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Home made LED & loco tester - Hints & Tips - Reference Area. - Your Model Railway Club
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 Posted: Tue Aug 7th, 2012 07:25 pm
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upnick
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Hi All,

I have made a simple LED tester for when i'm doing my projects saving getting a power supply out ...... the basic structure is 60 thou plasticard sheeting.

I combined the box its housed in with a length of track for a loco to be simply tested to check the wheel to motor contact is being met.

It would be useful at a show where the loco could be run briefly as a demo ... i have no control over speed but the on/off switch used while holding the loco gently is sufficient.

Overall dimensions for this one with N scale track are .....
6 inches X 2 1/4 inches X 1 1/8 inches.

of course for OO or larger it would have to be scaled up.




Switch on the left controls power to the track & on the right for testing LED'S via the two studs from Peco's probe & studs a resistor is fitted already in the box.




Inside the box simple wiring to two 9 V batteries enough power for each use seperatley.



With the lid screwed on access to changing the batteries is easy with a sliding door on the top from  more plasticard.

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 Posted: Thu Aug 9th, 2012 10:44 am
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Campaman
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Very neat, I just carry a 9V battery when going to exhibitions, car boots, second hand sales etc. Its just the right size for 00 wheel sets for testing.



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Andy
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 Posted: Wed Aug 22nd, 2012 12:53 pm
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upnick
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Campaman wrote: Very neat, I just carry a 9V battery when going to exhibitions, car boots, second hand sales etc. Its just the right size for 00 wheel sets for testing.

Hope  you dont carry it in your pocket with change as  some years ago i  remember one of my customers buying some loose new rechargable AA batteries he placed in his pocket which had some change in it . 

On the way home he had to pull  over &  remove his jacket which had a singed pocket :roll:

An old   one of these connectors is ideal snapped on  to prevent this ;-)

http://compare.ebay.co.uk/like/170845248425?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar&cbt=y

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 Posted: Wed Aug 22nd, 2012 09:45 pm
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Geoff R
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Very neat, but I would put a diode in series with that resistor for your LED tester if I were you.

9 volts is quite sufficient to blow an LED junction if connected in reverse - especially some of the very small high intensity ones. Almost any diode will do as you appear to be using a 1k resistor and are therefore limiting the forward current to less than 10mA.

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 Posted: Wed Aug 22nd, 2012 10:36 pm
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upnick
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I'll look out for a diode Geoff  is there a  particular one i  should be looking for (value)  to use.

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 Posted: Thu Aug 23rd, 2012 02:55 pm
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Geoff R
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Anything will do as the current is so low. The key is the "reverse voltage" rating which is rarely less than 50v whatever the diode. If you need to buy/find one, then the 1N4001 series are very common as they are often used for converting AC to DC. There are usually several inside Hornby/Bachmann controllers if you happened to have an old one.

You will also find similar diodes in lit coaches to ensure lighting from analogue control regardless of loco direction.

If purchasing at a show or local Maplins or similar, you will find that they are just a few pennies each.

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 Posted: Thu Aug 23rd, 2012 04:06 pm
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upnick
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Hi  Geoff,  

Many thanks for the information  very useful,    while i'm in  question mood  what is used in  flicker free lighting sets i  have seen could you tell  me as i've heard they are a common component that keeps the lighting on  for a while when  the coaches are stationary  storing a  small  current for a while.  

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 Posted: Thu Aug 23rd, 2012 07:02 pm
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Geoff R
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Nick, this usually consists of a small circuit featuring an electrolytic capacitor which stores energy rather like a rechargeable battery but for only a short period of time.

 Modern materials allow a fairly large capacitance to be provided within a relatively small package so it is quite easy to store enough energy to power several LEDs for short time periods such as when power is lost by a loco running over dirty track or even over an insulated frog.

The same principle is used for "stay alive" DCC decoder circuits.

 A resistor is used within the charging circuit to ensure that the capacitor does not cause problems for the track supply. Diodes are also included to ensure a single polarity supply to the capacitor and LEDs regardless of the polarity of the track if running with a DC controller. These diodes will usually also mean that the same circuit will operate successfully if the track is powered by a DCC controller.

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 Posted: Thu Aug 23rd, 2012 08:20 pm
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upnick
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Cheers Geoff,  answers my question  but still pondering if i should make my own  flicker free or go for DCC  concepts units  although they seem  expensive.

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 Posted: Thu Aug 23rd, 2012 09:02 pm
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Geoff R
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Sounds like you are looking for a solution for DC operation, yes? One of these:-

http://www.rapidonline.com/Electronic-Components/0-1f-5-5v-5mm-Memory-Backup-Capacitor-11-2160

with a couple of other components will keep LEDs lit without power for up to an hour.

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 Posted: Thu Aug 23rd, 2012 09:56 pm
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upnick
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Geoff R wrote: Sounds like you are looking for a solution for DC operation, yes? One of these:-

http://www.rapidonline.com/Electronic-Components/0-1f-5-5v-5mm-Memory-Backup-Capacitor-11-2160

with a couple of other components will keep LEDs lit without power for up to an hour.


Great solution for   DC  Geoff but i  am DCC ;-)

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 Posted: Fri Aug 24th, 2012 07:17 pm
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Geoff R
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Not a problem. They will work fine on DCC as well, but I therefore don't understand your earlier comment about keeping lights on when coaches are stationary? If on DCC track there will always be power present, and therefore the coach lights will be on all the time. Did you have something else in mind?

 

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 Posted: Fri Aug 24th, 2012 10:33 pm
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upnick
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Hi  Geoff,  

No  I hadnt naything else in mind  i  have fitted LED/SMD's  to some coaches &  they  flicker at times  on  DCC   i  was after a way to stop  this at an  economical  cost but you've answered  my question with the memory back up  capacitor.

Yes of course there is power on  all  the time on DCC my error  so  for an  electrically challenged individual  (me)  could you  tell  me  where to  place the capactior in  this diagram please.



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 Posted: Sat Aug 25th, 2012 03:52 pm
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Geoff R
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Nick,

I have added to your drawing:-


Forget the memory back up capacitor for this solution. You will not need that much capacitance to achieve flicker free on DCC and as they only withstand a voltage of 5.5volts, you would have to add a voltage regulator and also lose two thirds of the available power for your LED circuit. They really only come into their own for lighting without power and when the voltage on the track is variable as in DC control.

A couple of questions about your circuit. You show + and - as the power entry points. Do you really mean that?? If running on DCC you will only get + and - if you have a full wave rectifier between the DCC power on the bogies and this circuit. If you have NOT got a rectifier in place, you will need to include one; or at least 4 diodes to achieve the same thing. This is because although the LEDs work with either Direct Current or Alternating Current (as occurs with DCC), the electrolytic capacitor must have DC only.

You show a 1k resistor, but you also show the LEDs in series. How many LEDs are you putting in series? It depends on what sort of LEDs you are using, but the tiny surface mount ones you can buy in a strip are usually wired in groups of 3 in series with 180 ohms and are suitable for 12volts supply.

I have included a 220ohm resistor and a diode in my circuit. The resistor is to limit the inrush current when power is first applied and avoids it being seen as a short by the DCC controller. It will only take a brief moment for the capacitor to charge up to the supply voltage. The diode is to stop the charge stored in the capacitor feeding back onto the track when power is lost. Instead, all of the charge will continue to supply the LEDs until power is once more applied through the resistor and diode.

My assumptions are that the DCC track voltage is about 16.5v peak to peak AC. After rectification, you should have around 15v DC feeding the above circuit. If the LEDs take 10mA from a 12v supply, then there will be approx 3 volts dropped across the diode and 220 ohm resistor leaving 12v on the capacitor.

If you use a 1000uF capacitor, then I reckon you will get half a second or so from it before the LEDs dim. I think that ought to prevent flicker.

http://www.rapidonline.com/Electronic-Components/Forever-85-C-1000U-16V-Radial-Lead-Aluminium-Electrolytic-Capacitor-11-0290

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 Posted: Sat Aug 25th, 2012 06:34 pm
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upnick
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Hi Geoff,   

I  will   try the  method you've been kind enough to  set out for me in the diagram & let you know how it goes.

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