Transistor Comparator

Introduction: Transistor Comparator

About: Recycled Planets is a project funded by various environmental organisations to promote sustainability.

This article shows you how to make a simple transistor comparator.

My circuit is an improvement to the design shown in this article:

https://hackaday.io/page/7030-transistor-comparator

Supplies

Components: general-purpose transistors - 3 (1 spare). 10 kohm resistors - 5, 1 kohm resistor - 1, matrix board, insulated wire, 1 mm metal wire.

Tools: pliers.

Optional parts: solder.

Optional tools: soldering iron, wire stripper, multimeter, USB oscilloscope, voltmeter.

Step 1: Design the Circuit

I have drawn this circuit in the old PSpice software.

Calculate the Ve voltage Thevenin equivalent:

(https://www.allaboutcircuits.com/textbook/direct-current/chpt-10/thevenins-theorem/)

Veth = Vs * ReB / (ReA + ReB)

= 3 V * 10,000 ohms / (10,000ohms+ 10,000 ohms) = 1.5 V

Reth = 10,000 ohms || 10,000 ohms = 5,000 ohms

Calculate the comparator input current at a maximum input voltage of 2.5 V:

Ib = (VinMax - Vbe1 - Veth) / (Rb1 + (Beta + 1) * Reth)

= (2.5 V - 0.7 V - 1.5 V) / (10,000 ohms + 101 * 5,000 ohms)

= 0.3 V / 515000 ohms = 0.58252427184 uA = 582.52427184 nA

Ro is used for output short circuit protection.

Step 2: Simulations

Simulations show that the circuit is working well.

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