Schematics for "MH-Tiny"? Can it be made to run at 3.3V

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I recently acquired one of these "MH-Tiny" boards via Amazon and China for a pound or so. It's on sale by dozens of shops - but I cannot find the schmatic anywhere. I was, perhaps mistakenly, under the impression that the circuit board was open source hardware. I'm not overly concerned about this, but I realised after I'd bought it that it was 5V with a regulator to match, and I realised I really wanted to attach some 3.3v peripherals. I know the chip can operate at 3.3V, presumably I also need to check the BOD fuses appropriately, but I'm a bit stuck. So yes, there are other options llike throwing it out and using a different part, but I wondered if I could provide my own regulated 3.3v to the 5V pin, bypassing the regulator. I wondered if anyone had comments on this? 

You may have the opinion that I really shouldn't have bothered, that I should have used some other board. You're welcome to express that opinion and you might be correct, but if anyone knows if it can be made to operate at 3.3v I'd like to know.

I am reasonably knowledgable about electronics from the mid-eighties but I'm well behind modern trends. So please be nice.

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I wondered if I could provide my own regulated 3.3v to the 5V pin, bypassing the regulator.

Maybe.  It's what I would do.  At a pound each, you should probably just try it.

There are some details here: http://mh.nodebb.com/topic/47/mh... (and it looks like they've posted schematics for other boards in the past.)

In my observation, the voltage regulator on these "Arduino Derivative" boards from China is one of the shakiest components on the board.  Even if it were a brand-name component, it's inadequately heat-sinked (also true on official Arduino Nano boards), and it often NOT a brand-name component - just some 1-cent VR clone chip, that may or may not be the correct chip for the board :-(  Pretty much all of the actual problems I've seen reported on "Nano Clones" involves them "not working" when connected to external power rather than USB.

 

Also, the AVR used may not operate correctly at 16MHz at 3.3V... (it's out of spec.)

 

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Hello,   This Tiny88 board gets its power from one of two places.  There is +5V and ground(earth) on the USB connector.  The Tiny88 gets its power from here when a USB cable connects it to a PC.

The USB V+ goes into a Shottky diode, which drops it to about +4.5V.   This is the main power voltage; connected to Vcc on all the Tiny88.

 

The other place is the Vin "via" (plated through hole).   Vin is between +6V and +12/+15V.  It goes directly to a low-drop-out +5V voltage regulator (AMS1117).  The regulator output is connected to Vcc. 

 

To convert the Tiny88 to +3.3V, first obtain a supply of +3.3V AMS1117 voltage regulator ICs to replace the +5V regulator IC currently on the Tiny88).  Ebay is cheap with these parts:https://www.ebay.ca/itm/20pcs-AM...

 

Now the rework section.  There are two operations.  One is to replace the +5V regulator IC with a +3.3V IC (in the same IC package size).  The other is to connect the USB V+ (+5v) to the input of the +3.3 regulator chip, which disconnects its +4.5V from the Tiny88's Vcc bus line.

 

Lift the cathode side of the Shottky diode off the PCB.  Use a razor blade or box cutter blade after heating and solder-wicking the cathode side, which usually has a silver line.  Keep the other side of the diode (the anode) soldered to the PCB.  This disconnects the USB V+ from the Tiny88's Vcc line.   The cathode of the surface-mount diode should be suspended above the PCB pad (Vcc) and not be connected.  Use a ~3cm piece of thin wire to jump from the cathode metal tab of the diode to the input pad of the +3.3V regulator (which is also connected to Vin).  Wire-wrap wire AWG30 works well.

 

When powered by the USB cable,  USBV+ supplies +4.5v to the input of the 3.3V regulator.  Vcc is 3.3V.   When powered by Vin, Vin (~9V) gets regulated down to 3.3V for Vcc.  The +9 on Vin does not go into the USBV+ (5v) because it is blocked by the Shottky diode [the cathode voltage at +9 is greater than the anode at +5, no current flow in that direction].   This configuration allows +3.3V Vcc, +9V on Vin, and bidirectional USART communication over the +5V USB connector.

 

If you "just connect +3.3V to Vcc on the Tiny88", it will work at 3.3 Vcc until you plug in the USB cable to either program the Tiny88 or use the Serial Monitor.  Then USBV+ will raise the Vcc voltage to +4.5V.   Same with Vin.  If you put +9V on Vin, then the voltage regulator IC will put +5V on your +3.3V Vcc.

Last Edited: Sun. Sep 1, 2019 - 09:37 PM
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About  http://mh.nodebb.com/topic/47/mh-et-live-tiny88-16-0mhz  ... yes, I'd found that already, and as you can see it stops short of a schematic. However, if 3.3v means that it can't operate at 16Mhz I think I'll have to stick to 5V operation, and keep it for another task that doesn't have 3.3V issues.

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Wow - that's very detailed and I appreciate the effort you've made describing it. I'll be honest, my eyesight and manual dexterity aren't really up to it. I think I'll take a different route with a component that's designed for 3.3 operation in the first place. Thanks for the info though and hope someone finds it helpful in future.

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Welcome to the Forum.

 

All good advice above.

 

I'll just add an additional comment or two.

 

Are you building 1 PCB for your own, non-mission critical project, or building lots of them as a commercial endeavor?

 

If it is a commercial project, or mission critical, then keeping it all in spec is much easier and wiser.

 

Otherwise, just give it a try!

 

Interestingly, I just did a project that places an Arduino Nano on a larger PCB and runs it at 16 MHz, at 3.3 V, by back-feeding the 3.3V into the 5 V rail.

 

The data sheet says the operating freq is linear from voltage X to voltage Y, and when I checked I was over clocking it by 15% (or whatever it was).

 

Its for my own use, room temperature, and it won't get plugged into a USB Port because I put a 3-Pin header in front of the USB port to block it!

 

Remember that the data sheet spec's are for what the manufacturer guarantees the chip will meet.

All chips are suppose to meet that spec.

Many chips will outperform the stated spec, but you don't know which ones or by how much.

 

Typically one would have to be careful about the analog circuitry accuracy, and perhaps about the EEPROM writing times, etc.

 

The other issue to be aware of is that some voltage regulators have a built-in diode from Vout to Vin, so that Vout can not exceed Vin by more than one diode voltage drop. 

Some regulators don't have this internal diode, and some voltage regulator's data sheets suggest adding it as an external component if there is a chance that Vout won't drop faster than Vin when power is turned off.

 

On my recent project I added an external Schottky diode from the Vout (5V rail) to the Vin, to "protect" the voltage regulator on the Nano.

 

In any event, it is sometimes reasonable to tinker and play with the hardware.

 

Good luck with your project!

 

JC 

 

Edit: Typo

 

 

Last Edited: Mon. Sep 2, 2019 - 02:24 AM
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Again, thanks for the advice. This is purely a fun/personal project and while it would be delightful to come up with a "better mousetrap" that makes me rich, that's not my goal here.

 

Mulling it over, I wanted a device with a relatively large number of digital lines (about 20), a few analog devices, and a few digital out lines. It needs to be battery powered and the processing speed wasn't particularly critical. I had a Nano clone to hand, so thought the MH TINY was worth a look. Looking at it now, if I want 3.3V and low current consumption from the outset, I might as well get myself a DIP packaged ATTINY88 or whatever, throw it on a breadboard, and use 3.3V from the start.

 

I studied electronics and graduated in 84, then spent the majority of my working life in Software and Training, so getting back into hardware after 35 years makes me feel a bit like Rip Van Winkle. My physical coordination skills were never the best and my eyesight isn't what it was, so ... I need to keep it simple.

 

Thanks all!

Last Edited: Mon. Sep 2, 2019 - 12:08 PM