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Here's your next project called the 'Tic Tac Tone Generator - and Other Things'. It's called that, because it is primarily a different kind of oscillator. It's not like the previous Test Box MVB circuit - it uses 2 transistors of opposing polarities (NPN as opposed to PNP - the Test Box used 2 x NPN transistors - same polarity.) This circuit has only one capacitor and one (fixed or variable) resistor, plus an output device.

However, you will find that this basic circuit configuration will perform many simple tasks, but we're only going to look at three of them - a basic metronome (a ' tic' - 'tic' kind of sound, used for timing and punctuation during music practice,) an LED flasher - useful if you want to make those fake red lights in your model police car come to life (yes you can run 2 of them off a 9 volt battery). Then there's the 'Tone Generator' part of it - you can add a morse code key and practice learning your code, in case you want to become an amatuer radio station operator... called a "Ham".

You could build 2 of them and use a 4 wire telephone cable to place the speakers at opposite ends of the home/school room and send each other Morse Code, and see how many 'dots and dashes' you can read. Or you could just enjoy 'tweaking' the tone generator (use a variable resistor to vary the frequency) so that it makes all manner of funny noises... The choice is yours...

Step 1: How the Tone Generator Works...

This is a different kind of oscillator and we will have a brief look at how it works, in general, and then apply those basics to several different scenarios. This circuit looks a little 'bare bones' at the moment but it can be configured to do several different tasks. It is essentially an oscillator, which uses two transistors of opposite polarity. The BC 557 Q2, is a PNP transistor, and needs a -ve potential on its base lead in order for it to turn on. WHAT? I thought it was the other way around? Well - yes - it IS - for the BC547, which is an NPN transistor - the one you used in the Transistor Amp project #2.

So now you have to deal with a 'mirror image' in regard to the 2 transistors involved in this particular circuit - sort of... By the way, the current flows in the opposite direction in an NPN transistor, than it does in a PNP, and that's why Q2 seems to be 'upside down' while Q1 is 'right side up' - right? Confused? Don't worry - we'll sort it out as we go along.

Back in the early days of electronics, scientists and inventors believed that current flow began at the positive (+ve) terminal of the battery or power supply, and travelled down through the circuit and into the -ve terminal. That's why all the 'needles' on the diode and transistor devices point towards the -ve polarity of the power supply in a given circuit. The rule of thumb still holds - "needle points negative", which is still a good way to remember how to correctly wire an active "semiconductor" device into a circuit, however, the current (electron flow) actually flows in the other direction (-ve to +ve) up through the circuit elements.

OK - just to sum it all up - at 'startup' when power is first applied, Q2 will be 'off' (non conducting) and Q1 will turn on when its 'base' lead is sufficiently biased 'on' at +0.5 of a volt, by C1 charging up via R1 and the output device (speaker, LED or low value resistor to ground/-ve rail.) When Q1 turns on, its collector lead will be 'pulled' down close to ground/-ve rail (a virtual short circuit,) and this will provide the -ve bias that transistor Q2 (BC557) needs on its base lead, in order to turn on. When Q2 turns on, capacitor C1 discharges via the output device, which activates, and at the same time, turns Q1 (BC547) off, and then the cycle begins again.

Complete Parts Set
I'm giving you the Jaycar catalog numbers for the whole parts list in all of the variations of the oscillator circuit and this will allow you to build all three versions and tinker with the different component values on the list. You may have to invest in a set of plastic drawers and some sticky labels, so that you can store all of your parts and know where they are. A set of 12 drawers would be a minimum - don't worry, they won't stay empty for long, if you're serious about hobby electronics!!! One word of warning - miswiring transistors and diodes up to a 9 volt battery can 'cook' them, even if they appear to be wired correctly. A circuit needs a device (called a load) to develop its output voltage across. Failure to provide a large enough load, may damage some components. If your diodes or transistors ARE running hot (put your fingers briefly on each one,) then switch the circuit off immediately and check all wiring - including battery polarity (red and black wires from the battery connected to the right parts of the matrix board...)

Parts List

R1 - 220k resistor - RR 0628

R2 - 39R (ohms) - RR 0538

VR1 - 500k (lin A pot)- RP 3622

C1a - 100nF Greencap - RG 5125

C1b - 10uF electrolytic - RE 6066

Q1 - BC 547 - ZT 2152

Q2 - BC 557 - ZT 2164

Led 1 - Red LED - ZD 0150 (5mm)

LDR - Light Dependent Resistor - RD 3485 (this is a very sensitive device - well worth the money!)

1 x AAA battery holder with flying leads - 1 x 9 volt 'battery snap' - 1 x mini 8 ohm speaker - AS 3004 - mylar cone

1 x ceramic earphone/transducer PKM44EW by Murata (or your standard crystal earphone will do for starters.)

Final notes: feel free to try different coloured LEDs and also buy some smaller and larger caps - eg: 47uf, 22uF and 4.7uF electrolytics, as well as some fixed resistors - eg: 47k, (but nothing smaller) 150k, 270k - the above parts list is the bare minimum.

Most importantly - only use the 39R (39 ohm) resistor as an output device where specified - use it anywhere else and something may go "pop" - OK? Have a play with the LDR in scenario 3 (the actual noise making 'Tone Generator') instead of a fixed resistor (R1) or variable pot (VR1) - you can plug the small loudspeaker in as the output load, and play it like a mini Theremin!

Step 2: Building the Tone Generator...

(Note: the above layout is a 'generic' one only - it's meant to be a guide - NOT an actual practical circuit - OK? :) )

Lay all the parts out on a clean surface. Take the matrix board and look at the parts layout and beginning with the resistor and capacitor, begin to build the generator accordingly. Start with the R1 resistor and then the C1 capacitor. Install the 2 transistors last and then wire in the desired output device (depending on the role the oscillator will play) a mini loudspeaker, or an LED or another low impedence device wired between Q2's emitter and the -ve end of C1 join and down to ground/-ve lead.

By now, you should be experts at parts insertion, connections and even circuit 'debugging' (finding and fixing wiring faults,) so I'm not going to go into too much detail with the actual build, as such. More detailed attention will be paid to the different "incarnations" for this circuit in later steps. For this project, I've chosen a fixed on/off switch which can be mounted in the 'flap' of the white lid for the Tic Tac box. The previous project only incorporated a momentary on/off switch, due to the intermittent use of the Test Box. It is envisaged that the Tone Generator may be switched on for prolonged periods, and thus the need for a different kind of power switch.

Note that some versions of the TT tone Generator may operate on a +1.5 volts supply, but others (loudspeakers and LEDs) may need a 9 volt battery for them to operate more efficiently (brighter or louder) so you may have to be happy with an external 9 vo lt battery connected 'outside of the box' - OK? Good - you can secure it to the bottom of the box using one of those thick red coloured rubber bands, or use 2 pieces of double sided tape squares.

Step 3: Modifying the Tone Generator...

Here are 2 scenarios and the outcomes from each circuit variation:

Scenario #1: Metronome: the speaker will click on and off regularly, according to the value of resistor R1 (or the setting of VR1 - a variable resistor - commonly known as a 'pot'.) So in this version, VR1 would actually be the 500k variable 'pot'. C1 would be the 10uF electrolytic capacitor (watch the polarity of that one!) Make sure that the positive lead (+) of C1/10uF goes to the base of Q1 - BC547, and the negative (-) lead goes to the emitter of Q2 - BC557

Scenario #2: the LED will flash on and off at regular intervals, as a warning light, a marker for a hazard in the dark etc. So you can just use the Red LED, the 10uF cap and the 220k fixed resistor, or another fixed value for a faster or slower but steady flash rate. Using a "bright" LED (larger brighter LED) you could use it to indicate hazards in the dark (dug up footpaths or fallen tree branches etc).

If you use the larger Tic Tac box for this version, the circuit board AND the 9 volt battery will both fit inside the box - see the picture above. A number of amber coloured LEDs could be wired in series/parallel (you'll have to experiment with the numbers,) and fitted on to a Lego roadside layout to act as 'warning' lamps, alongside the traffic barriers (the ones with zebra stripes on them,) to give some realism to a crash scene, or to a "roadworks" layout - trucks, graders, bulldozer Lego etc...

Step 4: Tic Tac Tone Generator Extras...

Inserted update: The new pic shows the Tone Generator (a new build on another board,) with the loudspeaker option employed, and an adjustable 'pot' on board, making sure that the board can fit inside the box. The 'pot' is actually a 20 turn precision device, scavenged from an old analogue TV set tuner. the 'pot' used to control the voltage on a varicap diode (varactor). there were 7 of them, preset on a pc board, and I got the lot! To buy them nowadays does not cost much, but I still saved $15 to $20 in scavenging them.

Scenario #3: the resistor R2, will complete the circuit to ground, allowing another device to perform an outcome. for example, the capacitor can be replaced by the ceramic transducer (seeing it operates on principles similar to a capacitor,) and so the low value resistor - a 39 ohm one, becomes the actual output load, while the transducer does the 'clicking' or the tone generation. Both variations will work, according to the parts you have on hand - and that's what I mean by versatile design and usage. I'll post modified circuits over the next few days to make it all clear.

Whatever configuration you have set the circuit up for, and whatever values of R1 and C1 you have decided on, or whatever output device you have chosen, (providing the design of the circuit and the components are compatible with how the oscillator works,) the circuit will perform an outcome and then reset itself. Current will flow through R1, charging C1 and turning Q1 on first - then Q1 will turn Q2 on, thus activating the output device. - every time. That's the basics of how it works and you can see that there is a bit of diversity that is built in to this simple but effective oscillator circuit (because that's what it is - an oscillator...) Well, all the best for now, and I'll be back soon with some circuit mods and updates ASAP - enjoy... mk484

<p>Hi there 'Danger' - are you going to make one? If so, please post a pic or two and tell us how you got on yeah?</p>
<p>Nice, I love that it all fits in that tiny box!</p>

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Bio: Started as a hobbyist at 9 - built my first crystal radio on one of mum's prized cutting boards (eeek) - Worked in 2 electrical/electronics ... More »
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