Use of P1 to replace worn DCC controller?

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  • Thank you @kwinn I need to read up on how AC works.

    It seems I was wrong about the trains needing 30vac at the tracks. And I think I can jury-rig something to drive an h-bridge. I am reading through the NMRA standards and I see this:
    In no case should the peak amplitude of the command control signal exceed +/- 22 volts. The minimum peak value of the NMRA digital signal needed to provide power to the decoder shall be +/-7 volts measured at the track. Digital Decoders intended for "N" and smaller scales shall be designed to withstand a DC voltage of at least 24 volts as measured at the track. Digital Decoders intended for scales larger than "N" shall be designed to withstand a DC voltage of at least 27 volts as measured at the track.
    I misread my supply for the Bachmann remote. It gives 16vac, not dc. My multimeter reads 22vac from it. I shall hunt down a 15vdc wall-wart supply.
  • AwesomeCronkAwesomeCronk Posts: 941
    edited 2020-01-27 - 15:54:10
    I have finally managed to find some time to work on this project! I have been drawing an assembly-level schematic for the remote. All of my custom symbols look like the real deal, just a thing of mine. I find that it helps to match pins properly if I understand where they are physically. I need to find out if the Prop's 3.3v IO is enough to trip the LM298N at 18.5V operation. I also need to find out how I am going to connect the LCD.
  • The Prop 3.3V should not have a problem driving the L298 Dual H-Bridge Motor Controller since the logic power supply voltage is +5V. The 18.5V is for the high current and voltage of the H bridge power supply.
  • So, I do need to poke 5v into the +5v pin on the board? It has an onboard 5v regulator that normally does this, but I have disabled it as it only works up to 12VDC input.
  • Peter JakackiPeter Jakacki Posts: 9,220
    edited 2020-01-27 - 02:59:22
    Why don't you replace that cheap but huge switcher module along with the 7805 with a cheap 3-pin R-78 5V switcher? Then you can feed 5V where you need to including directly to the 5V of the Prop mini?

    Btw, the LM298 is rather a bulky and ancient chip and there are much better solutions. However in regards to the whether you can switch it with 3.3V levels just look in the datasheet under Vih - Input High Voltage. In this case it specifies 2.3V min which means you can.
    300 x 300 - 14K
  • AwesomeCronkAwesomeCronk Posts: 941
    edited 2020-01-27 - 02:58:04
    Because I’ve already bought these and have exceeded my purchase limit for the next few months. :smile:

    Also, I got 15 or so of these for $15, alongside the fact that I can adjust them. I am confused on the prop mini’s 5v pin. I thought at first that is was an output. Then I forgot it existed and plopped on a 7805. Now I am thinking that I need to nix the 7805 and draw from the mini’s onboard regulator.
  • Peter JakackiPeter Jakacki Posts: 9,220
    edited 2020-01-27 - 03:09:28
    Because I’ve already bought these and have exceeded my purchase limit for the next few months. :smile:

    Also, I got 15 or so of these for $15, alongside the fact that I can adjust them. I am confused on the prop mini’s 5v pin. I thought at first that is was an output. Then I forgot it existed and plopped on a 7805. Now I am thinking that I need to nix the 7805 and draw from the mini’s onboard regulator.

    Just because it says "regulator", it is not magic. You can operate the LM2940 directly off a much higher supply but you it will have to dissipate more power (heat) for the same current. Now you draw more current externally as well, and you can end up with smoke. It may be that the software you have running on the Prop mini may not end up drawing much current (more cogs, more speed = more current), and the LCD without backlight only requires milliamps, so you could run the mini directly up to 26V and power the LCD from the mini's 5V output (could force this as an input too). But if the regulator is too hot to touch or you are also powering a backlight then you will need an external 5V switcher (why aren't you at least setting the switcher to 5V output directly?).
  • The 7.5V is good as is. What is a simpler solution is the +5V on the prop-mini module can be used for powering the LCD module. Don't need any separate +5V regulator.
  • I looked at the prop mini’s product page:
    Voltage requirements: Regulated 6.5–12 VDC though VIN pin

    That’s why I chose to set the buck converter’s output to 7.5v, as that should account for most minor fluctuations.

    Should heat be a problem, I have two 5v laptop fans that I can look at. I don’t know their current draw, but that may be low enough to put next to the lcd.
  • Peter JakackiPeter Jakacki Posts: 9,220
    edited 2020-01-27 - 14:17:20
    Even 12V is a bit high if you are drawing too much current. It's simple, the LM2940 silicon can handle up to 26V at up to 1A but you will never be able to draw 1A with 26V input since Vin-Vout = 21 x 1A = 21W = lots of smoke. But even so this regulator comes in different packages which limits things even more, even though it's the same regulator. I may spec a product for a recommended voltage range too but that's a lot like a dry clean only label. If you know what you are doing, you are not limited by that "label".

    So 6.5-12VDC is not the limit of the regulator but simply a safe guide given by Parallax for most applications, not all. Understand the difference.

    Why 7.5V anyway? The LM2940 is a low drop-out regulator so you only need 5.5 or so for it to regulate to 5V. That means you can draw more current from the mini's 5V output without stressing the regulator overly.
  • I made some progress today. I got Chris Gadd's 4bit driver working. I also got the LM2596 properly adjusted. The display draws about 8.3uA in operation. Next step is testing the L298N and the DCC driver.

    Ronald J. Portugal, inventor of the solderless breadboard, I salute you!
  • You mean 8.3ma, not 8.3ua. I'm guessing that it doesn't use a backlight which would draw a lot more current.

    While you salute, I despair :)
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