But wait, it is supposed to do three things and I am counting the IC checking as just one. What else can it do?
- It can be used as a simple signal generator with an adjustable output square wave
- It can be used as a continuity tester which in addition to checking for low resistance paths test diodes and LEDs. ( Yest it tests LED too!, it makes them light if they are good ).
As an aside if you do analog electronics or work with sensors you should find out about op amps they are a very versatile circuit component.
In my case I built the circuit on a breadboard and used an external power supply. But you could build it in a more permanent form, ( a good idea ) and I did that too.
Step 1: Tools and Materials
- Power supply: You need a dual supply positive and negative with respect to ground, this can be 2 9 volt batteries, a bench dual supply or a single supply with a “splitter”.
- Breadboard: For casual or temporary use, or your choice of build infrastructure.
- Misc components Resistors, caps.... see sections below for component values.
- 555 timer and op amps - These are what you will test when used as tester, or to power the other applications.
- An Oscilloscope is a nice to have tool for the build and use, but is not necessary.
Step 2: Theory: Overall View
I will walk you through the circuit, first at a high level, then in additional steps in detail. Please refer to the block diagram for this step.
The 555 Timer is often used as a square wave generator. In the tester we set it up this way and look for the square wave output, if it is there then we declare the 555 circuit working. For other purposes we use this as our basic signal source.
The square wave is then fed to the input of one or more of the operational amplifiers. Each input of the amplifier is deemed working if it outputs a square wave.
The signal generator is simply the square wave output, this is a common signal from signal generators. For a nice bipolar square wave follow this up with a single op amp stage. Add a potentiometer attenuator to get a wave of adjustable amplitude.
For continuity testing we feed a square wave to a circuit under test and look for the wave pushing current through, using a detector made from a small speaker and a pair of LED's.
Step 3: Theory: 555 Timer
The output of the 555 time is passed to the op amps via a capacitor ( C_BLOCK ) so that the wave will be AC with respect to the ground of the dual supply, this makes the output of the op amp bipolar( swings both above and below ground ). We also add the resistor ( R_LIMIT ) to protect against short circuits at the output, and R_GND to make sure the output center value stays centered on ground.
A slight note on the square wave, some would say it is not a square wave but a rectangular wave, one side is twice as wide ( in time ) as the other. This is related to the timing equations for the 555 and that I have chosen 2 equal values for the two resistors in the timing circuit.
- 555 Timer by amandaghassaei Step 5: 555 Timer: Astable Mode https://www.instructables.com/id/555-Timer/
- LM555 Timer http://www.ti.com/lit/ds/symlink/lm555.pdf
Step 4: Theory: Operational Amplifier
- http://www.ti.com/lit/an/sboa092a/sboa092a.pdf go to page 15
- Some Useful Operational Amplifier Circuits http://www.facstaff.bucknell.edu/mastascu/elessonshtml/OpAmps/OpAmp3.html
- Voltage Follower http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html
Step 5: Theory: Detector
For the continuity tester we want the detector to detect the signal only if a current of a few ma is flowing indicating a low resistance connection. ( this same detector can be used for IC testing and the signal generator).
So here is a detector that is sensitive to a few volts at a few ma. It is just two LEDs connected in parallel back to back in series with a small speaker salvaged from a computer mother board ( or microwave oven or other piece of old electronics ). Finally use a current limiting resistor ( 400 ohms is pretty good ) When voltage is applied AC will make both LEDs glow, and a buzz will come out of the speaker. You can look at the detector or just listen for it. One of my speakers had a fairly high resistance, if this is the case for you can try paralleling it with 100 ohms or so, that worked for me.
Step 6: Theory: Potentiometer Attenuator for Variable Output
- Potentiometer Theory of operation From Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Potentiometer#Theory_of_operation
- Electronics Tutorial about Attenuators The Attenuator http://www.electronics-tutorials.ws/attenuators/attenuator.html
Step 7: Build It
But I just used usual breadboard building techniques. The op amp section is built over and over, once for the single op amp, twice for the dual op amp, and 4 times for the quad op amp. The rail connections ( the long ones across the top and bottom of the breadboard are used for the + side of the power supply, the – side of the power supply, the power supply ground, and the square wave output of the 555. Connect the square wave to the input of each of the op amp circuits.
The power to the circuit is a bit unusual because op amps like dual positive and negative supplies. The simplest way to supply this is with 2 9 volt batteries, but many bench supplies ( including the one I use see: Almost a Power Supply by russ_hensel https://www.instructables.com/id/Almost-a-Power-Supply/ ?ALLSTEPS ) are dual supplies which is what I use. Suit yourself, or you can even split a single supply for use with op amps ( see:
- Split rail power supply from a single rail supply by Harrymatic https://www.instructables.com/id/Split-rail-power-supply-from-a-single-rail-supply/
- Creating a Virtual Power Supply Ground http://forum.allaboutcircuits.com/showthread.php?p=84026
- C = 100 nF
- R1 = 25K
- R2 = 25 K
- C_BLOCK = 22 microF
- R_LIMIT = 400
- R_GND = 30k
- R_LED depends on your speaker, if not used try 400 ohms, adjust for reasonable brightness
- Put resistor in parallel with speaker if led does not light
In the picture the circuit for the single op amp is a bit messed up with the output from the 555, follow the schematic which is correct.
Step 8: Use It - Testing IC's
To test a 555 timer insert it and look at the output ( just after the capacitor C_BLOCK ). You can use either the detector or an oscilloscope to look at the output. Note that the signal direct from the output pin swings from ground to almost the positive supply, but it never goes negative.
To test an op amp first insert a good 555 timer, check its output. If it is OK insert the op amp ( any one of the single, dual, or quad ). ( Op amp pin outs are pretty standard, check to see if yours is an exception ). Then check the op amp outputs. If you have your square waves you are OK. ( these waves should swing above and below ground because of the DC blocking capacitor C_BLOCK. )
Step 9: Use It - Continuity Testing
A diode tested with this tester will light only one of the LEDs if it is good ( you can tell the orientation of the diode by which one lights ). If the diode is an LED itself the tester should have enough current so it lights on 1/2 of the square wave output. This diode test does not verify full specifications of the diode, and does not subject the diode to much revers voltage. You can also use it to test LED's they are diodes and will light no matter which way they are connected.
Testing a capacitor may or may not turn on the detectors LED's on depending on the capacitance connected and the frequency of your 555 ( test with known good components can give you a guide to interpreting the detectors reaction )
Step 10: Use It - Signal Generator
Insert a good 555 timer and a good op amp. Feed the op amp a signal from the 555 and feed its output to the attenuator. Take your final output from the attenuator, the wiper ( middle ) of the potentiometer. The other connection is, of course, the circuit ground.
Step 11: Use: the Detector
AC ( like the outputs here ) make the speaker buzz and the LEDs light. So you can here or see the output. Note there is a jumper in the detector which is broken ( removed ) when continuity testing.
Step 12: Variations and Comments
- Remember a breadboard is not a good permanent build, you can do better. See the picture of one I have made just for continuity testing.
- If you already have a signal generator you can use it with the detector to make a continuity tester.
- Rather than move the detector around between the output pins you could have one master output pin and connect the different outputs with momentary push buttons.
- Of course for any project an LED indicating the power is on is nice.
- The 555 square wave could be made variable by making either the timing capacitance variable ( probably with a switch ) or the timing resistor variable ( probably with a pot ). You may also just vary the timing components to get a frequency of your choice.