The HJW Electronics Choccy Block Flashing Lamp Flip-Flop: A No Soldering Alternate Flashing Light Kit

A complete kit of all the electronic parts shown is available, and the details of how to buy are shown right here:

Sales Outlets

the UK you can buy directly from me using the email address with payment via Paypal or BACS.  The current price is £9.95 plus £1.95 for 2nd class post or £2.95 for 1st.  I can offer sensible discounts and post multiple units in one packet if you're paying via BACS to cut costs.

The kit is also available on various online marketplaces, where overseas shipping options are available to some countries.

OnBuy listing under EAN 5060971140025:  Choccy Block Alternate Flashing LED Light Flip-Flop Kit Of Parts on OnBuy

Amazon listing under EAN 5060971140025:
  Choccy Block Alternate Flashing Light Flip-Flop Kit Of Electronics Parts on Amazon

Etsy listing, the best option for international shipping: 
Alternate Flashing Light Flip-Flop Choccy Block Terminal Strip Kit of Electronic Parts on Etsy

Etsy shop top level for other kits and components:  UsefulComponents Etsy Shop

HJW Electronics CBFF-1 Alternate
                  Flashing Light Flip-Flop KitHJW
                  Electronics Trademark Logo


This is a light flashing circuit which alternately flashes two LEDs or two lamps.  It is based on a circuit called a flip-flop or astable multivibrator and it has two additional transistors to allow you to flash two sets of several LEDs or lamps wired in parallel.

Choccy Block
            Flip Flop Kit Of Electronic PartsSolderless Flashing
            Light Kit Completed

Picture of the flashing light flip-flop schematic diagram.  Click on any picture to enlarge.

            flashing light flip-flop schematic

Flashing Light Flip-Flop 1 Bill Of Materials

Electronic Parts Included in the Kit.  Click on the links for details and pictures.

Qty    Refs          Type              Value

2      R1,R2         Resistor          4.7K 0.5W
2      R3,R4         Resistor          68K 0.5W, alt values 47K and 100K
2      R5,R6         Resistor          470R 0.5W
2      C1,C2         Capacitor         10uF NP
2      Q1,Q2         Transistor        BC549C
2      Q3,Q4         Transistor        BD140-16


1                    12-way 3A Terminal Strip
1                    4 X AA Battery Holder
2                    2 X M.E.S. lampholder
2                    2 X 6V 0.1A lamps
1                    D1 red wired LED with resistor
1                    D2 green wired LED with resistor
0.5                  100mm black single core wire
1                    100mm red single core wire
4                    100mm orange single core wire      

Link to PDF of Flashing Lamp Flip-Flop Schematic
Link to PDF of Flashing Lamp Bill Of Materials



I supply a kit of all the electronic parts in the list above, or you can buy your own.  I've assumed that you have a small screwdriver for the terminal screws,
four AA batteries, some small pliers, some small wire cutters for trimming the component leads and some means of stripping the insulation from the wire.  A craft knife is good for that.  It is best to mount up the circuit in an enclosure or nail it down to some wood when it is all working, and the holes in the terminal strips make that easy to do.


Use normal sense and don't short circuit the batteries, i.e. take care not to connect the red wire directly to the black wire.  Modern alkaline batteries produce a fair amount of current and the wires and batteries may start to get warm.  Do not use rechargeable batteries.  Rechargeable batteries can produce large currents if accidentally short-circuited.

What do you need to know before you start?

This is a fairly simple circuit which you can make pretty much just by looking at the pictures.  The components list has links to photographs of the components and some further information.  I've included a full electronic explanation later on.  If that doesn't make any sense, don't let it put you off just building the circuit.

How To Build It: Follow The Pictures and Schematic Diagram in Three Stages

There are three main stages to building up the circuit, and these were chosen so that you can see where the earlier components go in the pictures before they are partially covered up by the later stages.

Stage 1, Fit the Transistors, Wires links and Resistors

Stage 1 Assembly Top

Bend the leads as shown and assemble Q1,Q2,Q3,Q4,R1,R2,R3,R4.  Strip about 5mm of insulation from the the black wire link and the red wire link and assemble them as shown.  Don't tighten the screws too much at this stage as you may want to re-adjust the assembly.  It's best if you can get the leads in the right places so that they go right under the screws and don't get stuck up the sides where they may be loose later on.  The following pictures show some more angles and the leads going right under the screws.  Note that the transistors are fitted the opposite way around on each side of the circuit.  Q1 and Q4 are fitted writing-side up.  The resistors can be fitted either way round but it will make checking easier if you put them with the coloured bands facing the same way as in the pictures.

Stage 1 flip-flop
            assembly bottomStage 1 Flip-Flop Assembly Side

Stage 1 Flip-Flop
            Assembly Resistors Side

Stage 2, Fit the Capacitors and Resistors

Stage 2 Construction
            Top View

Now fit C1,C2 the two 10uF electrolytic capacitors and R5,R6 resistors.  The capacitors bridge several connections so you need to bend the wires so that they don't touch any others.  These are non-polar capacitors so they can connect either way round.  The marks C1x and C1y in the picture show where the wires of C1 go in.

Stage 2 Assembly
            Bottom View

Stage 2 Assembly
            Side View

Stage 2 Assembly
            Side View 2

Stage 3, Connect the LEDs and Battery Holder

Stage 3 Assembly,
            LEDs and Battery Holder

Strip back some of the insulation from the LED and battery holder wires and fit them in as shown.

Stage 3 Assembly
            Bottom ViewLED Connections Side View

Stage 3 Assembly Wide
            ViewStage 3
            Assembly Power Wires Side View

Thatís It, but Check and Secure

Have a final check around comparing against the pictures and make sure that the wires are secure.  You can now fit four AA batteries with the negative ends towards the spring contacts and the LEDs should immediately start to flash alternately, a little faster than once per second.   

Filament Bulbs and More LEDs
If the LEDs are working you can connect the two filament bulbs if needed.  The single core orange wire should be put through the hole in the contacts on the holders and wound tightly around and crimped down with pliers so that it forms a tight bond which doesn't wobble.  The bulbs provided are 6V 0.1A but the transistors can cope with up to 0.5 Amps.  More LEDs can be added noting that each one needs a resistor in series.  For 6V operation of standard 20mA LEDs that will be a 220 Ohm resistor.

The Completed Flip-Flop in Operation, YouTube Video

Astable Multivibrator
            in OperationThe Completed Flip-Flop in Operation

Changing the Flashing Rate or Duty Cycle

Using 68K for R3 and R4 and 10uF capacitors results in a flashing rate of slightly more than once per second.  The flashing rate varies in inverse proportion to the resistance, so you can increase the flashing rate by changing them to 47K or decrease it by using the 100K resistors.  Using a lower value resistor on one side only will change the duty cycle and give a shorter flash on that side.

12V Operation

Operation on 12 Volts is possible from small batteries or a current limited supply if you increase the resistor in your LEDs to 560 Ohms or change the bulbs to 12V ones.  The maximum current is 0.5A so check the rating on the bulbs.  I don't recommend using 12V supplies from a car or car battery as they are very high current and accidentally shorting them can cause fires.

How It Works While Running

Forget about Q3 and Q4 for now.  They have only a small effect on Q1 and Q2 and can be considered separately from the switching action.  Looking at the oscilloscope pictures further down the page might help here.

Imagine that there are no capacitors C1 or C2 connected.  Transistors Q1 and Q2 are high gain amplifiers.  A small current flowing through the base to the emitter will cause a larger current to flow from the collector to the emitter.  Resistors R3 and R4 provide just such a small base current from the positive rail.  The base voltages are about 0.7V which is what you normally see when some small current is passing through them in the direction of the arrow on the transistor symbol.  If there were no capacitors C1 and C2, this is just what you would see as two independent circuits with no interconnections.  The collector of Q1 would be at a low voltage pulling current through R1, as would the collector of Q2 pulling current through R2.  Imagine first that C2 has been connected.  For a short time it will pull a bit of current out of Q1 base as it is effectively connected to ground by Q2s collector before everything settles down again. 

Now we require a leap of faith.  Imagine that the right hand side of C1 has somehow been charged to minus 6V and connected to Q2 base.  This turns Q2 off completely, Q2 collector rises to 6V pulled up by R2, and suddenly dumps charge through C2 into Q1 base, turning it on even harder and causing a small voltage blip on Q1 base.  Meanwhile, that minus 6V on the right of C1, (Q2 base) is rising all the time, charging up slowly through R4.  When it reaches the turn-on voltage of Q2, about 0.7V, Q2 collector voltage drops quite rapidly pulling current out of Q1 base turning it off, Q1 collector starts rising and dumps charge into Q2 base making the collector turn on harder and creating that blip of voltage on Q2 base.  Because C2 was charged up to 6V on the right, and that end has now been pulled to zero volts, the left side of it is now at minus 6V turning Q1 right off.  Meanwhile, R3 is now slowly charging up the left hand side of C2 from that minus 6V ready to turn on Q1 base again.  As Q1 base starts to turn on, Q1 collector falls rapidly pulling current out of Q2 base via C1 and ending up with the right side of C1 at minus 6V.  That's where we started out and you can see that the cycle must continue.

As Q1 starts to turn on fairly gradually as the base voltage climbs slowly from about 0.5V to 0.7V via the fairly weak resistor R3, its high gain and the threshold voltage (Vbe) around 0.7V is what causes a more rapid drop in Q1 collector voltage turning Q2 off rapidly via C1, and the rising Q2 collector voltage on the other side reinforces the Q1 turn on via C2.  It's this positive feedback loop that causes the transistors to switch, or flip, quite sharply once the threshold has been passed.  Hence the name. 

Oscilloscope Pictures Of The Circuit in Operation. 

In order to get a clear trace on an analogue oscilloscope I fitted 0.01uF capacitors so that the circuit was oscillating at about 900Hz.  The vertical range is 2V/division and the horizontal is 0.2mS/division.  Zero volts is marked by the underlining in the CH1/CH2 text.

Q1 and Q2 Base Voltages

Q1 base and Q2
            base in Flip-Flop Circuit Oscilloscope Picture

This shows that the base of the transistors and one side of the capacitors really do spend a lot of their time rising from almost six Volts below the zero volt rail while charging through the timing resistor.

Q1 and Q2 Collector Voltages

            multivibrator transistor collector voltages

You can see that the traces drop much faster as the transistors pull down strongly when switched on, and rise more slowly as the stray capacitances of the LEDs etc. charge up through R1 and R2

Q1 Base versus Q1 Collector Voltages

Q1 Flip-Flop
            Base versus Collector Voltages Oscilloscope Picture

Q1 base versus Q2 Collector Voltages

Flip-Flop Q1
            base versus Q2 collector voltages

This last picture has the probes across one of the capacitors and shows the place in time where the voltage across the capacitor C1 gets reversed for a short period.  That is when the CH2 trace is still down near 0V but the CH1 trace has risen significantly above 0V, just before Q1 starts to turn on.  The little jump and squiggle on the top trace at this point is when Q2 turns off and the rising collector voltage pushes positive charge from C2 into Q1 base to turn it on even harder.

Q3 and Q4 Output Transistors.

Q3 and Q4 are used to isolate the low current switching action of Q1 and Q2 from the effect of the main loads, the LEDs or the lamps. Q3 and Q4 are PNP medium power transistors, unlike the small signal NPN transistors used for Q1 and Q2.  PNP transistors are used so that it is the positive supply rail that is turned on and off, with the current returning possibly via a common ground wire.  Also, some current is required to drive through the bases of Q3 and Q4.  If they were NPN, that current would have to be supplied as a positive current though R1 and R2 which would be fairly weak.  Using a PNP allows that negative current to be supplied by Q1 turning on which is a much stronger source.  When Q1 is turned off there is no current through Q3 base as R1 and R5 are pulling the voltage up toward the positive rail.  When Q1 turns on, current flows through Q3 base in the direction of the arrow on the symbol, down through Q1, again, in the direction of the arrow.  This causes a large current to flow down from Q3 emitter to collector, through the LEDs or lamps.  Q3 typically has a current gain of about 100 at 100mA collector current, so the base needs a current drive of about 1mA.  Q1 can do this easily thanks to its own current gain of about 300, even though its own base is only driven by the 68K resistor R3 (most of the time) giving about 70uA.  R5 limits the base current of Q3 to about 10mA.  Because the output transistors are either completely off or are turned on quite strongly except when they're switching, they don't dissipate much heat even running at 0.5A, which they only do for half the time anyway.  There is no need to attach them to any kind of heatsink.

It's possible to put LEDs with series resistors or indeed low current bulbs in the locations where R1 and R2 are.  Then you can just use two transistors.  This has some disadvantages.  You need an equal load on each side. 
If one of the bulbs breaks, the circuit stops.  If the bulb is too high power rated, the small transistor won't pull the collector all the way to ground.  As discussed, that helps the circuit to oscillate in a clean way.  You can use bigger transistors, but their lower gain means lower values for R3 and R4 which means bigger capacitors.

How It Works While Starting Up

Thinking about what happens when the circuit is first starting up can make your brain hurt.  You can see that I had to employ a leap of faith to get started in the description above, and hopefully convinced you with the arguments about the transistor gains and the reinforcing action of the cross-connected capacitors during switching.  It's possible to imagine though, that when starting up, you could end up up with a draw in the race to switch first where the circuit would be stuck in some middle state.  In practice small differences in the values of the components and transistor gains makes one side switch on first.  Even if everything was perfectly matched, tiny amounts of electrical noise would ensure that the balancing act was pushed in one direction, then that side would win out due to the positive feedback connection and the normal cycle would start.  People have written entire PhDs on the subject of oscillator start-up.

More Information

According to Wikipedia the original flip-flop circuit was implemented with valves early in the twentieth century, though I once made one with two relays so I think it probably goes back even further.  I would not be surprised to see something equivalent in clockwork or you could probably even devise a scheme which used flow of water with alternate tipping buckets. 

By using three or more switching transistors and it's possible to make a running light effect, but I remember that getting it to start up correctly was a bit more challenging.  It's probably time to switch to using digital gates at that stage of complexity.


Q:  Doesn't this looks a bit like that circuit in the 1979 Ladybird Book, "Simple Electronics?"
A:  Yes it does, though this is better.  Even in 1979, using AC128 germanium transistors was a bit a crime against engineering.

Q:  Can I drive relays instead of lamps?
A:  Yes.  You'll need ones with 5V coils or 12V coils if you switch to a 12V supply.  It's usual to put a reverse biased diode across the relay coil to avoid the back emf damaging the transistors when it turns off.

Q:  It just won't work and I've tried everything.  What can I do?
A:  My email address is shown below.  If you can send me well-focussed, good quality pictures of the main strip from several angles and of the whole thing, I can usually get you going.

Q:  Why bother with the extra driver transistors?  Isn't it just more parts and more difficult to build?
A:  A bit, but you can drive real things with this circuit, not just two pathetic LEDs.  Also, the driver transistors isolate the load from the actual flip-flop circuit so that, for example, mismatched bulbs won't cause the circuit to operate in a lop-sided manner.

Should you have any problems or questions, my main email address is shown below.  This address has been the same since 1997 and unless I'm on holiday beyond mobile coverage, it is checked daily including the spam folder.


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  © Henry J. Walmsley 2014.

Edit History

26-AUG-2022: Removed references to my ebay shop, having been "binned-off" from that site for telling the truth about their b
usiness practices to the UK vice-president Murray Lambell. Twice.
28-MAY-2023: Added OnBuy and Amazon buying links.
28-SEP-2023: Added HJW Electronics logo, pack shot, and some minor edits.