Simple Logic Probe Circuit using Transistors

The construction of a useful instrument known as a voltage tester is explained in this article.

It can detect whether there is a positive voltage at a specific point in an electronic circuit by using two transistors.

When it detects electricity it beeps and illuminates with indication sign.

Anyone interested in learning about electronics will find this to be an informative tool.

What is a Simple Logic Probe Circuit:

An electrical testing tool for detecting and examining digital circuits is a logic probe circuit.

It provides important information about how a circuit operates by helping to find out whether the logic levels are high or low at certain points.

The probe is especially helpful for identifying and fixing digital systems including integrated circuits, microcontrollers and other digital devices.

Circuit Working:

Parts List:

The circuit design and construction details of the twin transistor positive voltage sensor are included in this complete explanation of how it operates.

The configuration of transistors Q1 and Q2 is a Darlington pair.

High input impedance from this connection increases sensitivity to signals with positive voltage.

The circuits supply of power is made simpler by using Q2s common collector output to power the LED and piezo buzzer.

When a positive voltage is found at the input the logic probe turns on.

This is carried out by connecting the probes negative lead to the PCB getting examined common or negative supply.

Any positive voltage above around 1.5V logic high will cause the probe to react.

This threshold keeps the circuit from becoming active.

Both Q1 and Q2 activate when a positive voltage over the threshold is detected.

As a result the LED illuminates and the piezo buzzer is activated producing both visible and sound indications.

Formulas and Calculations:

The following formulas are for Simple Logic Probe Circuit using Transistors:

Calculate the estimated resistor value:

Voltage Difference (Vdiff):

Subtract the LEDs forward voltage (Vf) from the supply voltage (Vcc) to calculate the voltage drop across the resistor.

Vcc – Vf = Vdiff

Use ohms law:

Once Vdiff and the needed LED current (If) have been established use ohms law to calculate the resistor value:

R = Vdiff / If

How to calculate LED resistor:

Assume one uses a red LED that has a supply voltage of 9V and a Vf of 3.3V.

To choose a desired LED current (If) of 15mA for the safe operating current see the LED datasheet.

V_diff = V_cc – V_f = 9V – 3.3V = 5.7V

Using ohms law:

R = V_diff / If = 5.7V / 20mA = 285Ω

How to Build:

To build a Simple Logic Probe Circuit using Transistors follow the below mentioned steps:

Put the Darlington Pair together:

• Connect the base of Q2 to the emitter of Q1.
• Make sure the Q2 emitter is grounded.
• Use the piezo buzzer to connect Q1 and Q2s collectors.

The base resistors:

• Join Q1s base and the input signal by connecting R1.
• The LED cathode should be connected in series with the R2 current limiting resistor.

Connection to the Power Supply:

• Connect the piezo buzzers positive terminal to the positive voltage source Vcc.
• Join Q2s emitter to the ground.

Connection of the Probe:

• Connect the negative probe lead to the PCB getting tested as common or negative supply.
• Connect the input signal to the probes positive lead.

Take note:

• For accurate requirements and ratings do not forget to check the datasheets of the selected components.

Conclusion:

One useful tool for quickly checking the status of digital signals in a circuit is a logic probe circuit which helps in detecting component failures or improper signal levels.

It provides professionals and designers a simple way to monitor electronic circuits.

References:

Logic probe