Simple Logic Probe Circuit using Transistors

This article for Simple Logic Probe Circuit using Transistors tell how to make one useful tool called voltage tester; also it uses two transistors for checking if some place in circuit have positive voltage.

Further, when it find electricity it makes beep sound and small light turns ON.

Here, people who want to learn electronics, this tool will help them to understand better.

What is a Simple Logic Probe Circuit:

Logic probe circuit is one electric testing tool to check and see digital circuit; which helps to know how circuit work by showing if logic level is high or low in some place.

Also, this probe is very useful to find and fix problem in digital things like ICs, microcontrollers and other digital device.

Circuit Working:

Parts List:

Components	Values	Quantity
Resistors	560k, 1k 1/4 W CFR	1 each
Semiconductors	Transistors BC547	2
	LED (any 20mA 5mm)	1
	Piezo Buzzer	1
	Red Probe (as shown in the diagram)	1
	Crocodile Clip (as shown in the diagram)	1
	Battery 9V	1

To begin with, this circuit explains in full details how twin transistor positive voltage sensor work and how to make it.

Also, transistor Q1 and Q2 connects like Darlington pair; and this type of connection gives high input impedance so it can feel small positive voltage better.

Q2 have common collector output which makes easy to give power to LED and buzzer.

Furthermore, when input gets positive voltage the logic probe turns ON and to use it connect negative wire of probe to ground or negative of circuit to test.

Hence, if input voltage goes more than around 1.5V of logic high then the probe start working and this voltage limit stops the circuit from turning ON by mistake.

Also, when voltage goes over limit then both Q1 and Q2 turn ON and then LED light up and buzzer makes sound so we see and hear the signal.

Formulas and Calculations:

Here, is some formula for making simple logic probe circuit using transistors:

How to find resistor value:

Voltage Difference (Vdiff):

Firs,t subtract LED forward voltage (Vf) from supply voltage (Vcc).

Vcc – Vf = Vdiff

Use ohms law:

After we know Vdiff and how much current (If) LED need then use ohms law:

R = Vdiff / If

Example to find LED resistor:

Suppose we use red LED supply voltage should be 9V and LED forward voltage (Vf) is 3.3V.

And if we want LED current (If) to be 15mA then check datasheet for safe current.

Vdiff = Vcc – Vf = 9V – 3.3V = 5.7V

Now use ohms law:

R = Vdiff / If = 5.7V / 20mA = 285 ohm

So we need 285Ω resistor.

How to Build:

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

Make Darlington Pair:

• First, connect base of Q2 to emitter of Q1 and Q2 emitter must connect to ground.
• After that, connect piezo buzzer between collectors of Q1 and Q2.

Base Resistors:

• Then connect input signal to base of Q1 using resistor R1.
• Also, LED cathode connect in series with resistor R2 to limit current.

Power Supply Connection:

• Next, connect positive side of piezo buzzer to Vcc of positive voltage.
• Now Q2 emitter connects to ground.

Probe Connection:

• After that, negative probe wire connects to PCB ground and input signal connects to positive probe wire.

Important Note:

• Finally, always check datasheet of all parts we use, as this helps to choose right values and avoid damage.

Conclusion:

Overall, this Simple Logic Probe Circuit using Transistors is good tool to check digital signals in circuit fast.

Moreover, it help to find bad parts or wrong signal levels and also it is useful for professionals and designers working with electronics.