Simple Logic Probe Circuit using Transistors

This article shows you how to build a cool tool called a voltage tester.

It uses two transistors and can tell you if there is power positive voltage at a certain spot in an electronic circuit.

It beeps and has a light to show you when it finds power.

This is a handy tool for anyone who wants to learn about electronics.

What is a Simple Logic Probe Circuit:

A logic probe circuit is an electronic testing tool used to troubleshoot and analyze digital circuits.

It helps identify the logic levels high or low at different points in a circuit, providing valuable information about the circuits operation.

The probe is particularly useful for debugging digital systems, such as microcontrollers integrated circuits and other digital devices.

Circuit Working:

Parts List:

Here is a detailed explanation of the working of the dual transistor positive voltage sensor, including the circuit diagram and construction details.

Transistors Q1 and Q2 are configured in a Darlington pair.

This connection provides a high input impedance, enhancing sensitivity to positive voltage signals.

The LED and piezo buzzer draw power from the common collector output of Q2 simplifying the circuits power distribution.

The logic probe activates when a positive voltage is detected at the input.

This is achieved by connecting the negative lead of the probe to the common or negative supply of the PCB under test.

The probe responds to positive voltages higher than approximately 1.5V logic high.

Below this threshold the circuit remains inactive.

When a positive voltage exceeding the threshold is detected, both Q1 and Q2 turn on.

This results in the illumination of the LED and the activation of the piezo buzzer providing both visual and audible indicators.

Formulas and Calculations:

Compute Resistor Value (approximately):

Voltage Difference (Vdiff): To get the voltage drop across the resistor, subtract the forward voltage Vf of the LED from the supply voltage (Vcc).

Vcc – Vf = Vdiff

Utilize ohms law: After determining Vdiff and the intended LED current (If), compute the estimated resistor value using ohms law:

R = Vdiff / If

How to calculate LED resistor:

Lets say you are using a red LED with a Vf of 3.3V and a supply voltage Vcc of 9V.

You choose a desired LED current (If) of 15mA (refer to the LED datasheet for safe operating current).

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

Using ohms law:

R = V_diff / If = 5.7V / 20mA = 285Ω (rounded to nearest standard value)

How to Build:

Assemble the Darlington Pair:

• Connect the emitter of Q1 to the base of Q2.
• Connect the emitter of Q2 to the ground.
• Connect the collectors of Q1 and Q2 together with the piezo buzzer.

Base Resistors:

• Connect R1 between the base of Q1 and the input signal.
• Connect R2 current limiting resistor in series with the LED cathode.

Power Supply Connection:

• Connect the positive voltage supply Vcc to the positive terminal of the piezo buzzer.
• Connect the ground to the emitter of Q2.

Probe Connection:

• Connect the probes negative lead to the common or negative supply of the PCB under test.
• Connect the probes positive lead to the input signal.

Note:

Remember to refer to the datasheets of the chosen components for accurate specifications and ratings.

Conclusion:

A logic probe, circuit is a handy tool for quickly assessing the status of digital signals in a circuit aiding in the identification of faulty components or incorrect signal levels.

It provides a non intrusive means of monitoring digital circuits during development and troubleshooting.

References

Logic probe