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Tester Probe Circuit

Many people think multimeter can test any electric part.

But this not always safe especially for semiconductor.

This article for Tester Probe Circuit show how to make one easy and safe tester only for semiconductor.

This tester is easy to build and uses small current which is good to test many type of semiconductor.

What is a Tester Probe Circuit:

The tester probe circuit is small tool for checking and testing.

It helps to know how fast is electric circuit is working or not.

Also it uses to check if electric parts and device are working good or not.

Circuit Working:

Parts List:

CategoryDescriptionQuantity
Resistors150k CFR 1/4 W1
180k CFR 1/4 W1
10Ω CFR 1/4 W1
10k CFR 1/4 W1
100Ω CFR 1/4 W1
CapacitorsCeramic 47nF1
Electrolytic 1µF 25V1
SemiconductorsTransistors BC5471
Transistors 2N29071
Diodes 1N4148 (optional)2
Speaker1
Battery 9V1
Tester probes red1
Tester probes black1

T1 and T2 make low frequency LF oscillator which control by voltage.

Speaker work as load in this oscillator.

Oscillator frequency depend on C1, R1, R4 and outside resistance between test wires.

R3 is collector resistor for T2 and C2 give low frequency filter for this resistor.

How Oscillator Works:

Frequency of oscillator can change for testing.

No need to watch tester always because speaker give sound when testing.

Safety Features:

This circuit is safe for part we test.

Diodes D1 and D2 protect tester parts from bad circuit.

Power Saving:

If test wires do not touch anything then circuit uses very small current.

Battery last long almost same as shelf life of battery.

Formulas:

Oscillation Frequency:

The frequency (f) of oscillator and time for charging/discharging have same relation:

f = 1 / (Tcharge + Tdischarge)

where:

Tool Checking:

We can study the circuit to know which parts changes the frequency and how much it can go up or down.

Charging Time:

When voltage on capacitor C goes low one BJT turns ON and let capacitor charge.

Charging follow path through some resistors.

How fast it charge depend on current and resistance value.

Discharging Time:

When capacitor voltage reaches one level BJT changes state and turn OFF.

Then capacitor discharges through another path with different resistors.

Discharge speed also change with resistance in that path.

How to Build:

To build a Tester Probe Circuit follow the below mentioned connections steps:

How to test:

Below mentioned are the few methods for testing the Tester Probe Circuit:

Power ON:

Connect Measuring Leads:

Find Test Points:

Touch Test Points:

Listen for Sound:

Disconnect and Try Again:

Check Results:

Turn Off:

Conclusion:

This homemade Tester Probe Circuit gives safe and clear way to test semiconductor without breaking anything.

Follow the safety rules while working with electronics.

References:

Test probe

Datasheet 2N2907

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