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for a couple of years I have been trying to make this one oscillator circuit. 10/10 times it does not work. The best result I can get is the led being constantly on and dimmable with the pot resistor. amplification with out oscillation???

I know I could make the circuit in eagle cad, put together on a pcb and make it work. what am I missing????? I have tried trouble shooting the circuit and the best conclusion I can come up with is something is not working right or being bypassed around the two transistor ans capacitor. enter image description here

enter image description here

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  • \$\begingroup\$ 1) The load must be a light bulb. I don't think that a LED will work. 2) Increase the value of R2 to 10k or 15k. This is a classic old circuit and I'll have to take a closer look at it to refresh my memory about how it works - but R2 looks like a problem. \$\endgroup\$ – Dwayne Reid Mar 31 '15 at 3:12
  • \$\begingroup\$ What LED? It may not work with an LED, the schematic shows a filament lamp. Also R2 is not necessary (or even desirable). I wouldn't put in R3 either. kevin \$\endgroup\$ – Kevin White Mar 31 '15 at 3:12
  • \$\begingroup\$ If you have to use an LED then replace L1 with a 1K resistor and put the LED in series with a 100 ohm resistor from the collector of Q2 to ground. \$\endgroup\$ – Kevin White Mar 31 '15 at 3:16
  • \$\begingroup\$ Thanks for all the help guys!!! ill have to try those solutions out and get back to you. There was this other schematic that had the same set up but it was for a tracking light or something, can't remember if I tried it. \$\endgroup\$ – Chris Manning Mar 31 '15 at 3:18
  • \$\begingroup\$ @KevinWhite why do you say that R3 should be omitted ? I believe its purpose is to ensure that Q2 turns off properly when Q1 turns off. \$\endgroup\$ – efox29 Mar 31 '15 at 3:23
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The circuit is more complex and more fiddly than is apparent.
There are a number of factors that make this so but:

C1 polarity reverses and
C1-Q1_base junction is below ground for a significant part of the cycle.

This is an intended part of the design, but not one that most people are aware of or that is overly intuitive without some thought as to how the circuit works.

Timing occurs on capacitor charge and discharge but mostly on the latter as the on pulse is usually a low percentage of the duty cycle. When Q2 turns off the right hand end drops from near V+ to near V- and its left hand end falls to below ground. The capacitor is arguably shown with the wrong polarity for normal use, but in this case the design limits the +degree of reverse polarisation of the capacitor.

The product of the betas (current gains of the two transistors has a critical maximum value. Too HIGH betas kills operation

The current in R2 when Q1 Q2 are on multiplied by B1 and B2 sets the maximum stable on state current in L1. If the supportable current is > actual trhen the circuit is liable to be stable in the on state. It's not exactly at the limit exactly when
Ir1 x B1 x B2 < Il
as there are other scaling factors but it tends to be proportional to this expression. That means that eg if a circuit works with lowish beta transistors it may stop working if higher beta transistors are substituted.

Note that it can be oscillating at high frequency so the LED appears always on but isn't.

The main timing occurs by having Q1 base driven NEGATIVE wrt ground and charging up via the R1/R2 divider. He has used a low value cap and relies on R1 being adjustable to the crucial point that allows a long charge time due to "just enough drive".
Better is a larger cap (100 uF good, more better probably, a larger value of R2 and maybe 1 1M pot.

An LED can be used - an eg 1k or even 100R across the LED to start may get you going.


One cell LED driver:

Here is a variation that I devised 15++ years ago.
It can hardly have been original even then, but I've never seen an older version. (Somebody will have).

0.7V < Vin 1.5 V probably

Can be used on higher voltages with extra components.

L1 = something lying around - probably 100's of uH to some mH OK.

R2 and C1 set flash rate and can be adjusted for a slow flash or apparently a steady output due to high frequency. The LED can be in either position shown but not both at once.
The LED2 position is probably the best as the LED gets Vin_batt + the Inductor ringing voltage.
LED1 gets inductor ring voltage only.

enter image description here

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  • \$\begingroup\$ Thanks for the help!!!!! Thats a little bit to take in at first, but the circuit makes more sense. So what your generally saying is that, my transistors beta's/amplification has to be with in the right range for the circuit to oscillate and transistors to turn each other on and off. right?? and c1 ones polarity isn't supposed to be reversed causing C1-Q1 base junction being below ground for most of the cycle?? I kind of understand what you were talking about with the first highlight part. Thank you!!!!!!! \$\endgroup\$ – Chris Manning Apr 3 '15 at 22:50
  • \$\begingroup\$ @ChrisManning - The capacitor polarity reversal and 'below ground' feature are an intentional part of the design - just not very obvious on casual inspection. Unless people think circuit operation through in detail and/or test one in operation and/or simulate it the feature is easily missed. | Re Beta - yes - too much beta can be hard to deal with - not usally the case - with high beta it takes very little Q1 base drive to hold the whole circuit on the whole time. Increasing load current helps prevent this. Hence my suggestion to try starting with an R across the LED and other's comments ... \$\endgroup\$ – Russell McMahon Apr 3 '15 at 23:39
  • \$\begingroup\$ ... about not working with LEDs. Higher R2 also helps but then other factors also occur. Overall a tricky circuit that works well when it works but needs some cajoling. \$\endgroup\$ – Russell McMahon Apr 3 '15 at 23:40
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Back to back Joule thieves. One is an NPN and the other is a PNP. they both use the same power supply but reverse polarities. But the outputs both go into a Bridge rectifier to run an LED. You may try an 8 ohm and 1200uH audio transformers 2 and 2 transistors. With a bipolar capacitor of high capacity.

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