Discrete Chain Link Oscillator

The following project will describe how to turn 5 discrete components into a building block which when serially connected will form an extendable flashing lighting chain.

Just a simple discrete solution without the aid of a microcontroller. Even so each block can have a different coloured LED and these can be assembled within the chain in any colour combination required.

Not a replacement for a Neopixel just different.

Point LED red*

BC849 SOT23*

*equivalent components may be substituted.

47K 1206

470R 1206

10uF/16V Electrolytic

21 AWG solid core Enamelled copper wire or flexible wire.

Eagle PCB software

FlatCAM

Candle GRBL

3018Pro or similar

Single sided copper clad PCB board

Alternative Assembly - Stripboard

NPN Transistor (ZTX337)*

3mm Low Current LED*

*equivalent components may be substituted.

47K resistor

470R resistor

10uF/16V Electrolytic capacitor

One set of components per block is required. Just multiply the component (5) by the number of blocks.

Stripboard.

I have no affiliation to any of the suppliers you may have your own preference, the links are only provided for reference and more detail.

The circuit is based on a transistor configured as a fixed bias, common emitter mode switch.

In this configuration RB is connect to the +V supply and the base, we need the base emitter voltage (VBE) >0.7 for the transistor to be switched on and VCE will be <200mV

With the junction of RB and the base connected to 0V then VBE=0V and the transistor is turned off VCE=+V. Therefore with a switch connected from the junction of RB and base to 0V we can switch the transistor on and off.

With the switch open we have a current path via RB with a transistor being a current amplifier we need sufficient current at the base to fully switch the transistor on but how do we know what is sufficient current.

If we connect an LED to the output in addition to a resistor we can see when the transistor switches on.

Using a 5V supply the LED which has a forward voltage VF of ~1.6V and the difference between this and the supply is 4.3V. Imax=V/R therefore if 5mA is through the LED then RLED=860R

Assuming a gain of 100 for the transistor it could be more of less, for the existing components If its significantly less than Imax will be lower and may not light the LED but if it is greater than we will not get more than Imax as its limited by the components and the supply.

Therefore, IB = IC/Gain = 50uA and RB = (5-0.7)/IB = 86kR

With these values and a switch we can turn the LED on and off.

However, this is a manual operation and we want the LED to switch on and off automatically for as long as required.

If we consider a serial Resistor/Capacitor network with a 5V supply and the switch in parallel with the capacitor.

Apply the power and the capacitor will charge up via the resistor until is equals ~5V. this charging will take some time which is calculated by RC, if we take R=86kR and C=10uF the time (Time constant), will be 0.86 seconds.

In reality the capacitor is only charged to 63% of 5V in this time and over the next 37% charges more slowly and to all intents and purposes takes 5 time constants to fully change or 4.3 seconds.

Returning back to the switch if we now close it this discharges the capacitor in RC seconds but as the switch is effectively 0R then time = 0 seconds. In the circuit transistors replace the switches.

Now we have switching control for the LED and a timing element we have our basic building block.

On its own nothing will happen but if connected to a duplicate building block with feedback we have the classic Multi-vibrator.

Why is the building block half a multi-vibrator when we can just build a full multi-vibrator.

In the classic configuration we have a closed loop that appears tied together but if we reconfigure this circuit you can see that its 2 identical sub blocks and we simply connect them together in a chain of sub blocks serially with a single feedback path

In connecting the 2 halves of the Multi-vibrator together how do they then work.

If we make the assumption T1 is switched on the capacitor C1 at its collector will be near zero (VCE), the other end of C1 is connected to the base of T2 via R3 having charged up to VBE>0.7 via R3 switching T2 on and connecting C2 at the collector to near zero and this will start to charge via R2.

However, as C2 was already at 5V and the other end is connected to the base of T1 at 0.7V the voltage across it is 4.3V.

With T2 switched on the rapid change creates a negative voltage of -4.3V at the base of T1 ensuring it is well and truly switched off.

This causes the voltage at the collector of T1 to go high pulling the base of T2 negative via C1 and so the process repeats.

With equal values of R and C in both halves of the circuit the mark space ratio is 1:1

The frequency is calculated from 1/T = 1/1.38(RbC)

Be aware that component tolerance particularly for the capacitors +/-20% may result in differences in timings . These can be minimised by using components of identical type and construction.

Each individual Link Chain Block (LCB), only consists of 5 components, these can be quickly assembled on relatively small pieces of Stripboard or even smaller PCB's which are then linked together.

The stripboard prototypes have connectors fitted to illustrate the ease with which these can be reconfigured.

However, there is no reason why each LCB could not be hardwired for simplicity.

We have the elements of the Link Chain Block its time to design a PCB this design is created in Eagle.

The first thing will be to create the schematic and this will use all surface mounted devices (SMD).

The resistors and capacitors will use the1206 package type. The 1206 package is 3 x 1.5 mm.

While the LED and the transistor will be SOT23 (Small Outline Transistor), 3 x 1.75 x 1.3mm

The intention will be to hand solder the components in place, a magnifier, soldering iron with 0.4mm tip and a steady hand will be required.

For the LED and the transistor its not necessarily critical to use a specifically named part particularly if the part you have is not listed the main criteria is that the size and pinout match.

Once the schematic is complete its a good idea to run the Electrical Rule Check (ERC), to check for any errors at this stage. If there are any, resolve them before continuing.

Next we layout the components on the board in their final physical locations.

With so few components its easy to lay them out with plenty of space to accommodate hand soldering and in a logical flow left to right from the input to the output and at the same time have everything mounted on the top of a single sided board. Board size is 19 x 17mm.

Other considerations are the manufacturing process which will be DIY board milling rather than through commercial means.

As such changing the track width from 6mil to 24mil will result in a more robust finished board.

Once the design is complete running the Design Rule Check (DRC), will let you know if there are any issues that need attention.

For the final part of the PCB design Gerber files will need to be produced..

The milling operation will be performed by a CNC3-3018pro.

In order to control the tool to mill the board the top layer Gerber file previously generated is required.

The file in this case is loaded into FlatCAM.

From the Project tab select the Gerber file and then the Selected Tab to edit the Gerber Object details for Tool diameter, number of passes, pass overlap and combine passes if more than one pass.

In this particular case Tool diameter 0.2mm, 1 pass which negates selecting overlap and combine then select Generate Geometry.

From the Project tab select the ISO file and then the Selected Tab to edit the Geometry Object details for Cut Z, Travel Z, Feed rate and Tool diameter as required.

In this case -Cut Z 0.3 (as cutting down in to the board), Travel Z 2mm (the travel over the board when not cutting), Feed Rate 120 then select Generate.

From the Project tab select the ISO_CNC file and then the Selected Tab to edit the CNC Job Object details as required and select Export G-Code

This will create the milling path file.

The previously created G-Code file is loaded in to Candle (or similar application), in preparation for milling.

Secure the board to be milled to the bed with a flat supporting board beneath it, to prevent damage to the working area if over cutting should occur. I used a piece of 5mm Perspex sheet.

You could Z prolife the area to be milled to compensate for any unevenness at this stage.

As the boards being created were <20 x 20mm I just drew a grid on the copper board, one grid unit per board and placed the cutting tip in the centre and used the shadow method.

With the tip over the board a shadow is cast which becomes progressively clearer an sharper and closer to the tip as the gap reduces, when the tip just touched the board the shadow tip is coincident with the cutting tip on the board.

Alternatively you could connect a meter in diode or resistance mode between the board and the cutting tip and listen for the tone or 0R when the tip just makes contact.

At which point you can Zero Z, jog the tip to the top left corner and Zero XY or just do all the Zero setting in the top left corner. For boards this size I have had no issue with either option for zeroing.

Start the job and wait for it to complete.

For the next board position the tip in the next grid and zero using the appropriate method.

Repeat for as many boards are required.

Separate the boards from the main sheet by cutting along the grid lines.

As a result of the prior milling process burrs will be created along the cut edges these could result in bridging and short circuits and these need to be removed.

At this stage I use a brass brush to both clear the board and remove the burrs by brushing the surface the bristles being soft to both polish and deburr without damaging the board surface.

Then clear with IPA or Methylated Spirit prior to soldering.

As previously described the components will be hand soldered.

Prior to assembly perform a continuity check between the interspaced copper and the isolated tracks in case any shorts exist. If any are found a sharp needle will fit between the cut line to remove it if reapplication of the brush along the cut line does not remove it.

In this case resistors were attached first, followed by the LED & transistor, then the capacitors.(leaded versions used at the time of assembly), interconnecting wire was 21 AWG enamelled copper wire.

During assembly the SOT23 LED was replaced by a Point LED which a a 2 pin package and fits between the required pins.

At each stage perform a continuity check from the interspacing copper to the isolated track just in case a solder bridge has developed.

Better to find faults during assembly rather than when power is applied.

Now that you have assembled as many boards as are required for the intended use all that remains is to connect them together.

I would recommend starting with a two board configuration to enable you to check each board in a minimum configuration just in case there has been an assembly issue. Be sure that LK1 is connected on the first board and LK2 is connected on the last board.

Once you are confident that all the boards are without issues you can proceed to connect them together.

The minimum supply requirements are ~2V at 2mA per pair, below this the LEDs will flash progressively faster until they stop flashing prior to going out. ~2V is the end operating voltage of a 3V/200mA/Hr (CR2032) coin cell so should operate continuously for ~100 hours subject to component tolerances.

There is plenty of score to vary the component values to adjust frequency and LED intensity for reducing power consumption.

The more elements in the chain the shorter the battery.

Alternative battery options up to a 9V (PP3) are not an issue but not recommended above this due to the likelihood of damage.

As an alternative to a PCB a stripboard version is described.

Using the layout, components are mounted on a 6V x 13H hole piece of stripboard.

1: 2 x 6 pin right angle SIL pin headers are mounted on each short edge of the board, these allow each board to be connected to each other .

2: Track cuts are required to isolation areas these can be cut with a track cutter or 3mm drill bit.

3: Two wire links are required to connect to the feedback track depending on whether the board is at the beginning or the end of the chain.

4: Fit the resistors one for the transistor base and the other connected to the collector for the LED.

5: Attach the capacitor.

Finally the semiconductors (LED and transistor), ensuring they are correctly orientated.

A minimum of two boards are required but if you only need two then stick with the Classic Multi-vibrator configuration.

Thanks for reading and hope you found it informative.