Category Archives: Circuits

Op-Amps or Operational Amplifiers

741 Op-Amp Self assemble kit

741 Kit at Amazon

The “XL741” Discrete Operational Amplifier kit from Evil Mad Scientist Laboratories is a faithful and functional transistor-scale replica of the muA741 op-amp integrated circuit, the classic and ubiquitous analog workhorse. An Operational Amplifier, or “op-amp” as they are commonly known, is one of the most useful and common types of analog electronic circuit building blocks and the XL741 kit was designed by Eric Schlaepfer, in collaboration with Evil Mad Scientist Laboratories is lovingly designed to resemble one. The XL741 kit is sold as an easy-to-build soldering kit. It includes the circuit board, resistors, transistors and capacitor that make up the electrical circuit as well as printed assembly instructions. The kit also comes complete with the “IC Leg” stand, and 8 color coded thumbscrew terminal posts. Basic electronic soldering skills are required, but no additional knowledge of electronics is presumed or required and it should take about an hour to build. The XL741 circuit is a direct implementation of the “equivalent circuit” from the original Fairchild muA741 datasheet. It comes with terminal posts and solder points so that you can actually connect to it and build up classic and functional op-amp circuits.

Op-Amps or Operational Amplifiers

A operational amplifier has two inputs an inverting input that is 180 degrease out of phase of the output and a non-inverting input that is in-phase with the output. An op-Amps input impedance is very high. Buy Operational amplifiers

List of common Op-amps:

LM324 (quad opamp):

LM324 opamp

Low Power Quad Operational Amplifier
The LM324 can operate on a single voltage power supply. this op-amp has 4 independent operational amplifiers in a single chip.


LM741 opamp

The 741 op-amp is a common general purpose Operational amplifier.
This op-amp is useful as it has short circuit protection and high gain over a wide voltage( up to 18V max).


LM308 opamp

This opamp is now obsolete
the LM308 was a precision operational amplifier. It had a wide voltage range up to 18V and could run off an unregulated power supply.


LM386 opamp

The LM386 Operational amplifier is a Low voltage Audio Power amplifier and is some times refered to as a “audio op amp”. it also has a wide voltage range from 4V-12V or 5v to 18V Low distortion and Low current drain. This op amp was designed to be used as a low voltage product to be used in consumer products the Gain is set internally 20 20 to keep external components to a minimum, lowering product costs. The LM386 is an ideal product for basic audio circuits.

Good reads:

Op-amps and Linear Integrated Circuits – by Ram Gayakwad Op-Amp Circuits Manual: Including OTA Circuits – by R. M. Marston Operational Amplifiers (EDN Series for Design Engineers) – by G.B. Clayton and Steve Winder Operational Amplifiers: Theory and Design – by Johan Huijsing Operational Amplifier Noise: Techniques and Tips for Analyzing and Reducing Noise – by Art Kay




Types of OP amp Circuits:


opamp comparator

A Voltage comparator will determine witch voltage is greater, the result is the outputvoltage. If the non-inverting input(+) is greater than the inverting input(-) then theoutput voltage will be positive from the op-amps positive supply.

Inverting amplifier:

opamp inverting amplifier

In this configuration the Input signal is connected to the Inverting input on the Op-amp and the output is fed back 180 degrees out of phase into the inverting input(negative feedback) this reduces the gain of the amplifier. If Rf = Rin then the gain will be -1 and the op-amp will act as a simple inverter. The input impedance(z) is relatively low and is determined by Rin.

Non-inverting amplifier:

opamp Non inverting amplifier

In a Non-Inverting Op-Amp Input signal is used as a reference voltage on the non-inverting input of the op-amp. and the inverted input is basically referenced to ground. This circuit has a very High impedance(z) somewhere between 1M to 1T ohms.

Differential amplifier:

opamp Differential amplifier

Vout = (V2-V1)(Gain)

A differential amplifier will determine the difference between two input voltages andoutput the amplified difference that is determined by the external resisters in the circuit. If all the resisters are equal then the Gain will = 1. To adjust the gain you will need to change the Resistance of both the input voltage dividers equelly.

Voltage follower:

opamp voltage follower

Vout = Vin

An opamp voltage follower has a very useful characteristic as the input Impedance is extremely high and in-essence will isolate the input signal voltage from the outputsignal. this will reduce the loading effect on the signal source.

Schmitt trigger(“thermionic trigger):

opamp Schmitt trigger

Comparator implementation of a schmit trigger using positive feedback loop.

Inverted Schmitt trigger

Inverted Schmitt trigger.png

Non-Inverted Schmitt trigger

non Inverted Schmitt trigger


Simplest LED Flasher Circuit

Simplest LED Flasher Circuit


Need to flash an LED on and off but do not want to make an entire 555 circuit?

Why not try this simple circuit 

It consists of two resistors, a capacitor and one NPN transistor that’s it nothing else other than the power supply and LED

I tweaked the values to get a nice flash out of it.

In my implementation I have a 4.3k resistor connected in series with the emitter on a NPN transistor then the collector has been connected to a LED with a current limiting resistor on the LED to ground

In parallel with the transistor and LED resistor there is a 330uF capacitor



LED flasher NPN transistor
Single NPN transistor LED flasher Breadboard
LED NPN single transistor flasher
LED NPN single transistor flasher


In this circuit a 1k resistor charges the 330uf capacitor until the voltage became large enough for the emitter-base junction to avalanche this discharges the voltage in the capacitor through the LED and current limiting 100 ohm resistor.


Follow this link to Dick Cappels, the creator of this modern wonder.

PDF datasheet for the 2N3904 NPN transistor but almost any general purpose NPN will do.




7447 BCD to 7 segment display driver

7447 Binary coded decimal to 7 segment display driver

7447 Binary Coded Decimal counter


the 7447 decodes BCD 0-15 to there decimal equivalents plus 6 distinct character’s for letters A-F. the font is set to have tailless 6 and 9 characters. If required look at the 7448 the nice feature of this IC is that it has a “following display blanking” pin (Pin4) and will display 0 if it is the highest active unit but will display ” ” nothing if not needed, so in a 4 digit system you will have as an example ” 52″ and not “0052”

Tie Vcc (pin 16) to +5v and GRD (pin 8) to 0V also to get this operating as a single display tie all /(LT) /(RBI) and /(BI RBO) high.


7447 Font table

7447 font display output