QUANTSUFF's Ultimate Flasher page

QUANTSUFF'S CIRCUIT PAGE.
The evolvement of the One transistor flasher circuit

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We had started with the 3-coil Mk2 light and combined it with an RC timing circuit to make it flash, but we added a unique twist. When Q1 turns on, it uses the stored energy in C1, but Q2 will bypass the capacitor to charge the coil directly from the battery. This allows us to use a smaller C1 and still get the superior performance. I even posted a proof of concept video here. I was preoccupied with making a coil that everyone could duplicate so I designed, and tested this with an air core, and it did not like ferrite or toroids. I have the construction details of the air coil further down on the page.
    The video demo also showed that the flasher will still work without the parts associated with Q2 (the P2 coil and R2).
    A point was raised about possibility that capacitor C1 was exposed to voltages of about -0.5v, but according to Cornell Dubilier, "[electrolytic] capacitors can withstand continuous application of 1.5 V reverse voltage."

Each burst is a flash of the LED. (Left) A single coil gives a 10mS blink, while (Right) with 2 coils driven, it is 6-times longer, and brighter as well.
In the circuit above Q2 acts almost like a parasite - it turns on when Q1 does and picks up about 50% of Q1's load. But because it is connected directly to the battery, it does not affect the charge in the timing capacitor, and, as a result, we can make do with a smaller capacitor. If we continue this line of investigation, how much of Q1's work can we offload to Q2?
    Well, the answer is almost everything! In fact, we can eliminate Q1 completely, and we'll have the improbable looking circuit below. Here, Q2 charges up the coil through P2, but it is discharged through P1, just as if Q1 was there! All capacitor C1 does now is supply the bias voltage to turn the LED on, which means not only do we get a nice bright flash, the capacitor can be an entire magnitude smaller. Simply adjust R1 to keep the R1*C1 time constant the same.

Change R2 to 1K and you can use a 2N4401 or PN2222; for 2N3904 R2 should be 1.5K. R3 can then be as high as 100-ohms.

The tools here can help you work out how to wind air-core coils.

As it turned out, we could take this a step further and this final iteration of the SJT flash circuit brings us full circle. Everything to the right of T1 is the basic SJT. The extra winding on the left side does nothing but supply the bias drive to the transistor, allowing us to reduce the size of the timing capacitor even further. Another advantage is that now the LED will be driven at maximum brightness for the entire cycle. Thanks to WatsonsEblog for the idea to put C on B+ to avoid any undershoot voltages. A short video is here on YouTube.
    The flash time is approximately R*C seconds, C can be 2uF to 30uF and you can adjust R from 47K to 1Meg. R2 is 1.5K for BC337 and FJN (2SD-) 965; 2.2k for PN2222 and 2N4401, and 1K for medium amplifiers like MPS651 and ZTX694. Use a 470pF for C1 if you are using 2 LEDs in parallel. The battery should be bypassed by a 5uF capacitor.

A closer look at the layout (left), showing the 3-wires which comprise the coil. The Green is a #32 wirewrap, used for the secondary; the Yellow is #26 for Q1 and the enamel (magnet) wire is a #24, used for Q2, which carries more of the current. The pen behind that is the form I used. Note also the plastic-ties used to hold the wires tightly.

Several coils using different kinds of wire - they will all work as long as you have 40+turns of each wire.

This may give you an idea of how powerful the flash is - the circuit, using a BC337-25 transistor is lighting up 10 different LEDs.

Click to enlarge For further experimentation, it IS possible to omit the winding going to diode D1 to get the circuit on the left. However it is not very stable, and performance varies with the impedence of the power supply.
    Better results can be had by doubling the number of turns for the secondary, in fact the circuit will then run even without R1. However, fabricating an inductor with two coils with uneven number of turns is probably no easier than making one with three equal coils.

See my other Projects on Instructables.com

Questions? Suggestions? Send me email!

	We had started with the 3-coil Mk2 light and combined it with an RC timing circuit to make it flash, but we added a unique twist. When Q1 turns on, it uses the stored energy in C1, but Q2 will bypass the capacitor to charge the coil directly from the battery. This allows us to use a smaller C1 and still get the superior performance. I even posted a proof of concept video here. I was preoccupied with making a coil that everyone could duplicate so I designed, and tested this with an air core, and it did not like ferrite or toroids. I have the construction details of the air coil further down on the page. The video demo also showed that the flasher will still work without the parts associated with Q2 (the P2 coil and R2). A point was raised about possibility that capacitor C1 was exposed to voltages of about -0.5v, but according to Cornell Dubilier, "[electrolytic] capacitors can withstand continuous application of 1.5 V reverse voltage."
Each burst is a flash of the LED. (Left) A single coil gives a 10mS blink, while (Right) with 2 coils driven, it is 6-times longer, and brighter as well.
	In the circuit above Q2 acts almost like a parasite - it turns on when Q1 does and picks up about 50% of Q1's load. But because it is connected directly to the battery, it does not affect the charge in the timing capacitor, and, as a result, we can make do with a smaller capacitor. If we continue this line of investigation, how much of Q1's work can we offload to Q2? Well, the answer is almost everything! In fact, we can eliminate Q1 completely, and we'll have the improbable looking circuit below. Here, Q2 charges up the coil through P2, but it is discharged through P1, just as if Q1 was there! All capacitor C1 does now is supply the bias voltage to turn the LED on, which means not only do we get a nice bright flash, the capacitor can be an entire magnitude smaller. Simply adjust R1 to keep the R1*C1 time constant the same. Change R2 to 1K and you can use a 2N4401 or PN2222; for 2N3904 R2 should be 1.5K. R3 can then be as high as 100-ohms.
The tools here can help you work out how to wind air-core coils.
	As it turned out, we could take this a step further and this final iteration of the SJT flash circuit brings us full circle. Everything to the right of T1 is the basic SJT. The extra winding on the left side does nothing but supply the bias drive to the transistor, allowing us to reduce the size of the timing capacitor even further. Another advantage is that now the LED will be driven at maximum brightness for the entire cycle. Thanks to WatsonsEblog for the idea to put C on B+ to avoid any undershoot voltages. A short video is here on YouTube. The flash time is approximately R*C seconds, C can be 2uF to 30uF and you can adjust R from 47K to 1Meg. R2 is 1.5K for BC337 and FJN (2SD-) 965; 2.2k for PN2222 and 2N4401, and 1K for medium amplifiers like MPS651 and ZTX694. Use a 470pF for C1 if you are using 2 LEDs in parallel. The battery should be bypassed by a 5uF capacitor.
	A closer look at the layout (left), showing the 3-wires which comprise the coil. The Green is a #32 wirewrap, used for the secondary; the Yellow is #26 for Q1 and the enamel (magnet) wire is a #24, used for Q2, which carries more of the current. The pen behind that is the form I used. Note also the plastic-ties used to hold the wires tightly. Several coils using different kinds of wire - they will all work as long as you have 40+turns of each wire.
This may give you an idea of how powerful the flash is - the circuit, using a BC337-25 transistor is lighting up 10 different LEDs.
Click to enlarge	For further experimentation, it IS possible to omit the winding going to diode D1 to get the circuit on the left. However it is not very stable, and performance varies with the impedence of the power supply. Better results can be had by doubling the number of turns for the secondary, in fact the circuit will then run even without R1. However, fabricating an inductor with two coils with uneven number of turns is probably no easier than making one with three equal coils.