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Replacing light bulbs with LED's for illuminating a radio's scale

Discussion in 'LEDs and Optoelectronics' started by Richard9025, Jun 24, 2017.

  1. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    Hi !
    In my old big radio , the light bulbs that were supposed to light up the scale burned
    So , I want to replace them with some LED's that can illuminate all of the radio scale .
    The schematic showing how the light bulbs are connected :

    LEDS.jpg

    BULB 1 , 2 and 3 are burned , the power led still works (35 years )
    Can you give like ebay/amazon/tme links to the leds that can replace the burned bulbs , or at least specs ?
    Thanks !
     
  2. Alec_t

    Alec_t

    2,686
    710
    Jul 7, 2015
    A problem with LEDs is that they are very directional compared to incandescent bulbs. Each bulb may therefore need to be replaced by several LEDs aimed in different directions to get enough light spread. Alternatively a single LED could be behind a diffuser; but this would reduce the brightness and require compensation by using greater LED current or a high efficiency type. Experimentation will be needed to get a reasonably uniform spread of light.
    Impossible to give LED specs when we don't know the bulb specs or how the bulbs are positioned :rolleyes:.
     
    Richard9025 likes this.
  3. safetywatto

    safetywatto

    3
    1
    Jun 24, 2017
    The equivalnce wattage ratigns on LED bulbs confuse the hell out of me
     
    Richard9025 likes this.
  4. Externet

    Externet

    710
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    Aug 24, 2009
    Bulbs 1,2,3 are supplied by AC. To use LEDs there, would be convenient to insert a 1N4001 diode in series to S4; and resistance values for R22,23,24 changed to 1KΩ/0.25W. Observe polarities on LEDs.

    You will end with insufficient illumination spread for the dial after the surgery. Try 6V light bulbs of the same dimensions instead; they may last another 30 years. If you find 12V bulbs that fit, will be less bright with longer life.
     
    Richard9025 likes this.
  5. duke37

    duke37

    5,231
    718
    Jan 9, 2011
    Instead of a series diode, use two leds in inverse parallel, each will protect its butty from excess reverse voltage.
    With 10V AC you could use two pairs in series.
     
    Richard9025 likes this.
  6. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    More info :
    First how long is the scale (30cm ruler also has inches)
    30cm.jpg

    Now the position of each bulb :
    FIRST BULB
    1st.jpg

    SECOND BULB

    2RD.jpg

    THIRD BULB

    3rd.jpg

    Now , these bulbs are covered with two metal pieces to reflect the light across all the scale
    One metal piece :

    placed.jpg



    How it should look like with all the bulbs working (a photo from the internet)

    maxresdefault.jpg

    And how it looks after someone replaced the bulbs with leds (also from internet) :

    1306083282.jpg


    There is not much info about the bulbs , there is more about the led's
    How I made that drawing from above
    There is a interconnection schematic in the service manual that shows where tha bulbs are connected
    BEC = BULB
    RETEA = POWER (power like from the wall wart , not power from a cb radio or amplifier or smth else)

    interconnections.jpg

    What is in the orange rectangle from above is this :
    LEDS.jpg
    The orange rectangle zoomed in
    snippet.jpg

    The leds that indicate what band it is (12 total green led's) are some MDE1531 , there is one red led for the fm stereo even if there are two fm bands (2 fm , 1 lw , 1 mw and 6 sw)
    About the specs of the light bulbs inside . I will remove one and measure it
    If I remove the metal pieces , I can fit some leds there
    Now you know much more info
     
  7. Alec_t

    Alec_t

    2,686
    710
    Jul 7, 2015
    So, the dial is edge-lit. The LED effort shown looks to have 2 LEDs at each end of the dial. More LEDs would clearly give a better spread of light. If it were me I'd try probably 6 per end.
     
    Richard9025 likes this.
  8. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    See if the following diagram is good
    IMG_20170623_104411091 - Copy.jpg

    Also note that the radio scale is ALL GLASS , and on top of it is the "filter" showing the band , numbers , etc)

    So these 2 metal pieces were reflecting the light into the glass , so that all the letters can be illuminated
    On the photo number 7 on my previous post , he removed the metal pieces , because the blue leds dont equally light up the scale

    Between the metal piece and the rest of the radio are 4 mm
    If the drawing is above , what type of LED can I use , they must be under 4 mm , so some ultra-common 3mm leds . And , what voltage and miliampers rating should they be ? Dont forget the 10Ω/2w resistors .
     
  9. Externet

    Externet

    710
    145
    Aug 24, 2009
    Diagram is good. Color is your preference. Probably 'warm' white will go better with the age of receiver. There is also more efficient types that give more light with the same current.
    Aim the LEDs beam spaced towards the edge of glass ends. Avoid under and over aiming the glass, or place masking.

    Perhaps 1cm+ distant to avoid the concentrated spots and give more uniformity. You have to experiment. Adding a piece of diffuser as a matte-clear adhesive tape at the glass edges can help.
    LEDs come in several beam angle flavors, you should investigate the data sheet for the ones you can source. That will affect the optimal distance choice from glass edge.
    Luminosity will change with series resistors value, Aim for 10-15mA . Start with 1KΩ resistors and lower 100Ω steps without exceeding 75% of rated current, as they will be running on AC.
     
    Richard9025 likes this.
  10. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    I'll use some white LEDs , 3mm 3.2V 12mA (3V-5mA 3,2V-12mA, 3,4V-20mA)

    For DC is : The supply voltage minus The forward LED volts divided by the forward LED current and for AC , the result is divided by how many leds are in inverse parallel

    So , for the following circuit :

    LED.jpg

    R is 10v-3.2v divided by 6x 0.012A = 6,8:0,072 = 94Ω
    So I can use 100Ω for R22 and R24 and for R23 , because there are only 2 leds , not 6 , a 270Ω resistor

    See if the calculations are correct .
    Thanks !
     
  11. duke37

    duke37

    5,231
    718
    Jan 9, 2011
    That diagram would work if all leds were identical. It would be better to put them into pairs so that one led does not have to take all the reverse voltage.

    R = (10- 9.6) / 0.012 = 33
    This however is too low to define the current with voltage or temperature variations.

    The resistor will set the current and will be easy to calculate using DC, however, you have AC so there will be no current during part of the wave and in particular only half the wave will be active. I would select the resistor to give the correct brightness. Compare with 20mA DC.

    Have you thought about glueing the leds directly to the edge of the glass to put most of the light into the scale?
     
  12. Alec_t

    Alec_t

    2,686
    710
    Jul 7, 2015
    If you were to use amber/yellow/green LEDs instead of white their Vf would be lower so you would have more 'headroom' to define their current better with a dropper resistor, assuming the LED arrays are as in post #8.
     
  13. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    All the leds are identical
    33 ohms ? kinda low
    The scale is all glass , yes i thought of that , but first lets get the electrical part done
    Just make a drawing on how to wire them and some specs if its incorrect what i did before,
     
  14. duke37

    duke37

    5,231
    718
    Jan 9, 2011
    The leds may be similar but they will not be identical and will be at different temperatures.

    I would try two inverse parallel pairs in series and a single resistance.

    At a rough guess (10-6.4)/0.0020 = 180
    See how the brightness compares with 20mA on DC.

    Use the circuit as in #10 but delete two diodes and cross connect each side as in #8.
     
    Richard9025 likes this.
  15. Externet

    Externet

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    145
    Aug 24, 2009
    AC reaches peaks (and valleys) of 1.41 times RMS = 14.1 Volts. for the 10 Volts AC.

    Nothing wrong with asumming 10% extra if secondary of transformer puts 11V AC RMS instead.
    That raises peaks to 15.5 V. Call it 16V for safe calculation.

    The 16 V when used with three 3.33V Vf LEDs leaves 16-10=6V to use for the current calculation.

    6V divided by 0.015 A = 400Ω for the schematic on post #10. Go for 390Ω 0.5W standard value.

    Perhaps you like side looking LEDs----> http://www.mouser.com/Optoelectronics/LED-Emitters/Standard-LEDs-SMD/_/N-ec21j?P=1yp37eaZ1z0s07o
     
    Richard9025 likes this.
  16. duke37

    duke37

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    Jan 9, 2011
    If you want to calculate properly, then work out the time of the peak of the sine wave above 6.4V. Integrate the voltage during this time to calculate the average voltage to use to calculate the resistance. Perhaps you went to school.:)
    I would do it by trial and error.
     
  17. duke37

    duke37

    5,231
    718
    Jan 9, 2011
    I have had a brain storm and decided to do some calculus. 65 years is too long to leave it dormant.

    Take a supply of 10V, peak of 14.1V.
    Take drop of two leds totaling 6.4V.
    Work out when the voltage is high enough to pass current. 0.476 to 2.67 radians.
    Integrate sine wave between these limits. 14.1 * 1.78 = 25.13
    Subtract the lower rectangle (6.4 * 2.2 ) from 25.13 = 10.96
    Average height of active area = 10.96/2.2 = 4.98
    Average height over one cycle = 4.98*2.2/3.14 = 1.74
    V = IR
    R = 1.74/0.02 = 87
    So use a 91 or 100Ω resistor.
    The current will be high over part of the cycle but will average about 20mA.

    It would have been easier to get my computer working on LTspice:)
     
  18. kellys_eye

    kellys_eye

    4,281
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    Jun 25, 2010
    Last edited: Jul 2, 2017
  19. FuZZ1L0G1C

    FuZZ1L0G1C

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    Mar 25, 2014
    Regarding the diffuser, I've been thinking of an idea, open to suggestions:
    For camera flashguns, and on a larger scale, fluorescent light-fittings, you get a perspex diffuser.
    These come in different thicknesses and pattern densities, from "moire-pattern" fine to bumpy.
    As they don't block light, instead diffusing it, (maybe..) it will do the job.
    LEDS: what about also strip-lighting top & bottom edges, as "LED" photo is dimmer on face.
     
  20. duke37

    duke37

    5,231
    718
    Jan 9, 2011
    You do not need a difuser, what you need is the light going into the edge of the glass. The light stays inside the glass due to total internal reflection When it encounters the print, the light can escape. Light bulbs must be placed close to the edge but leds will be better since the light source can be placed closer and glued into place.
     
    FuZZ1L0G1C likes this.
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