Tic Tac Test Box... a Simple Audio Signal Injector!





Introduction: Tic Tac Test Box... a Simple Audio Signal Injector!

About: 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 stores as a teenager - Became a College kid in 1980 studied electr...

Well folks - here we are again! And here's the next step in building the Nic Nac Tic Tac series of projects. The reason this 'Test Box' is necessary, is that if you build a project, and it doesn't work (for example - you built the radio, and then the amplifier, but can't get any sound in your earphone/earbuds) then you need to trace through the circuit and see just where it all went wrong.

This Instructable will show you how to build a simple, 2 transistor "signal injector". When you connect up the ground clip and touch the probe onto a circuit input or output, it will inject an audio tone of around 1 khz (1 kilohertz = 1,000 hertz or cycles) which is well within human hearing capabilities. You can then start at the back end of your radio/amplifier or other audio project, work your way to the front, and see just where the signal drops off or becomes lower in volume.

Once again, the circuit is built on a piece of matrix board (about twice the size as the other Tic Tac projects) but will fit inside the smaller Tic Tac box, so that you have a continuity in the size and shape of each finished project. It is powered by the smaller 1.5 volt AAA battery, so is quite 'kid safe' when it comes to how many 'volts' little Johnny or Suzy are playing with...

So let's look at the circuit diagram for the signal injector and learn a bit more about transistors and other simple components...

Step 1: How the Test Box Works...

Our signal injector is patterned on a famous circuit known as a "multivibrator" (MVB). This circuit was first invented back in 1942, and involved two valves (tubes) which could switch state continuously (one valve on and the other one off and then reverse that state - back and forth and so on.)

Our version of the circuit, called an "astable multivibrator", relies on the fact that no two transistors are exactly alike in their characteristics. Given the 'mirror image' nature of the circuit, (2 resistors one capacitor and one transistor per 'side',) even when the components used are exactly the same values, the MVB circuit will always consistently start with one or the other transistor turning on first, before the other one can.

How It Works:

Let's assume that transistor Q1 will start the electronic ball rolling. It has a 22k load resistor (R1) wired from its collector to the +1.5 volt supply rail. Transistor Q2 is 'off', when Q1 is 'on', so Q1 will'conduct' current from the battery -ve up through the C - E junction of the transistor, via the 22k load resistor and return back via the battery's +ve terminal.

In so doing,Q1's collector/emitter 'conduction', shorts the left hand end of C1 virtually to ground, allowing capacitor C1 to start charging via the 150k resistor R2 . Resistor/capacitor (RC) networks, use a time constant, so that for a given value of R and C, the time it takes the capacitor to charge up to 2/3rds of its possible charge/storage rate can be known. By knowing the answer to the time constant equation, we can predict what frequency the astable multivibrator will operate at.

When the charge on C1 via R3 220k reaches around 0.5 volts, Q2 will turn on, and C2 will begin to charge in the same manner, and so on. Capacitor C3 will block any DC voltage on the collector of Q2, but will allow the generated audio tone to be fed into another circuit, and return via the clip lead to ground, thus completing the circuit.

Parts List For Tic Tac Test Box

Here's the parts list with Jaycar catalog numbers:

R1, R4 - 22k - RR 0604 x 2

R2 - 150k - RR 0624 x 1

R3 - 220k - RR 0628 x 1

Q1, Q2 - BC547 bjt - ZT 2152 x2

C1, C2, C3 - 10nF - RC 5840 x 3


1 x piece of matrix board to suite - HP 9562 - buy the large square and cut it into smaller pieces for other projects.

1 x 1.5 volt AAA battery - PH 9260

2 x alligator clip - black/red - HM 3020

1 x Red push button switch - PB 1 - SP0710

1 x small Tic Tac box (of course!)

Various lengths of thin, insulated hook up wire - basic tools - side cutters, sharp instrument, small pliers.

Step 2: How to Build the Test Box...

There is a manner in which to build these projects so that you can do it easily and make fewer mistakes. Usually it is best to install all the resistors first, as they are usually mounted in a low profile position - flat against the matrix board.

Then you mount other components such as the 3 capacitors, and lastly - the 2 transistors, as they are the highest profile components on the board. If you do this any other way, you may succeed, but make mistakes, so let's begin.

The Board

Lay all the parts out on a clean surface, and beginning with the 4 resistors, take R1 (22k) and fold its lead out wires at right angles (90 degrees) to its body, and do the same with the other 3 resistors too. Insert R1, 2, 3 and 4 where shown on the board layout. Bend the lead outs slightly, so that the resistors won't fall back out again when you tip the board over to make the connections. When inserting the resistors, bear in mind that colour codes are read from top to bottom or left to right.

Next, take the three 10 nF (nano-Farad) capacitors (the little blue ones) and insert them where shown. This is the tricky part. If you get this wrong, the multivibrator won't work, so look closely at the diagram. Do the same as you did with the resistors - bend the leads slightly out from underneath the board, so that they can't fall out when you tip the board over.

Finally, take the first transistor and pay particular attention to its lead outs (see diagram above). These devices MUST be inserted correctly into the matrix board. If you incorrectly install them, you could 'pop' them internally, and they won't be any good for future use. Look at the diagram (showing a 'top view' of the transistor casing) and splay the three leads slightly, so that they will insert in the holes (marked in black on the diagram), underneath each transistor, as indicated on the board layout, above. Bend the leads slightly underneath the board, so that the transistors won't fall out when the board is flipped over, while you make the connections.

When you've finished installing all 9 components, begin at one end of the matrix board (now over on its back) and slowly and carefully, begin to twist the leads together where they meet at a joining point. Common joining points on the board are shown as follows:

all the 'top' leads of R1/R2/R3/R4/ are joined together along the top of the board. You can run a single uninsulated wire (underneath the board) from one side to the other, and then connect each 'top' lead of each resistor to that single wire. Join the RED wire from the battery holder to the right hand end of the wire.

Lead out wires from the 'bottom' of all four resistors, are connected in the following manner:

R1/C1/Q1 collector lead - R2/C1/Q2 base lead - R3/C2/Q1 base lead - R4/C2/C3/Q2 base lead - C3/Probe lead or probe bolt. Run a second uninsulated wire from left to right at the bottom of the board (underneath the board) Connect the BLACK lead from the battery holder to the left hand end of that wire to one tag of the push button (momentary on) switch PB1, and from the other tag of PB1, connect another wire back to the bottom 'common' ground wire, at the bottom of the board.

When you've finished the last lot of twisted wire joins, you can then take your side cutters, and trim off any excess wires from the component leads. Take care to ensure that you don't cut the leads in such a way that a join will come undone!!!

Step 3: Making the Probe and Finishing Off...

The Probe

Attach the two wires for the 2 clips (RED and BLACK) or to one clip (BLACK) and the head end of the bolt, used as a probe. Make sure that you leave enough 'slack' in each of those wires, so that you can remove the matrix board and battery holder should you need to do so (e.g., so you can change the battery over.) That completes the construction of the board and all other component parts, so now, just gently place the board and battery holder side by side, inside the Tic Tac box and insert the white lid, sealing the circuit inside the box.

The Test box needs a suitable probe so that you can easily touch it onto test point around the circuit undergoing the test. In the parts list there are 2 clips specified (that's because Jaycar sells them in pairs) and you can elect to use them both - RED for the probe tip, and BLACK for the ground return lead. If you elect to use an actual probe tip, you can fashion one, as shown in the diagram. Simply take a thin threaded bolt, and make a hole in the clear plastic end of the Tic Tac box. With the wire from the free end of C3 wrapped around the INSIDE part of the bolt (just underneath the head) place a small washer over that, and THEN insert the whole threaded shaft so that it protrudes outside the box, and then thread the nut onto the shaft from the outside, and tighten it. It may also be wise to use a piece of plastic insulation sleeving (stripped from the right sized piece of wire or cable) as a 'sleeve' placed over the probe, leaving just the end (5mm or so) exposed. You could also obtain a short length of 'heat shrink' plastic, and do the same.

Step 4: Testing the Test Box

Now that you've finished building the Tic Tac Test Box, you are ready to 'test' it out! The Test Box emits an audio tone which you can use to trace a signal path through one or more pieces of equipment. So, let's assume that you have built both the Tic Tac radio and the Tic Tac Transistor amp. Remove the earphone from the output of the Tic Tac radio, and connect both leads up to the input leads of the Transistor amp - active to active, and ground wire to ground wire.

Hopefully, both projects are working properly (seeing you have worked so hard to make them!) Remove both matrix boards from their boxes, and then connect the clip lead from the Test Box to the common ground lead. Then connect the Test Box clip lead (or the probe) to the active output/input leads of the radio and the amp. You should hear a radio program, and another tone, superimposed on that program - yes? YES!!! It works!

Or not... don't worry if it doesn't work at first. It's probably just a wiring fault - they are easy enough to make, but you can go back over the 'how to build' part and check the circuit layout, and make sure that each wire and component lead is in the right place...

Well, that's it for the Tic Tac Test Box - see you in the next project - the Tic Tac tone Generator - a circuit that can do a few more tricks... Until then...

Step 5: Tic Tac Tone Generator... Sneak Preview...

Gee - it looks just like another project in a Tic Tac box - well yep - you're right, but this one ain't just any old project. Essentially it's a basic oscillator circuit, but it has some versatility which you can configure, to suit your intended outcome. It can be a 'tick tock' type of metronome (or was that a 'Tic Tac' metronome? ... chuckle :) )

It can blink an LED on and off at varying rates, or it can work at a steady (but still adjustable) tone/frequency output, so you can use it to practice Morse Code (with the addition of a simple, home made morse key.)

Or, you can have a bit of 'slap stick' fun and 'buzz' your friends while they try and take a nap - laugh in the back room, while you watch them trying desperately to swat that non existent mozzie (mosquito) zzzzZZZZ - slap - zzzz zzzzzZZZZ - GOTCHA!! - but they only got your transducer - no mozzies buzzing around at all - ha ha ha!



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    16 Discussions

    Hey rtmessenger - i don't sell kits, but the basic parts list can be bought online or from any decent electronics store. Not sure where you come from so cannot reccomend any...

    I'm working on project to enable a tiny button that when pressed peforms morse code sound for my phone. Just exploring.

    This circuit could do that, but I'm not sure how you could plug it into your phone (cellphone?) There is another project (Tone Generator) which can generate morse code into a speaker - go to the Tic Tac Tone Generator:


    and look at that one - maybe that one would be more suitable for you? It is a simpler circuit - less parts, and more adaptable. mk484

    Hi pacbatskator - this circuit produces an audio tone at the probe. When you want to test an audio circuit, (like a small amplifier) you attach the clip lead on the ground wire of the circuit, then use the probe to see where the signal stops in different places along the signal path in the circuit

    Hi Victor8o5 - thanks for that - want to share with me some of those apps?

    Hi all - thanks for all the looks and favourites - I appreciate all of that - just don't forget to post in with pics and feedback on what you've built (it should look more or less the same - grin :) )


    Of course, it goes without saying, if you can solder, then feel free to go ahead and do so on any of my Tic Tac projects, but just don't get any "burnt fingers" while doing so :) mk484

    Hi JRF! They're meant to be a set - a radio, an amp and a tester, so that kids/newbies will have all they need to get a start, and have some fun... I've got 10 designs all up and am thinking of putting it all in an E-Book...

    i love the idea of reusing existing boxes for electronic projects. Sure we all know the Altoid tin but TicTac's give good possibilities as well.
    I have used the larger tictac box for an ssr project:


    and i have been using those slightly opaque boxes that screws come in:


    2 replies

    Hey diy-bloke - that's a great project and a good use for the Tic Tac boxes too. I like them because, unlike the Altoids tin, they are plastic insulators and are also very workable in drilling and/or punching holes in. The older style boxes used to be made of hard plastic and they would crack all too easily, making them unsafe and therefore unusable. Great stuff - go for it...


    The project generates a fixed frequency tone, which you can inject into an audio circuit (say a simple amplifier,) and then you move the probe from one test point to another (usually the input of a transistor stage,) until the signal disappears, indicating where an electronic fault may have occurred.