There is a certain amount of skill required to design a circuit but a lot of skill to simplify it.
There are lots of transistor tester circuits on the web and they all do the one thing - identify a PNP or NPN transistor. Some provide the gain and even the maximum operating voltage but in most cases we only want to know the c, b, e pin-out and if the transistor is NPN or PNP.
That's what this tester does.
You fit an NPN or PNP transistor into one of the circuits with the leads around the correct way and the LED or LEDs will illuminate.
The 1k5 resistor limits the current into the base so the feedback winding can produce a voltage that adds to the supply voltage and turns the transistor ON harder to produce part of the cycle.
The operation of the circuit is quite complex because it involves the action of a transformer and this is always quite complex.
When current flows through a single turn of a coil, it produces magnetic flux and this flux flows around the wire so that it all comes out the centre of the turn. When more turns are added, the density of the flux increases as each turn adds more flux.
The supply for this project is from a battery and it produces flux called STATIONARY FLUX when a coil is connected to a voltage called a DC supply.
But when it is initially connected to a DC supply, the flux gradually builds up from zero to a maximum.
During this time the flux is called EXPANDING FLUX. This takes on a few microseconds to occur.
When the coil is removed from the DC supply, the magnetic flux collapses and it is called COLLAPSING FLUX.
Thus we can get 3 different types of flux from a DC supply.
When the flux collapses, it cuts all the turns of the coil and produces a voltage in each of the turns that is opposite to the applied voltage.
In addition, this flux collapses very quickly and the voltage produced is much higher than the applied voltage.
Finally, there is one more important fact you need to know about magnetic flux.
When the flux is expanding or collapsing, it cuts the turns of the other coil in the circuit and produces a voltage in the turns.
BUT when the flux is stationary, it does NOT produce any voltage in the other coil. It is only EXPANDING or COLLAPSING flux that induces a voltage in a second coil. The first coil is called the PRIMARY coil and the other coil is called the SECONDARY COIL or FEEDBACK coil.
This means the PRIMARY coil produces a voltage in the FEEDBACK WINDING when the PRIMARY is connected and when it is removed, but when it is fully connected and producing STATIONARY FLUX, the FEEDBACK WINDING is not producing any voltage.
This is the secret behind the operation of the transformer and you do not know the magnitude of the voltage produced by either of the coils until you build the project.
On top of this, you do not know how fast the circuit will oscillator until it is built. These are all characteristics of the coil, battery voltage, value of resistor, type of transistor, layout and many other things. We are not going onto any of this as you can see the result and measure the frequency.
The two coils of wire turns the circuit into a very complex arrangement that needs a high degree of understanding to see how it works. We will try to simplify everything by getting you to build the circuit and take measurements.
Two separate circuits are needed because an NPN transistor needs a positive voltage on the base while an PNP transistor needs a voltage that is less than the emitter.
Combining the two circuits would need a switch to change the positions of the coils and a coil of wire is simpler and cheaper to add to the PC board.
You can increase the brightness of the LED by placing a 100p to 4n7 across the 1k5 resistor. This changes the waveform from 8v to 7v but increases the frequency to more than 1MHz and the width of the spike is increased.
The capacitor effectively stops some of the spike being lost or being driven into the 1k5 resistor and now turns on the transistor more so a greater current flows. This current produces a healthier waveform when the coil collapses and delivers more energy to the LED.
The frequency increases because the transistor turns ON faster.
A kit of components is available from Talking Electronics.
All the components are included in the kit and everything is identified on the board.
The only fiddly job is making the two transformers.
This is the last job. Make sure all the other components are fitted to the board before making the coils.
Fit the NPN transistor to the board.
Wind 40 turns on a pen or shaft 12mm dia (0.5in) and make sure the shaft is tapered so the coil can be easily removed.
If the shaft is parallel, add a match or the lead of a resistor to the diameter and wind over this at the same time. Remove the wire and the coil will become lose and easily removed.
Before you remove the coil, twist the two ends together three times and cut the leads short.
Make 4 identical coils.
Place two coils on top of each other and keep them together by winding the 0.25mm wire around them as shown in the photo above.
Do not solder the ends of this wire together as this will produce a "shorted turn" and the transformer will not work.
Tin the 4 leads and fit two of the leads of one coil to the board.
Poke the other two leads down the holes in the board and the two LEDs should illuminate because the project is COMPLETE.
If the LEDs do not illuminates, reverse the two leads.
When the LEDs illuminates, you can solder them in position.
Carry out the same procedure with the PNP transistor.
Here is a photo of an NPN transistor fitted to the tester and
illuminating the red and green LEDs. The LEDs are very bright because
HIGH BRIGHT LEDs are included in the kit.
The cell is held tight by soldering two wires across the top and
bottom when it is in position.
(buy a number of kits and pay
(buy a number of kits and pay