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Sunday, December 4, 2011

Square wave and pulse generator

Simple square wave and pulse generator
A.K.A. 1960-ies style guerilla homebrew 6 Hz - 2 MHz function generator with 4 transistors :)

Introduction

Recently I needed a function generator for the workshop at the place where I work. Detailed inspection showed that we had four.

One is in good shape, made in 1979, and capable of producing sine, triangle and square waveforms. This one is retained and used as main function generator for testing purposes at my main office location.

Other three were not working, or working sporadically. They were all from 1950-ies or early 1960-ies, with electron tubes, of two different types. High voltage was ok on all of them, but output was lacking or intermittent. I decided to scrap them, keep the working tubes, transformers and metal for future projects, and make a new function generator using only transistors. This new generator will be used in the machine shop location.

Old vacuum tube based function generators

Old Krohn-Hite function generator images. Two non-working examples were found. They were of very solid build. Useful parts were kept for future builds.

Old EICO function generator. Much poorer workmanship quality, thin steel chassis plates, etc.

Transistors only? Why?

New generator was supposed to have all the bells and whistles, and use case and if possible other parts from the scrapped vacuum-tube generators. Transistor-only design might seem strange at this day and age. However, I wanted to challenge myself. And there was another, somewhat nostalgic reason: I have spent some years in a war zone, and I can appreciate how difficult it is to extract chips from boards (and get their datasheets) during times of war. Transistors were easily removed from equipment, and were not critical when in different role. BC108 will work just as well as 2N2222 in a LF amplifier for instance - often without any change in external components.

Therefore, transistor design has a practical second and third world use, IC based one much less so. No special chip? Tough luck. It's not going to arrive on a mortar round. So, for all the people in the world that are maybe now in the similar situation that I was some 20 years ago, it is going to be a simple and reliable, no frills build.

Use of a voltage regulator IC is not cheating. Device works perfectly well without it, frequency is just more stable with regulator. Frequency drift using unregulated power supply is only 2-3 %. Frequency drift with 5 V voltage regulator, measured in a 24 hour run, is always less than 1%. And anyway, 78L05 is a three terminal device :)

Complaint that's more to the point is the dual rotary switch with 5 positions. If you can't find it, you can use regular DPDT switch for two bands only. Bands can be "streched" by using smaller band shortening fixed resistors (1 Kiloohm in my schematic). This will increase total ratio of resistance change and provide wider range (and coarser) tuning. Other option is to use Megaohm dual-ganged base resistors with Darlingtons instead of regular transistors (see here, Figure 14). With this measures, it is possible to achieve say 10 Hz - 1 KHz band A and 1 KHz - 100 KHz band B. No DPDT switch / just lazy you say? Use one band only and you don't need a switch at all ...

You have no dual-ganged resistors? Ok, just find two regular potentiometers. Your tuning will now involve adjusting 2 pots, but that is a small price to pay for a working function generator, if nothing else is available.

Nothing else in this schematic is critical: change transistors, supply voltages, resistors, capacitors. It will work with a very wide range of components.

Sinewave and squarewave generators

Read More...
http://www.arthropodsystems.com/SquareWaveGenerator/SquareWaveGenerator.html

Great Work and good Craftsmanship!:) Now, if I just knew how to use one of these!:O Then I would be in good shape;) I wonder if something like this, would help me in building a True Sine Wave DC to AC Converter? I do have a 1950's - 60's Automobile Generator Tester. It has a Volt Meter (Measures up to 40v DC), Ammeter and a Generator Load Tester. And I do know how to us it! Well, most of the functions, anyway...;) It was given to me back in 1975 - 76. It still works, but the insulation on the wiring is aged and cracked now. So, you have to be very careful not to touch the wires together where the insulation if broken or to anything that can ground the circuit. This Article, reminds me... I really need to get that thing out and fix it up. My only other Electronics Tester, is a 1975 - 76 Radio Shack Multi Meter. It still works great. But, I can't see as well as I use to. So, I have a very hard time reading the Small Scales on the old Radio Shack Analog Meter. The ones on the Generator Tester are much Bigger though... And I do have uses for it, with my Autos and Electronics...

Don