Newb question - Single transistor oscilator - Page 1
Forum section: electronics / beginners

author: kalel
author_rank: Frequent Contributor
timestamp: June 22, 2017, 11:38:46 pm
content: By the idea from here:http://www.cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.htmlI tried doing something similar on a small breadboard.The circuit does work and will flash at a rate of maybe 2 Hz or so (not accurate guess by any means), butonlyif I hold the base of the transistor with my finger, while being grounded (feet on the floor).I'm assuming I either didn't hook something up correctly (quite possible, I haven't really used breadboards before, I wouldn't be surprised if I got that simple schematic wrong and shorted something), or the parts used simply won't work since every transistor is different.Parts are as follows:- Dusty old (mostly never used) mini breadboard- Wires providing around 12v- Unmarked resistor (approx 1k)- 16 V 100 uF cap- 3 mm red LED- NPN S9014Any idea what's going on? I still suspect the circuit would work similarly if wired point to point (with the same connections, of course).

author: Cliff Matthews
author_rank: Unknown
timestamp: June 23, 2017, 12:48:07 am
content: That circuit is unreliable. Check this page for ample description on why (under unconventional circuits):http://www.talkingelectronics.com/projects/FlasherCircuits/Page83FlasherCircuitsP1.html

author: Zero999
author_rank: Super Contributor
timestamp: June 23, 2017, 07:49:06 am
content: As mentioned above, the circuit is unreliable.When it works it's because the transistor acts like an avalanche diode, similar to a spark gap. It turns on, when certain voltage threshold (known as the reverse breakdown voltage) is exceeded, thus allowing a current to flow and turns off, when the current falls below a certain threshold. The capacitor charges, via the resistor until the voltage across it exceeds the transistor's reverse breakdown voltage, the transistor turns on, discharging the capacitor via the LED, until the current falls below a certain threshold, then it transistor turns off. The cycle repeats indefinitely.http://www.learningaboutelectronics.com/Articles/Relaxation-oscillator-circuit-with-a-transistor.phpThe trouble is, under certain operating conditions, the transistor can behave like a zener diode and become stable, just dropping the zener voltage. You might be able to get it to work by shorting the base and collector or changing the resistor value.This circuit also damages the transistor, which should be discarded after use. Build a proper two transistor oscillator which will work reliably and not damage the transistors.

author: kalel
author_rank: Frequent Contributor
timestamp: June 23, 2017, 03:14:03 pm
content: That's a pretty great explanation of the circuit. Thank you both!

author: bson
author_rank: Unknown
timestamp: June 24, 2017, 02:01:10 pm
content: The breakdown is actually between E-B, not E-C, and the collector only conducts when there is a current on B due to the breakdown. You should either use B directly (leaving C a NC) or join B and C so there is a distinct E-B breakdown current; the B current is critical (a transistor can'treallybe modeled as back-to-back diodes). Because of the extra diode drop on C vs B most current will come out of B anyway. If you leave it disconnected there is no base current - until you touch it.

author: ali6x944
author_rank: Frequent Contributor
timestamp: June 25, 2017, 03:10:04 pm
content: Quote from: kalel on June 22, 2017, 11:38:46 pmNoise on ur finger are maybe enough to bias the transistor...

author: rfeecs
author_rank: Frequent Contributor
timestamp: June 25, 2017, 05:24:19 pm
content: Quote from: bson on June 24, 2017, 02:01:10 pmNot sure what you are getting at here. The oscillator circuit works because of the BVceo (Breakdown Voltage Collector to Emitter, with Base Open). This has a distinct "snap-back" effect where the voltage across the transistor suddenly drops as the current increases, resulting in a negative differential resistance region.You can read about that here, starting on page 8:https://www.onsemi.com/pub/Collateral/AN1628-D.PDFQuoteIn this circuit and in the video posted previously, the transistor is inserted in reverse. But it is the same effect, except the transistor is operating in reverse mode.Another more conventional single transistor oscillator is made with a unijunction transistor:http://www.electronics-tutorials.ws/power/unijunction-transistor.html

author: kalel
author_rank: Frequent Contributor
timestamp: July 13, 2017, 06:49:59 am
content: Thanks for everyone's help. In the end, I got this simple (component and connection wise) circuit to work by increasing the voltage.I can't measure how stable it is, and since I didn't cut off the base lead, the transistor is collecting everything from the environment. Especially if there's a physical contact, e.g. by touching it with your finger, the oscillation frequency is increased.While not being stable for sure, I guess this is one of the simplest oscillators component-wise. There's just one transistor, one capacitor, and one resistor. It worked with my transistor at around 13.6v circuit input voltage (minus whatever drop there might be from the current drawn). I'm sure that each transistor type will be different regarding the voltage required.I know a 555 timer (tried one), or some other relaxation oscillator circuits with different components would work better (didn't try any, need the components), but sometimes you just want an LED to flash, and do so as simply as possible. The real challenge here would be getting a specific and stable frequency, which fortunately I don't need for the LED.P.S. it might damage the transistor as mentioned in this thread, but I don't know by how much this is, and it's probably not something that can be simply measured (e.g. would such a LED flasher be able to run for years, would the transistor break before other components stop working properly, e.g. the capacitor or the LED itself).