Flashing Led's

Question

Hello, I am new to this site and loving it. I did a bit of Electronics in 1995 where I made a flashing led circuit with 2 led's and it was cool. My other hobby is model trains and I would like to put 2 small led's in buffer stops and make them flash and run it off a DC train controller. Does anyone have a simple circuit that would do the job for this? I am basically a noob at electronics but I can follow a simple circuit.

Thank you.

Gary.

14rhb · Answer

Hi Gary, Welcome to Element 14 Community. Although a long time ago, you have already completed the first step on the world of modern electronics - the famous 'blinky' experiment . I'm assuming you want your LEDs to alternate. There are many ways of doing this from relays, microcontrollers, 555 timers etc but what will be easiest is probably to search for "Astable Multivibrator + transistor". One good link I have just found is this https://www.build-electronic-circuits.com/blinking-led-circuit/ (however I apologise to other E14 members as in my own comments I have stated that I don't like posting links off site). A little way down that linked-page is a circuit titled Astable Multivibrator using transistors. If you are in the UK you can get the parts from somewhere like CPC Farnell (nice and low cost delivery), Farnell UK or in America use Newark. The transistors are nothing special, so 2N3904 would be fine; there are many other suitable ones. Again a quick Google and you will likely find a full parts list. You need to adjust the values of R1 and R4 to suit the Power Supply you are using - for 12v I would increase them to something like 750 ohms. Keep asking if you are unsure of anything, and I'd really like to know how you get on (eg a photo of the finished buffer stop lights in action ) Rod

beacon_dave · Answer

Probably can't get much simpler than a flashing LED: https://uk.farnell.com/c/optoelectronics-displays/led-products/flashing-blinking-leds Size may be an issue however. 3mm is the smallest listed.

fmilburn · Answer

Hi Rod and Gary, I am more of a digital guy but I saw this and had to build it. The circuit is close to the first link that Rod gave above. The voltage source is 4 AAA, R1 and R4 are 330 ohm, R2 and R3 are 47k, the caps are 22 uF, and the transistors are 2N3904.

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14rhb · Answer

If you have loads of 1k resistors and don't mind using several of them up you could parallel two to get 500 ohms. Put three in parallel to get 330 ohms. Then put the double bundle in series with the triple bundle to get 830 ohms....probably about right for your 14v supply.

If you are buying a resistor for R1 and R4 I would choose something around 680 ohms for a 14.2v supply (14.2 v - 0.2 v drop in transistor, 20mA design current for LED ) and they should be a good brightness. That assumes a fairly standard LED current, if you know the part numbers of the LEDs you have then you could look up the If (the forward current they recommend). Again, if you are unsure, just ask.

Rod

[editted: see calculation below that included the voltage drop of the LED]

14rhb · Answer

If you have no other information on them you can usually just work out the resistance for 20mA @2.2v drop.

Actually in my calculation above I forgot the voltage drop of the LED: so 14.2v -0.2 - 2 = 12v to drop across the R1/R4

12v / 20mA = 600 ohms => 620 ohms typically

Rod

commanderkelly · Answer

Hello Rod, I have no clue what the part number is sorry I only have a pic I just took. not sure if that helps. Gary.

commanderkelly · Answer

Thank you for the info you have given me, I will do a photo for sure and let you know how it went. I will have to wait 2 weeks to get the stuff as I do not work so not got money right now. I do live in the UK so I will check the places you have told me to check. Gary.

commanderkelly · Answer

Hello Rod, I made the Flashing led circuit but got a prob, I made a video for you to check. https://www.youtube.com/watch?v=G2GYehn22DU&list=PL-kfiq8mHqQ2-8B3Lid3TlONV2XteJgby

shabaz · Answer

Hi Gary, It can do that, there's an example for a single LED here: https://www.arduino.cc/en/tutorial/blink Once you've tried that, then you'll have confirmed the software and hardware is functioning, and then you can extend it to more LEDs to achieve your aim. Extending to flash two LEDs alternatively is easy by slightly modifying this code. However, it wasn't clear what behaviour you were looking for. If it is to flash all LEDs identically or flash all LEDs in alternate pairs, then the code doesn't need to change further, you can connect many LEDs to the same connections on the Arduino, although you'll need to use some small additional circuit to provide the current for all the LEDs. Is this what you're aiming to do? Otherwise, if your aim is to make them flash at different rates, or alternately, or under control of individual switches or sensors, then that will need more code changes. The easiest way is to start with the blink example mentioned, and then try a few more examples, to get familiar with the capabilities of the Arduino. Then you'll feel confident to want to do more complex behaviour for the LEDs (e.g. controlled by switches as mentioned). The tutorials here: https://www.arduino.cc/en/Tutorial/BuiltInExamples will give you a feel for the capabilities of the Arduino (not all need to be tried, just a few that interest you).

commanderkelly · Answer

The Arduino UNO did it for me, just what I want. 1. tryed with 1 Led flashing2. added another led where both was flassing at same time. I want to Thank everyone who as helped me with this project really nice people, I am sure I will play with this to make them flash at different times.

14rhb · Answer

To everyone following commanderkelly initial blog, I must admit there are some strange goings on for such a simple circuit Like fmilburn I also breadboarded this to see how supply voltage affected the flash rate. I used 2N3804 transistors, 68k + 10uF/25v capacitors, plain old technology LEDs + 560 ohm current limiting resistors. These component values give a decent 1.22 Hz flash rate at 14v DC. The flash rate did vary, and it increased as the voltage dropped to around that required by the LEDs...as expected. Mid-voltages I saw a flash rate as expected, as seen in the blue plot below. This was based on the following equation: frequency = 1 / (0.38 * R * C) = 1.06Hz But at higher voltages the flash rate started to increase again and I wasn't sure why. I don't think it is to do with the capacitors: for some types of capacitor the capacitance value changes significantly with applied voltage, but that isn't the case for electrolytics. I ran the circuit without the LEDs and measured the frequency on an oscilloscope and it still exhibited this variance in frequency. That leaves the effect down to the transistors. Focusing on the voltages on the transistor, and specifically the base voltage (which goes negative during switching and the capacitor voltage is maintained but inverts) this reverse biased the base-emitter junction of the transistor....there is a rated value in the specification sheets for that of 6v minimum. I suspect when that is overcome the junction behaves like a Zener diode, something we did not expect. Now the timing will start to vary as we see in the blue plots above. There is a solution though. Adding a simple diode like the 1N4148 in series with the base of both transistors prevents this effect occuring as these diodes now increase that breakdown voltage significantly to approx. 75v. Plotting the frequency now gives the orange graph above. We can see the circuit behaves poorly at low voltages as it now has to overcome the forward bias of the new diode to start working. But at higher voltages it definately looks much better. Rod

commanderkelly · Answer

Hello 14rhb, I like what you are talking about here, I was thinking about a sensor also. Where the train say like driving up to the buffer stop and the sensor picks it up and the buffer spot LED starts to flash and when the trian get right up close the LED then stops flashing and stays on to let the train know to stop. I am not sure if that could be done or not and you might have other ideas. Gary. I have also ordered some 555 timers.

commanderkelly · Answer

Hello I tryed that circuit you showed me with the push butten, I also added a led from pin 13 to the breadboard with a 100Ohm Res. 1. Off 2. On

fmilburn · Answer

Hi Gary, If you are OK with them flashing at the same rate and more or less synchronized then it is fairly straightforward with an Arduino and will work just as Shabaz has described. The circuit will be simple. There can be 10 output pins connected to the LEDs with current controlling resistors of say 330 ohms. There could be 10 input pins for switching the behavior of the LEDs. When a switch signals the start a timer can be used to determine how long the blinking occurs. It is an interesting project. Once you get things working as Shabaz describes then report back if it is not clear on how to proceed.

jc2048 · Answer

Although datasheets always show it as either 5V or 6V, in reality reverse breakdown of the base-emitter junction can be a good bit higher than that - more like 7 or 8V. It's exactly the same effect as a 'zener' diode of more than 3 or 4 volts, merely the reverse breakdown of a pn junction by avalanching. The difference is that with an actual zener it's designed to cope with a certain power level and is properly characterised for voltage. With the transistor, it will start to do damage that will probably affect the transistor properties. On a 9V battery you can usually get away with it without problems, although technically you are outside the datasheet. With higher voltages, traditionally people used to put diodes in series with the emitters like this That's from Transistor Circuit Design published by the engineering staff of Texas Instruments in 1963. I'm not sure why that's better than having the diode in series with the base where you'd preserve the output swing. The effect on the timing would be the same. Perhaps there's a snag with doing that that I can't see.

14rhb · Answer

Hi commanderkelly Sorry to hear you were unwell - hopefully you are OK now. Thank you for keeping us informed of your project progress. It is always nice to know someone's goals are progressing and it sounds like yours are with extra LEDs around the model. You've got some great replies already and I'm so pleased to see your Arduino progress. Personally I too like the Arduino as you can very easily and accurately adjust the timings. In time you may also like to add a switch input - perhaps a microswitch that the train brushes against or better still a non-contact sensor....we can discuss that when you are ready. You would then be able to modify your Arduino code to perform an action based on that switch or to undertake an action (the buffer LEDs toggling) for a certain amount of time before turning off again. I see you have suggestion of using the 555 timer IC - always a staple device for such projects in the past and I think most of us have a fond spot for it. However it cannot be configured quite as easily - great if you are happy with the exact blink times and the 50:50 split between two LED but less so when the timing division is asymmetrical or hard to set. Setting up switches would then be more complex as well....not extremely, but just a bit awkward IMO.

commanderkelly · Answer

Hello and thanks for the info so far I will get a buffer stop or make one and all look at the links, as for how to set this up I have no clue but willing to learn. Gary.

jc2048 · Answer

I think I see why it's like that. I'm wrong to say that the effect on the timing would be the same. In the emitter, the diode reduces the voltage that the capacitor charges to by a diode drop and it also reduces the swing at the collector by a diode drop and the two effects then cancel to leave the timing more or less the same. That's not the case if you put the diode in series with the base. One thing that's complicating things for you and Frank is having the LED as a load. It means the capacitor doesn't charge all the way to the rail, when the transistor is off, and will add an offset to the timing.

commanderkelly · Answer

Hello Jon, I'll try what to said when I get some diodes all this info helps me as I am a noob lol. I did make a new video where I change r2 and r3 for a 68k resistor from 47k and it helped some. https://www.youtube.com/watch?v=qXv8IIqkNeU&t=0s&index=3&list=PL-kfiq8mHqQ2-8B3Lid3TlONV2XteJgby Gary.

commanderkelly · Answer

Hello and thank you I just want them to flash at the smae rate so for now this will do, thanks for your help. Gary