1206 package
100KR resistor 1206 package – Qty 3
10KR resistor 1206 package – Qty 3
5K1 resistor 1206 package
2K resistor 1206 package
1KR resistor 1206 package – Qty 4
470R resistor 1206 package – Qty 3
8k2 resistor 1206 package
82R resistor 1206 package – Qty 2
20k resistor 1206 package
2k2 resistor 1206 package
100nF capacitor 1206 package – Qty 5
10nF capacitor 1206 package – Qty 3
1uF capacitor 1206 package – Qty 2
20mm coin cell Battery Clip CR2032
All surface mounted components
May prove more cost effective to buy a range of values rather than individual values unless you already have them available. Some components may also have a MOL greater than the quantity specified in the component list.
No affiliation to any of the suppliers, feel free to obtain the supplies from your preferred supplier if applicatble.
Links valid at the time of publication.
Tools
Soldering Iron
Solder
Tweezers
Magnify
Know your tools and follow the recommended operational procedures and be sure to wear the appropriate PPE.
Step 1: Contactless AC Voltage Test
The Contactless AC Voltage tester consists of three BC849B transistors connected in cascade resulting in the final gain being the product of the gain of each transistor. Overall gain = Gain1 * Gain2 * Gain3.
For the BC849B transistor gain averages 220, giving an overall average gain of 10.48E+6. Due to gain variation between transistors within the same gain band the overall gain may be higher of lower than this.
The AC voltage being detected (~3*Vbe min), is isolated from direct contact with the transistor base via the source wire insulation and the resistivity of the air at ~2x10E16 (Ωm), subject to temperature and humidity resulting in a very low base current.
The antenna rather than a separate length of wire is an integrated copper trace coil on the reverse side of the PCB.
In this example Ib1 = 100pA would become Ic3 = ~1mA.
The resistors in the first two transistors limit the maximum collector current.
1MR < 3uA as VR1 = 3V – (Vce + Ve [the emitter is ~2* Vbe’s above ground]).
100kR < 30uA as VR2 = 3V – (Vce +Ve [the emitter is Vbe above ground]).
470R < 6.4mA (Subject to Vce and the LED).
Due to the low battery voltage of 3V powering the circuit and to reduce current consumption it is recommened to use low current high efficiency LED’s.
Usage simply requires turning the test tool on and placing it with the antenna in close proximity to the cable or wire connected to a mains AC supply upon which the LED will illuminate.
Step 2: 1kHz Square Wave Generator
The signal generator can be used for troubleshooting in audio and IF stages due to the infinite number of odd harmonic frequencies extending beyond the fundamental.
The square wave generator is based around a Timer (555 or equivalent), using one resistor and one capacitor to create a 1khz square waveform with 1:1 mark space ratio.
In this configuration the Control Pin is connected to a capacitor to decouple this pin from external noise, this pin can be used to vary the threshold voltage externally.
The Reset pin is connected to V+.
Trigger and Threshold are connected together and these are connected to one input each of the 2 internal comparators.
Each comparator has a 2nd input which is connected to one of the tap points of the three 5k resistors in series. The negative input of one comparator is set at 2/3 V+ whilst the positive input of the other comparator is set at 1/3 V+.
The midpoint of the capacitor, resistor (CR), network connects to the two connected points of Trigger & Threshold with the resistor connected to the output.
Decoupling capacitors (1uF & 100nF), are connected at the device supply.
If we start with the assumption that the output is High (it could equally have started Low), the capacitor begins to charge via the resistor until the voltage reaches 2/3 V+ this causes the Flip Flop to change state and switches the output Low. The capacitor discharges via the resistor until the voltage reaches 1/3V+ causing the Flip Flop to change state and the output goes High.
As long as power is applied the output will switch alternatively high then low indefinitely.
For the required frequency the CR values are 100nF & 7K1 (comprising 5k1+2k).
Frequency can be calculated from the following:
Frequency = 1/(1.4*CR) = 1/(1.4*100nF * 7K1) = 1006.03Hz. Measured as 1008Hz.
The output of the timer has a 1KR series resistor connected to prevent excessively loading the output if connected to a short circuit.
Step 3: Continuity/Diode Tester
The continuity/diode test as its name indicates checks continuity <=2R and diodes.
Indication is given by two LED’s one per function to the following table:
Continuity Diode Status
off off Open circuit.
on on continuity <=2R
off on diode or <1kR
The two parts of this test element are created using a dual operational amplifier with one amplifier assigned to each function.
The OPAMP is a micropower amplifer with a low operating voltage range of 1.8 to 5.5V ideal for operation with a 3V coin cell. This is a rail to rail opamp which will maximise the output range given the limited supply voltage.
Consideration was given to low bias currents (IB) and low offset voltages (Vos)
Continuity tester.
This uses one of the amplifiers configured as Non Inverting with Voltage Gain (Av) of ~221
Av = 1 + Rf/R2 = 1 + 220k/1k = 221
Due to the voltage being measured <10mV and the gain 221 consideration needs to be given to the impact of offsets on the output voltage.
Considering One, Vout_Ib = Ib*Rf = 200pA*220K = 44uV
Considering Vos, Vout_Vos = Vos*(1 + Rf/R2) = 5uV*(1 + 220k/1K) = 1.105mV
In this case the greatest influence is due to the Vos.
The non inverting input is connected to a 1k pull up resistor which when connected to the element being assessed forms a potential divider.
With an input resistor of 1R the input voltage Vin = (Ri/Rt)*Vs = (1/1001)*3 = 2.997mV
This gives an output of 2.997mV * 221 = 662.337mV.
The error due to Vout_Vos equates to (Vout_Vos/Vout)*100 = (1.105mV/ 662.337mV)*100 = 0.1668% or 1.668mR
The 1K resistor is 1% therefore 1K +/- 10R which equates to +8/-6.6mV change in the output or +12.1/-9.9mR
If we consider the effect of a reduction in the supply voltage from 3V to 2V
With an input resistor of 1R the input voltage Vin = (Ri/Rt)*Vs = (1/1001)*2 = 1.998mV a -50% change or -1mV/V.
We can reduce the impact due to supply variation on the potential divider by instead driving the device under test (DUT), with a current source. The one chosen is specified down to a supply voltage of 1.2V.
Although, the minimum supply limitation for the continuity/diode test is the OPAMP at 1.8V
Continuity Indication
This is achieved using a PFET where Vgsth is -1V to -3V.
With the input to the amplifers open circuit (OC), the input is pulled up to the supply by the input pull up resistor.
Therefore the output is ~3V and Vg wrt the supply is ~0V and the PFET is switch off.
With 1R connected to the input the output is 662.337mV.
The output connects to a potential divider with a 10K and 100k pull up to 3V with the centre tap connected to the gate.
The gate voltage (Vg) = (100k/110k)*(662.337mV-3V) = -2.13V (wrt supply), switching the PFET on illuminating the continuity test LED.
With a short circuit (SC), on the input the output is at ~0V and Vg = (100k/110k)*-3V = -2.73V (wrt supply). illuminating the continuity test LED.
Diode Tester.
This uses the other amplifier configured as Non inverting with a voltage gain (Av) of ~1.2
For a small signal diode with Vf ~0.6V the output is ~728mV.
Diode Indication
With the input to the amplifers open circuit (OC), the input is pulled up to the supply by the input pull up resistor.
Therefore the output is ~3V and Vg wrt the supply is ~0V and the PFET is switch off.
With a signal diode connected to the input (anode to input and cathode to 0V), and forward biased.
The output is at ~728mV (for an input Vf of ~0.6V), and connects to a potential divider with a 10K and 100k pull up to 3V with the centre tap connected to the gate.
The gate voltage (Vgi) = (100k/110k)*(728mV-3V) = -2.065V (wrt supply), switching the PFET on illuminating the diode test LED.
With a short circuit (SC), on the input the output is at ~0V and Vg = (100k/110k)*-3V = -2.73V (wrt supply). this also illuminates the diode test LED but in conjunction with the continuity LED, indicating a short circuit (SC).
Step 4: PCB
2 More Images
The PCB was designed using EagleCAD and the PCB was fabricated by OSHpark.
Its a double sided PCB with the main components mounted on the top and the coil fabricated as a continuous trace on the back which negates hand wound coils.
However, provision exists to solder an extension to the end of the coil to increase sensitivity if required.
Version 1 uses R1 to form a potential divider with the DUT whilst version 2 adds a current source to bias the DUT.
The PCB design included is for version 2.
Prior to assembly check the PCB for defects, paying particular attention to any vias (plated through holes).
Should an open be found it can be repaired by passing a wire through the hole and soldering on both sides of the PCB.
Although, a very rare occurrance it is easier to resolve prior to assembly.
The majority of the passive (resistors/capacitor), components are SMD 1206 type.
The IC’s are SOIC8 with the exception of the Current source which is in SOT223
I manually solder the components in the following order, although a hot plate could be used.
Mount the passive components.
R1 can be omitted if the current source is used.
The current source section has provision for additional decoupling and filtering (R25, C7 & C8), which can be populated if necessay although in use I found no issues and omitted these in the final build.
Mount the transistors and LED’s ensuring they are correctly orientated.
Mount the IC’s ensuring they are correctly orientated.
Connection to the PCB can be made at the header pin through holes with compatible pins and sockets.
Prior to operation check that the components are correctly positioned and orientated and that there are no unexpected open or shorts circuits. Both by visual inspection with a magnifier and electrically with a DMM.
Attachments
Step 5: Operation
Before conducting diagnostic tests on any powered equipment understand the risks both to the user and the equipment the need for PPE and if in doubt seek expert advice before commencing.
Power is supplied by a CR2032 coin cell and operation is initiated by a non latching push button.
Optionally, an external supply can be connected to the VS pin (1.5 to 5.5V), which is isolated from the coin cell by the switch and the reverse biased diode.
However, use the VS pin with care as exceeding the maximum input voltage will cause damage to the components.
Connection to the PCB can be made at the edge fingers or header pins.
Its recommened to solder the connections for the cont/diode test to negate the inclusion of addition resistance.
When using the contactless AC voltage indicator keep your fingers away from the coil at the back of the PCB to prevent false indication. Neither connect it directly to mains AC as this will cause damage to the components and the user.
The cont/diode tester is to be used on unpowered circuits only.
The signal injector is for use on low voltage powered circuits but subject to requirements may need to be attenuated, amplified and/or the output isolated with a suitable circuit (eg capacitor network), so as not to back bias the output which could result in damage.
